Understanding Exhaust Gas Analyzing

Jim Mobile Technician Southampton, Pennsylvania Posted   Latest   Edited  
Discussion
Driveability
Emissions
Lean Misfire
Excessive Emissions
No Start
Nox
Hc
Co
Co2
O2

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.

+13
Thomas Diagnostician
Saint Petersburg, Florida
Thomas
 

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....

+1 Ð Bounty Awarded
Brandon Diagnostician
Reading , Pennsylvania
Brandon
 

❤️❤️❤️

0 Ð Bounty Awarded
Mario Diagnostician
Weston, Florida
Mario
 

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!

0 Ð Bounty Awarded
Taurayi Owner/Technician
Harare, Zimbabwe
Taurayi
 

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

0 Ð Bounty Awarded
Geoff Diagnostician
Lahaina, Hawaii
Geoff
 

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.

0 Ð Bounty Awarded
Guillermo Owner/Technician
Blue Island, Illinois
Guillermo
 

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.

0 Ð Bounty Awarded
Martin Instructor
Burnaby, British Columbia
Martin
 

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

0 Ð Bounty Awarded
Jim Mobile Technician
Southampton, Pennsylvania
Jim
 

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

0 Ð Bounty Awarded
Randy Analyst
Denver, Colorado
Randy
 

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. diag​.​net/file/f2vuey3c7…

+1 Ð Bounty Awarded
Jim Mobile Technician
Southampton, Pennsylvania
Jim
 

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

0 Ð Bounty Awarded
Jim Curriculum Developer
Frederick, Maryland
Jim
   

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

0 Ð Bounty Awarded
Jim Mobile Technician
Southampton, Pennsylvania
Jim
 

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.

0 Ð Bounty Awarded
Martin Instructor
Burnaby, British Columbia
Martin
 

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

0 Ð Bounty Awarded
Randy Analyst
Denver, Colorado
Randy
 

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

+1 Ð Bounty Awarded
Jim Mobile Technician
Southampton, Pennsylvania
Jim
 

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

0 Ð Bounty Awarded
Martin Instructor
Burnaby, British Columbia
Martin
 

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

0 Ð Bounty Awarded
Cliff Diagnostician
Santa Maria, California
Cliff
 

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

0 Ð Bounty Awarded
Martin Instructor
Burnaby, British Columbia
Martin
   

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

0 Ð Bounty Awarded
Jim Mobile Technician
Southampton, Pennsylvania
Jim
 

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.

0 Ð Bounty Awarded
Jim Curriculum Developer
Frederick, Maryland
Jim
 

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

0 Ð Bounty Awarded
Jim Mobile Technician
Southampton, Pennsylvania
Jim
 

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

0 Ð Bounty Awarded
Jim Analyst
Denver, Colorado
Jim
   

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

+2 Ð Bounty Awarded