Understanding Exhaust Gas Analyzing

Jim Mobile Technician Southampton, Pennsylvania Posted   Latest   Edited  
Lean Misfire
Excessive Emissions
No Start

A half of a dozen times a year, I’m asked a question about exhaust gas analyzing. It is generally right before or after a technician is taking an ASE exam. I’m well aware that most technicians don’t have or ever used an exhaust gas analyzer, but just knowing how the gases react to the combustion chamber can be valuable information.

One of the first mistakes I generally see is the technician is exposed to a chart with all of the gases included.

Then if it happens like what happened to me, the next thing you hear is “See it is all right there”, I don’t know about you but that picture looks like a bowl of spaghetti to me. When someone finally decided to teach me what all of the individual exhaust gases were and how they interacted with each other, the picture became clearer.

Now it this point we can go way off on a chemistry lesson or concentrate on what the gases are telling us for diagnostic purposes. The first very important element is that some of the gases are PRODUCTS and others are BY-PRODUCTS. An easy way to think about this is, a product gas does not need for something to happen to be present. A hydrocarbon is a hydrocarbon with nothing else happening, which makes it a Product. Oxygen is Oxygen with nothing else happening, so again, this is a Product. In the case of Carbon Monoxide, Carbon Dioxide and Oxides of Nitrogen there has to be an action, like combustion taking place, so these gases are classified as By-Products.

When I’m teaching exhaust gas analysis, I always like to start off by teaching 2 Exhaust gas analysis. We have all heard the phase if you watch over the pennies, the dollars will take care of themselves, well in this case, the pennies are the Oxygen and the Carbon Monoxide. Now when you are using exhaust gas analysis, you must be sure you are dealing with the oxygen from the combustion chamber only, if the vehicle has an AIR circuit, it should be disabled, also make sure there are no leaks in the exhaust system to draw outside oxygen in​.​With that being said, Let’s take a look at our 2 exhaust gases, first independently, then together. 

Carbon Monoxide (CO) is a By-Product of combustion on the rich side of the air/fuel ratio, now keep in mind, it doesn’t have to be too much fuel, it could just be not enough Oxygen. So, from the stoichiometric point going lean Carbon Monoxide bottoms out, that is the reason that Carbon Monoxide is NOT a good indicator of a lean air/fuel mixture. You might know that it is lean, but not how lean it actually is.

The Next Product gas to use in your Diagnostic procedure is Oxygen (O2), There is approximately 21% oxygen in the air and how and if it is used in the combustion chamber is valuable information. As I stated already, you have to make sure you are dealing with oxygen from the combustion chamber, No AIR pump system or exhaust leaks. 

So now, we have the opposite effect, if the combustion chamber is running rich, you would have used up the available Oxygen content. If it is running lean, you will see the Oxygen content climbing the leaner the combustion chamber is. Now, if we put the two most important gases on a chart together, we will have an excellent idea of what is happened in the combustion chamber. 

So now, my out-of-the-gate diagnostics is if the CO is higher, then the O2, the combustion chamber is on the rich side of Stoichiometric, and if the O2 is higher, then the CO, the combustion chamber is on the lean side of Stoichiometric. PERIOD!!! 

Now let’s see what happens as we add some of the other exhaust gases to our chart.Carbon Dioxide (CO2) is a By-Product of complete combustion, The better the combustion chamber does its job, the higher the CO2, Which is the reason it is known for measuring the efficiency of the combustion chamber. 

In the Perfect world, the CO2 would actually peak out slightly to the right of stoichiometric (lean side). So let’s do a little recap on the gases so far, if we match the Carbon Monoxide (CO) and the Oxygen (O2) at their lowest readings, without even having to worry about the Carbon Dioxide (CO2) reading because it will be at its highest level. So as I already stated, if you take care of the pennies, the dollars will take care of themselves. 

Let’s move on to the exhaust gas that seems to cause the most confusion, Hydrocarbon (HC), a lot of techs think that high Hydrocarbon readings are an indication of a Rich air/fuel mixture only, NOT TRUE, if you have a high Hydrocarbon reading all it means is that the combustion chamber isn’t at stoichiometric. Let’s add the Hydrocarbon exhaust gas to our chart.

As you can see, the Hydrocarbon readings can be elevated on both sides of stoichiometric. The confusion with this seems to be that we can all understand the increase in hydrocarbon on the rich side of stoichiometric because we have more fuel than we can burn, but on the lean side of stoichiometric, we go into what we know as a “lean misfire” situation at approximately 17:1 air/fuel mixture. The reason for this is there is a portion of the combustion chamber that does not have enough fuel molecules close enough to each other for the combustion process to continue across the top of the piston. The next thing that is going to happen is, the exhaust valve will open and allow the unburned Hydrocarbon out into the exhaust system. My analogy on this for a better understanding is to think of a lot of sheets of paper crumpled up in a confined area.

With all of the paper close enough to each other, if we were to light any piece of paper the flame would continue burning by jumping to the next piece of paper, same with a spark plug lighting off the air/fuel mixture in the combustion chamber. But, if we were to have a large enough area without something to burn, the flame front would stop right there, and if we were in a combustion chamber with that situation, the Hydrocarbon on the other side would not continue to burn and the exhaust valve would open and allow the unburnt Hydrocarbon out into the exhaust system. 

If we go back and look at the last chart, if the Hydrocarbons are high along with the CO being high, the air/fuel mixture is on the Rich side of stoichiometric. If the Hydrocarbons are high with low CO, then the air/fuel mixture is lean. 

Now we come to our 5th and final exhaust gas Oxides of Nitrogen (NOx). Under the presence of HEAT Nitrogen and Oxygen Bond. The number one rule is, We don’t want to get rid of NOx, we just want to control this exhaust gas, we need heat in our combustion chamber for good combustion​.​Let’s add another chart with the NOx included now.

Now look what happens with NOx, it peaks out on the lean side of stoichiometric. Why? Because of the HEAT that comes with a lean air/fuel mixture. We as automotive technicians already know about this, we have all either used or saw an acetylene torch used in our shops. We light the torches on the acetylene, then slowing add the oxygen until we have a nice hot flame, after we heat the metal up enough, we push the lever to add a little more oxygen to the torch flame and the now the flame is so hot that the metal begins to melt.

So it is at this point that our flame on the torches are slightly lean of stoichiometric. This condition in the combustion chamber is what rises the NOx levels so high an also causes the exhaust valves to burn. But remember that at approximately 17:1 air/fuel, we can no longer support combustion, so the NOx drops at the point also. This is the reason that I call that section “FORT NOx” 

For a little better understanding of this, the next time you have you torches out, instead of turning the torches off the correct way, try adding a little more Oxygen to the flame. What will happen is there will be a loud POP and the flame will go out… —> LEAN MISFIRE.

Remember that the next time you have a vehicle that starts to POP through the intake when you mash the throttle down, if there is a fuel delivery problem, the combustion chamber goes into a lean misfire state also.

Thomas Diagnostician
Saint Petersburg, Florida

Thanks, Jim. I just spring for Snap On's new five has analyzer. Like Scopes and scan data, I put it on known good vehicles as well as the sick ones....

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Brandon Diagnostician
Reading , Pennsylvania


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Mario Diagnostician
Weston, Florida

Wow, excellent breakdown and writeup for this extremely important diagnostic strategy. I have to constantly revisit this as I do zero gas analysis with a 5 gas analyzer, unfortunately. I wish my shop would pick one up, but until then this is great writeup to stay sharp on the theory. Thank you, Jim! 

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Taurayi Owner/Technician
Harare, Zimbabwe

Awesome Jim, there is no better simpler way of explaining exhaust gases analysis than you did . Your teaching is exceptional my friend . Thank you ,❤❤❤ !!!

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Geoff Diagnostician
Lahaina, Hawaii

Yes, indeed. A lot of us (this whole State probably) have never seen, or used, one in person. I used to memorize the chart before the test, and get all the questions right. All made perfect sense to me. When I re-certified last, about four years ago, the questions weren't even there anymore. I was glad for that, One less thing to worry about.

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Guillermo Owner/Technician
Blue Island, Illinois

Thanks Jim, I really always got confused with understanding all of this in my past, but it's so important to understand that. I still get a little confused, but thanks for using the torch example. I will be looking and remembering this every time I use the torch. I will be re-reading this over and over again until I memorize it.

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Martin Instructor
Burnaby, British Columbia

Nice "KISS" approach Jim.

I'd like to add some thoughts if I may, for anyone just entering vehicle emissions analysis, focusing specifically on tailpipe emissions.

For me, during the earlier years of emissions testing, learning how to lower undesirable vehicle emissions was aided by having the opportunity to work on vehicles fitted with minimal emissions control system components. This meant often being of the non-catalyst, non-feedback variety with carburetors and distributor ignition systems. Being able to make physical adjustments and view the results of those changes as changes in the measurements of gases leaving the tailpipe provided some valuable learning opportunities. It was great working on vehicles that had only a PCV valve, not even a catalytic converter as on my '71 GMC C2500. Whatever goes in to the engine through the carburetor, exits the tail pipe unaltered by anything but the combustion process and PCV system functions.

In reality today, since most vehicles will have a catalyst, the emissions diagnostician should sample engine out post combustion emissions and use them to determine whether gases are in an acceptable ratio and quantity. This will identify whether there is an engine mechanical/control systems condition or an exhaust after treatment condition. Of course, relative compression and other non-intrusive tests may be incorporated into verifying mechanical integrity as discussed elsewhere.

Acceptable engine out exhaust gases into a bad catalyst, results in excessive tailpipe emissions. Unacceptable engine out exhaust gases into a good catalyst, may result in excessive or marginal tailpipe emissions and poor catalyst life.

There are of course many pre and post combustion influences that must be inspected and measured, but starting with the "KISS" approach reduces the confusion.

For me, also using the oxyacetylene welding and cutting torch to demonstrate the visual effects of the air fuel mixture has long been an effective segue into gasoline internal combustion dynamics. This was demonstrated and discussed before moving on to utilizing the most basic of vehicles, which shifts the focus to the fundamentals of tailpipe emissions and what takes place inside the closed combustion chamber.

BTW, I said "was", because exhaust gas analysis has been removed from apprenticeship. In BC the AirCare® emissions testing conducted at centralized stations operated by Envirotest, only here in the Lower Mainland that began in 1992. It was shut down at the end of December 2014. While analyzing exhaust gases has diagnostic value, it is no longer an apprenticeship learning requirement and "portable" gas analyzers have pretty much become a thing of the past.

However, that does not prevent useful discussion for those who need or desire skills focusing on gas analysis.

By using a non-feedback vehicle, the learner will not be faced with the challenges of computerized vehicle control systems that are attempting to regain control of a system resulting in confusion. That level of understanding can come later, once combustion is fully understood, adding various layers of complexity in vehicle systems, sensors and exhaust after treatment.

Efficient combustion of chemicals entering the combustion chamber can be affected notably by performing some very simple carburetor adjustments to vary vehicle air fuel ratios and making ignition timing adjustments. Monitoring the analyzer measurements, allows us to consider and interpret the resulting effects of changes on tailpipe emissions. This can readily be observed on both emissions analyzers and ignition oscilloscopes, when making incremental changes and monitoring the effects.

While the numbers displayed on the gas analyzer reflect the changes in AFR or ignition timing, seeing the effects of the changes on the ignition oscilloscope during the 1960s was key to my understanding of using an ignition scope in combustion diagnosis.

BTW, for accurate results, always make sure to perform analyzer maintenance as required. Leak testing checking the filters and calibrating the analyzer is necessary. It was mandatory here to be completed every third day of use for AirCare® diagnosis on vehicles . It should be the first piece of equipment turned on daily after the shop lights.

When presenting exhaust emissions content, it is important to consider everything that will support and affect combustion, from the air that we breathe in the shop, to the equipment used for diagnosis. Expect to see a handful of hydrocarbons and ~21% oxygen in the shop air, should be displayed on a calibrated analyzer. Nitrogen forms 78% of the air and ~1% other gas and whatever else is in the air that enters the engine.

Consider that all air, fuel, chemicals and any other elements entering the engine to support combustion, leaves the combustion chamber and exits from the tail pipe, whether or not combustion occurred. So, interpreting the analyzer readings and understanding the relationships of the gases during various conditions is important. Being able to create a variety of conditions by making adjustments or causing air leaks or ignition misfires, provides huge benefits when conducted on the most basic equipped vehicle. Exhaust emissions boil down to "simple" science and mathematical formulae that support "what goes in, must come out". Think of it as baking a cake from various "ingredients" mixed together. If the batter is perfect and the oven temperature just right, the end result is the perfect cake.

Deviate even slightly with the ratio of the ingredients in the cake mix or the oven temperature and what we are left with may be undesirable, much like excessive exhaust emissions from less than perfect combustion.

Some general rules of engagement, where the following can be considered as "indicators" to be used to observe tailpipe emission. CO is the rich indicator, O2 is the lean indicator, CO2 is the efficiency indicator and HC is the Misfire indicator.

Understanding the relationships of these gases and how they present together, is key to efficient diagnosis of what is occurring within the combustion chamber.

Increasing hydrocarbons (HC) will also increase carbon monoxide (CO).

Extremely excessive HCs are frequently the result of an ignition system misfire.

Leaning the Air Fuel Ratio (AFR) will lower HC and CO eventually to the point of a lean misfire, where HC will again rise because the fuel is not burning.

Engine lubricant diluted by fuel will result in elevated HCs and CO, readily checked by removing the PCV valve from the valve cover and noting analyzer readings, which will drop when high level contaminants are not being drawn into the combustion chamber from the crankcase through the PCV.

In perfect combustion, all of the oxygen should be consumed, so very little oxygen (O2) should remain in the exhaust. Likewise, (CO) levels should be very low when combustion is proper, with O2 and CO levels will be quite similar. The actual values will be dependent on vehicle equipment and emissions components.

Stoichiometric AFR for gasoline fueled vehicles has long been considered to be 14.7:1 (e.g. pounds of air to pounds of fuel) When a properly running engine achieves 14.7:1 to ~15:1 AFR, CO2 will also be ~15%.

CO2 as the efficiency indicator, will drop on both the lean and rich side of stoichiometric.

Ignition timing can have notable effect on tailpipe measurements, particularly HC, CO and NOx. Over advancing ignition timing will increase these values, while retarding ignition timing by as little as 2 degrees can result in a notable reduction in tailpipe emissions.

NOx can be notably affected and rises with excessive temperature in the combustion chamber. The most notable system for NOx reduction was the EGR valve, that is now almost extinct on gasoline-fueled engines with the advent of Variable Valve Timing (VVT) technologies.

An incorrectly rated or faulty engine cooling system thermostat can notably affect NOx emissions and other gases. Excessive compression from carbon deposit buildup can increase NOx and be rectified with combustion chamber cleaning. An inoperable Exhaust Gas Recirculation (EGR) valve or plugged passages and over advanced ignition and lean conditions can all result in excessive NOx. A catalyst that is unable to reduce NOx, is another possibility.

All of these relationships of the gases can be readily observed by performing various carburetor and distributor ignition system timing adjustments and that is why I advise learners to try this out on engines with minimal emission control devices, before advancing to feedback computer controlled systems.

A good catalyst can mask poor engine out emissions, so again, the focus really should be on engine out exhaust gas samples, not tailpipe samples, but we have to start somewhere and the standards list various engine capacities along with tailpipe values.

Whatever the situation, it is really all about the air entering the engine. If the engine has good pumping characteristics (mechanical sealing), a capable and properly timed ignition system and the air mass is accurately measured, metering the correct amount of fuel, should support proper combustion.

Most of the above adjustments or conditions can be readily effected on older vehicles, allowing for a clear understanding of exhaust gases. Once mastered moving on to more complex systems and understanding their influences will be less challenging.

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Jim Mobile Technician
Southampton, Pennsylvania

Martin, Great additional information. As I was writing my post on Exhaust Gas analyzing I realized that he could include a lot more information, But as you indicated, I just wanted to give the "KISS" information for those attempting to answer a test question with exhaust readings included. I have never stopped using my Exhaust Gas Analyzer, but as you indicated, you must know if and how the Catalytic Converter is changing the readings. An example of that is when you see very high NOx reading but at the same time the other gases are screaming lean air/fuel mixture. As I indicated in my post, when you are in a lean misfire state, there can and will be higher then normal Hydrocarbon reading. At that point the front (Reduction) section of the Cat can be lowering the NOx by separating the Nitrogen and Oxygen, but then the rear (Oxidation) section can be burning the Hydrocarbon and rising the temperature in the rear section causing the Nitrogen and Oxygen to bond again. One of the situations that I still use the Exhaust Gas Analyzer is on a Crank / No-Start, years ago, I could just look down through the carburetor and thumb the throttle to see if there is gas going into the engine. It is a little bit harder to look down into a fuel injector. What I now do is insert the Exhaust gas Analyzer probe in the tailpipe and crank the engine over for 10 to 15 seconds ONLY, then watch your screen, it will take about 10 seconds for a reading to be displayed. If there is 2 to 8,000 ppm of Hydrocarbon, the engine is receiving enough gas to start. Now I have heard others mention the same thing, but my mentor went one step further with this, He told me if I see Hydrocarbon to be in the correct range, then look at the Carbon Monoxide (CO) reading, He taught me that the Carbon Monoxide is an indicator of INCOMPLETE combustion, But it is NOT an indicator of NO combustion. So you will have a better starting point in the matter of a minute or so. e.g. - If there was bad gasoline in the tank, there would probably be some CO on the screen, due to some combustion, but not enough to get the engine started. If there is no CO on the screen, there is NOTHING happening in the combustion chamber. The fact that the vehicle is a No-Start, the Catalytic Converter isn't working because he hasn't reach operating temperature.

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Randy Analyst
Denver, Colorado

Hi Martin, Jim,

We still have an emission inspection program that includes gas measurements, both mass and concentration, so I work and teach this regularly. You guys have made some curious statements here. Most notably, the "torch" comments. Take a look at this chart, tell me if it aligns with what you are saying.


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Jim Mobile Technician
Southampton, Pennsylvania

Randy, The analogy I try to make for technicians is because most if not all have either used or watch a technician use the torches. When the torches are first ignited with the Acetylene only, it is a very rich flame with little heat energy. As you begin to introduce the oxygen, the heat energy rises, the tech can even hear the flames effect at this point. After we heat up the metal that we want to "cut", we press the trigger on the torch set to introduce added Oxygen, which increases the temperature and begins to melt "Cut" the metal we are working on. I realize there is only a narrow band that we are using, any additional oxygen will not support combustion, that is why I talked about the torch going out with a "POP" when we go pass approximately 17/1 air/fuel ratio.

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Jim Curriculum Developer
Frederick, Maryland

Jim, It is a cutting torch not a melting torch. The added 02 will consume the steel if the job is done right. The air is NOT mixed with the existing combustion process, it is directed to the heated metal that then ignites due to the added 02. There is a reason a good cut looks like a saw while a poor one looks like puddles and blobs..... Many of us never even learn that basic level which explains why we see the torch struggles in use. Years ago, on a 9000 acre grain farm, I had this coached into me by a local master welder that helped me out a lot! The idea of a carburizing, oxidizing or neutral flame setting based on the amount of 02 added to the mixing chamber. The "button" flow 02 directly to the newly created fuel......



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Jim Mobile Technician
Southampton, Pennsylvania

Jim, I AGREE with you 100%, it is just that there are some that want to argue that the torch is "melting" the metal and I was trying to get pass that and just use the analogy. Thanks and Hope to see you at Vision.

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Martin Instructor
Burnaby, British Columbia

Hi Randy. It is good that you have time to offer assistance. BTW, please give Lloyd my regards and a Happy New Year to you all. It is well-known and appreciated by many of us that you are one of the Master of theory and practice surrounding vehicle emissions training and technology.

FWIW, vehicle emissions and associated learning in automotive, is often considered a necessary "evil" rather than a topic of extreme interest. Students typically are mainly focused on performance gains, which of course often result in increased emissions and poor fuel economy. To dump the "holy grail" of emissions onto students at any age can quickly overwhelm them, so using props and "simple" vehicles can be a way to attract more interest.

My use of torches was simply to demonstrate the mixing of fuels for combustion. It was not about discussing the ultimate temperatures, stoichiometry and the how the fuels were mixed, but to sew the seed and lead the audience along a path in which little interest might otherwise be shown. It was simply about adding an oxider to a fuel burning under atmospheric conditions. This of course contrasts with the gasoline engine where fuel is added to the air and "air is the king".

Leading Level 3 apprenticeship students through the required gas analyzer calibrations and setup is now a thing of the past here, since gas analysis has been eliminated from the learning requirements. So, it has become virtually redundant to all except those few who might be interested. It's uncanny how it took almost 15 years for the carburetor to finally be flushed from the learning environment, but at first mention that exhaust gas analysis was to go by the wayside, it and the equipment virtually vanished overnight, leaving only "Describe Vehicle Emissions" content in its wake!

I will stand by my recommendations for using vehicles fitted with minimal emissions equipment to allow for manual adjustments to be made and observed on the gas analyzer and ignition oscilloscope. I used general "rules of thumb" as a guide to attract the learner to make the physical adjustments or create conditions on a basic engine and observe the results of their work first hand.

There are many theories, concepts and standards in automotive to discuss, challenge and prove or disprove. As technologies improve, we realize that some of what we first learned many years ago is no longer applicable, but to attract learners using the simplest representations works well to garner interest and lead to more advanced learning.

Randy, I understand from your own career as a technician and shop owner and in your current role that combustion technology, emissions and related technologies are of great interest and that you are skillful in your work. If you have some methods to share about how to better introduce emissions concepts and theories at a basic level for novice learners, that would surely be welcomed here.

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Randy Analyst
Denver, Colorado

I also present to entry level students at multiple community colleges around the area. I have a 3 hour presentation that revolves around live demonstrations using the students own vehicles. I really enjoy it and the students seem to love it. As you already know, most/many students are entering the field with a thirst for gaining power.:) I tap into that love of racing with the 5 gas presentation. When I get to NOx, that's when they really become engaged as more power produces more heat and heat is a critical component for NOx production. I then lead them to a discussion into AFR and lambda and where they want to dial in AFR for maximum power. After an agreement that somewhere on the rich side (keeping it simple) is the best choice, I relate that back to physics and heat.

As you can see in my chart, HC based fuels have higher flame front temperatures on the rich side of stoich and the same is true for gasoline fueled vehicles. That's why the most power is on the rich side. Makes sense, doesn't it? 

So then I ask them what would happen to their dragster if they run the mixture too lean? They will say engine parts melt, which is true and is the source of the myth. "Lean burns hotter and I know it because engine parts melt on the lean side!". Actually, teaching in service technicians is much harder. They can get down right angry at this point. Jim and I have more than one time had to end this part of the discussion due to heated exchanges and continue with the class.:)

The next part you should like, I use the torch to explain it all but I use it opposite as you guys do. I ask them what happens if I adjust the torch (ignore a cutting torch, just a torch as for brazing) and then just quickly touch the metal? What happens if I readjust the torch, either lean or rich, and then hold the torch on the metal? Of course the latter method produces more heat in the metal. Same concept happens with lean mixtures. Lean mixtures burn slower so the latent heat from the flame front has a longer time to transfer heat to the metal parts resulting in melted metal. Using the example the way you are using it is not correct. There is no way for you to measure the AFR of the torch and you are misinformed as to what it really is.

Next, we spend time dealing with 2 graphs, engine out O2 and flame front temperature, which then gets us to the actual NOx curve.

A few final thoughts;

I have never, not once, fixed a NOx failure by correcting an over temperature cooling system issue.

Avoid lean misfire discussions. Due to the many GDI systems out there and other various engine designs, it's too messy trying to sort out when an engine will misfire at what AFR unless you are teaching a very advanced 5 gas class.

High oxygen does not mean the mixture is lean and neither does high CO mean it is rich. I had a vehicle this morning with elevated O2 and it was not lean. It had a misfire and the mixture was correct, 0.8% lean but not an issue (an FTIR or FID may have shown the mixture to be lambda 1 or less). 


I also have graphs with CO around 2.5% and lambda is 1. 

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Jim Mobile Technician
Southampton, Pennsylvania

Good Stuff, Randy. Thank You. You are correct about the students with the Exhaust Gas reading. I had a nice kit from Emission Supply Co in Pa for installing pre-cat threaded ports. Showing the students pre and post readings on their own vehicles was priceless. Also we had a nice Mustang Dyno, putting the students vehicles on it with the scanner displayed readings on one 65" monitor and the exhaust gas readings on another 65" monitor at the same time really had them excited.

Another awesome lesson for the students was we had a vehicle that we were pretty sure it had a slightly leaking fuel injector, so we had a discussion on what procedure they thought we should do. They all had good ideas, I went over to the vehicle that had been sitting overnight, removed all of the spark plugs then went in the vehicle, turn the key to the on position then off without cranking. I took a short piece of heater hose and put it over the end of the Exhaust Probe, stuck it close to each spark plug hole and one of the cylinders HC was much higher then the rest.

Whatever it takes to get their heads thinking, is AWESOME.

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Martin Instructor
Burnaby, British Columbia

Good stuff as usual Randy. I use exactly the same discussion in class relative to rich and lean and the temperatures. Rich burns hot, but for a shorter time than the lesser heat from the leaner combustion, that travels into the system. Wrenching on and racing two stroke GP motorcycles years ago, very quickly taught me plenty about lean!

I absolutely agree on the part of never encountering a cooling system issue over high engine temperature that was the cause of any NOx failure that I encountered in the shop. However, I do recall published info circa 1978, when Chrysler products used a lower temperature thermostat in an effort to reduce NOx with slight elevation of HC and CO as a result. I had always struggled with a slight variance in the resulting cooling system temperature having a significant effect on combustion chamber temperatures.

As far as exhaust emissions variance as a byproduct of enrichment or leaning, we're really looking to see that in doing so there are effects that can be measured and visualized at the basic level. Given the complexities of combustion technologies today, we know is not transferrable.

Regarding air and fuel entering the combustion chamber, it all must leave the chamber. If there is a misfire, there is a significant amount of unused oxygen leaving the cylinder, far in excess of a lean condition.

There are often times in instruction where basic concepts are presented and practical application leads the student to that is the way it works. Carrying that through to the point where the students are required to successfully complete examinations that may have age old typical system questions written on flawed theories. It is a two-edged sword, where text books and publications are often riddled with errors and the examining bodies base questions on "mythstakes" and "mythbeliefs".

There are times when I have stood in class discussing how a system is believed to function according to published content, but later added, "this is how it really works". This happens across all areas of automotive learning. When taking students out into the shop to perform specific activities, I always invited them to view it as an opportunity to challenge and qualify or disqualify theories and concepts for themselves.

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Cliff Diagnostician
Santa Maria, California

I have been using an exhaust gas analyzer my entire career on a day to day basis since 1989. This is a great write up for those who've not had the pleasure of playing with one. You can get an immense amount of diagnostic information on how well fuel is being consumed by the engine, especially before the catalyst. I still remember using a vacuum gauge on carburetor vehicles to achieve the highest vacuum reading and then leaning it out by 50 rpm and that would typically get the emissions close to passing the emissions standards set by California at the time.

Thanks for sharing Jim. 

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Martin Instructor
Burnaby, British Columbia

Cliff, one of the first diagnostic tools that I learned to use when I started out in '68, was the "lowly" vacuum gauge. I lived in Birmingham, England and optimizing the ignition timing with a vacuum gauge by achieving maximum vacuum and then reducing it by one inch was the norm. With the ignition set to achieve maximum idle vacuum, pinging (pinking in UK lingo) was sure to occur. I lived through the 70s here in BC performing lean drop and propane enrichment adjustments that are now little more than distant memories!

FWIW, big box oscilloscopes of the day, often had a vacuum gauge integrated into the features, along with condenser testers and other now redundant functions.

While testing with pressure transducers has largely replaced vacuum testing as a mainstream diagnostic step, with the advantage of the capability of valve timing events being displayed, a high quality vacuum gauge can still offer some useful results. I find that it is much like carburetors these days. Techs run away scared of the complexities of carburetors and don't spend much time if any, exploring the use of basic tools such as a vacuum gauge.

My use of exhaust gas analyzers began in 1992, when BC was the first province in Canada to adopt emissions testing with the now defunct AirCare® BC program operated by Envirotest centralized test stations, that closed their doors in December 2014. The BC program emissions testing program was adapted directly from the Colorado State Standards.

Dealerships and aftermarket shops meeting standards could become designated repair facilities. Technicians certified by examination and successful completion of a core emissions category, plus one other mandatory category and two other optional categories. I maintained certification in gasoline, diesel and alternate (gaseous) fuels for some years, but my diagnostic focus shifted largely to diesel systems.

From the start of mass vehicle testing in 1993 which was almost a year later than our first certification exams, we dealt with NOx failures in addition to HC and CO. The latter pair were often the only "bad" gases of focus in some states or specific areas and Europe at the time. Things have sure changed over the years!

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Jim Mobile Technician
Southampton, Pennsylvania

Martin, You had me laughing a little. I had a student once that was trying to set the timing on his old Buick and could not see or find the timing mark. I threw a vacuum gauge on it and timed it exactly the way you noted, that student could not believe how much better his vehicle ran. Also, I had to send you a picture of my well used Propane Enrichment tool set.

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Jim Curriculum Developer
Frederick, Maryland

Jim, I actually like that you posted this topic. Learning this "now seemingly useless" stuff can be a great tool for understanding modern management systems. The topic is as deep as one chooses to make it. I have personally been cautious in how I shared the topic because i had a fear. Teaching a concept with a slight inconstancy could create a challenge to learning related advanced concepts later. One example would be a vehicle program intentionally targeting a slight deviation from lambda during specific operational conditions. With a dynamic environment an engine design may struggle with out gas composition or have an issue in transition. The programming team may apply deviations to control or lower a specific gas at that time to achieve the overall target on the FTP. Understanding the chart can help us "see" that relationship potential.

I also have never corrected a NOx failure with a thermostat or cooling system. In Maryland, we had central testing with IM 240 then the shop would diag and repair. The facility only provided the drive trace when a vehicle failed, yet it became a key part of the process. One could learn to read them like a story to gain direction very quickly. I was sorry to see it go......

Understanding that heat transfer is a function of temperature and TIME so a cooler event that lasted longer could cause more damage to components as well as allow more time for undesirable reactions. I never took upstream readings. Matter was neither created nor destroyed in the engine or cat as long as the system integrity was sound.

The other thing I will be very candid about is where much of the fuel for my emission diagnosis training came from. Maryland had a program to bring in training for techs in the program. It was free to us if we would just sign up and show up. Hours in class, and after class over a barley beverage, started to shape this hard, challenge everything type of student. Randy B and Jim K were two of the instructors as well as various presentations from Weber State. I am truly thankful for that. It was actually sad to see how few took advantage of the opportunity. The same faces most times just like we see at other events.......

BTW, I kind of like the crumpled paper. Add some different color paper for a third picture and talk about the effects of inert gasses.......

If I have misstated anything here, I fully expect my teacher to set me straight. (Randy)

Jim, we need to have a beverage in KC. i don't think we have had a chance to actually talk for years. I am looking forward to catching up and batting ideas around!

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Jim Mobile Technician
Southampton, Pennsylvania

Jim, You are so right about this subject can go as deep as you want to go. When I was writing this post, I realize that "Somewhere" I'll have to stop, The REALLY Great thing for me learning this was, my mentor (Tom McKenna) would teach me more and more about this as we were looking at Good and Bad vehicles, then there were times that he would just plain ass LIE to me and tell me to go to a vehicle and see if what he said was true or not.

This topic is one that most techs don't care about because the shop doesn't have an Exhaust Analyzer, But as you noted, just knowing what happens with the combustion chamber can answer some of the question about computer control of the fuel delivery circuit. We could go into the oxygen sensors, Fuel trim and the operation of the Catalytic Converter with this topic ( We will have to save that for that "Beverage" meeting in Kansas City. HaHa

I BTW did see ONE vehicle that the cooling system was the reason for a failure on the emission test, it was a Buick that started out OK, but in a relatively short amount of time the engine temperature went very high, it was an idle test that it failed. As soon as you started to drive the vehicle the temperature went down some. What it turned out to be was the coolant was so old and dirty, it actually took all of the 90 degree fins on the propeller on the back of the water pump off, the only thing left there was what looked like a Karate Throwing Star, so there was NO coolant being moved throughout the cooling system.

BTW - What I did in my Pa. shop when we went to the 2nd generation tailpipe emission machines, unlike most other shops that traded in their 1st generation machine, I purchased a huge box of Pre-Cat test ports from Emission Supply in West Chester, Pa. I then installed a THREADED sample probe on that old machine, install the test ports in just about every customer's vehicle at NO CHARGE, I then had access to hundreds of vehicles with PRE and POST Catalytic Converter readings at the same time by using the OLD machine PRE and the NEW machine POST.

Talk Soon, My Friend.

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Jim Analyst
Denver, Colorado

Hi Jim,

Thanks for the article!

I've read all the responses and the corrections are mostly there, but I'd like to put them all in one spot for future reference.

First, I know it's hard to write an article that is both factual and concise, yet still entertaining to read. Still, I wanted to be sure techs have the right ideas here. While using one or two interests gases to determine Lambda is possible (in this case CO and O2), there are so many caveats and "excepts" in there that I've come to the conclusion we shouldn't teach it that way. Where the mistake is I think is when the emphasis "period!" is used. It just isn't so - you really can't determine, with any degree of certainty, which side of stoichiometry we are on with those two values. Therefore, I think we should teach that the only way to determine Lambda (or AFR) from a gas analyzer is using at least five interest gases (CO, CO2, O2, HC, and NOx) and making the calculations. Not using NOx is probably ok, except in cases where NOx is very high.

The key point here is that using Lambda as an index is good for explaining fundamental relationships and causes, but is not good for using them to determine the true AFR outside the calculations used by a gas analyzer.

Next I know that producing graphs is a pain, but the HC line not correct. It is more of a "U" shape with the lowest point generally to the lean of stoic. In the graph there is a point on the lean side where the HC line separates and widens. None of the others do this. The point were it separates is the misfire point. At that point both combustion and HC concentration value becomes unstable; hence the widening cone. Yet prior to that it is also rising (again though, the lowest point should be lean of stoic). The explanation is not necessarily wrong, but it doesn't just apply to a misfire condition. It also applies prior to any misfire and is called bulk quenching, or sometimes bulk-phase quenching. In essence there are localized pockets of slow combustion (generally due to lean mixture) that are "bulk-quenched", causing incomplete combustion.

My apologies for those that think I am putting too fine a point on this.

Next, the NOx explanation is definitely not correct. The lean NOx peak is not caused by increasing temperatures - In fact combustion temperatures are DECREASING as Lambda is enleaned. As Randy's graph demonstrates, combustion temperatures are highest on the RICH side of stoic (the graph is in equivalence ratio [EQ]; not Lambda).

So what would explain this?

Well, the reason NOx is lower on the rich side of stoic is not because of a lack of heat - it's because of a lack of oxygen. The reaction ends when we run out of oxygen. Note the O2 curve. Now, as we enlean from maximum temperature (for gasoline an EQ of about 1.1, or a Lambda of about 0.9) the falling temperature is initially offset by the increasing oxygen. This continues until the peak on the lean side when the increasing oxygen is finally overcome by the falling temperatures and NOx begins to decrease. This is why the torch analogy doesn't work; because this relationship is true for all carbon-based fuels and maybe for all fuels.

Finally, increasing combustion temperatures are NOT responsible for thermal damage to parts (exhaust valves, etc.) during lean conditions. The reason is related to your wadded paper analogy. When we enlean from stoic there are fewer HC molecules in the chamber; hence they are farther apart. Because the molecular collisions have to now travel farther to cause reactions the speed of the combustion slows down. This increases the "residence time" of the flame on the metal parts. This is where a torch analogy does work and I've seen Randy B use it in presentations.

Imagine a torch on high heat (rich) and place it on a metal plate for one millisecond. Not much damage occurs. However, enlean the torch to a much lower heat value, but now place it on the metal plate for several seconds and it will begin to melt.

Thanks again Jim.

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