Airflow, VE, & Fuel Trim Diagnostics
Diagnosing Breathing Faults
ENGINE BREATHING EXAMPLES
The following information presents Volumetric Efficiency (VE) examples from our GM Training Manual to explain how simple, high-level tests can rule out or confirm entire categories of possible causes. Consider using our free Volumetric Efficiency Calculator or for the free app version, visit the Google Play store or iOS App store.
‘Old School’ Engine Low VE Example
The capture below shows a PID-gathering test drive for a GM 2003 5.3L V8. You’re lucky to get 85% VE out of these pushrod 2-valve engines, but this result is even worse. The peak values from the test drive prove that this engine is only breathing at 76.76% efficiency.
Fuel Trim PIDs were also included in this test drive. We always include them to quickly differentiate between real and fake breathing faults. The high Long Term Fuel Trim (LTFT) values show that there is a learned fault at high load and engine speed. VE is about 10% lower than expected, and LTFT is 10-15% higher than expected. The ‘VE vs. Fuel Trim Possible Cause Table’ (from the full Engine Breathing section) says that this is a MAF or duct fault, and it’s correct! A new MAF sensor brought VE up to 85% on this engine.
Single Bank Breathing Fault Example
The capture below shows a low MAF result for another 5.3L V8 that was in for a P0101 (MAF performance) and some Misfire codes (sorry Vortec, it’s nothing personal). Of course, we don’t know that the MAF result is low until the values are entered into a VE calculator. Normally the peak RPM and MAF PIDs occur at the same time on a test drive, but if the engine is running badly enough, you may need to pick a couple of points and recalculate.
The values at the cursors were used because the Fuel Trim is at its greatest deviation. And that’s the greatest clue! The VE calculator says this engine is breathing at 80.85%, which isn’t exactly the end of the world. However, Bank 1 Short Term Fuel Trim (STFT) shot up to +17%, and Bank 2 STFT dropped to -17%. This is a clear case of a clogged exhaust on Bank 2 (per the ‘VE vs. Fuel Trim Possible Cause Table’ earlier in the full Engine Breathing).
GM DRIVABILITY & CODE DIAGNOSTICS MANUAL
The VE examples and discussion above are from our General Motors Drivability & Code Diagnostics Training Manual, described in more detail below:
Due to the increasing complexity in engine management, technicians have to choose from far too many possible cause relationships. Every year we see fewer simple circuit codes, and more ‘condition’ codes, multiple codes, and no-code symptoms. Furthermore, GM has scrapped traditional flowcharts in favor of lists with links to every possible system that may have some role in causing the code or symptom. The result? Following the ‘instructions’ used to take hours, but now it may take days. More than ever before, our industry needs an intuitive, repeatable diagnostic decision-making process.
At ATG, we’ve solved the modern flowchart problem by skipping it in favor of a ‘High Level’ diagnostic approach. In practical terms, we offer over 400 pages of GM system information and tests, but always with decision-making tables to help you eliminate hours of testing before you even begin. The result is a layered diagnosis that looks something like this:
- Your first task is to choose a section in our manual that most closely relates to your symptom or code.
- Use the High Level tables in that section to assemble the best possible cause list.
- From the possible causes, create a list of the best tools and tests to eliminate the most possibilities with the lest effort.
- Test what remains! Use the tests in that section or references to other sections to perform the few remaining specific tests to arrive at the final diagnosis.
This strategy prevents you from performing any difficult or invasive tests until step 4. As technicians, our instinct is to dive right into the middle of a diagnosis, but peeling in from general indicators to specific tests has proven to be far more efficient and accurate. The manual applies these proven principles to the latest GM systems, but also includes a ‘relook’ at the way older systems are aging and failing.
The manual is divided into sections that cover the major categories of faults instead of specific systems. For example, there isn’t a section for ignition systems. Rather, there’s a section for diagnosing misfiring, which includes ignition system information and testing procedures if your diagnosis moves in that direction. This allows us to directly address your diagnostic concerns instead of simply providing facts about systems and leaving you to guess which ones apply. This ‘shop-up’ approach creates the following flow for the manual:
- Diagnosing Cylinder Contribution
- Diagnosing Engine Breathing
- Diagnosing Engine Mechanical
- Diagnosing Fuel Control
- Diagnosing Emissions Systems
Using the ATG High Level approach means that you will jump from section to section as needed, taking a different path for every diagnosis. For example, you will start with the Fuel Control section for a P0171, but this code may be caused by ‘too little fuel’ or ‘too much air’, which may lead you to look for vacuum leaks using the Engine Breathing section or even a stuck open purge solenoid using the Emissions section. We’ve developed the manual, and especially the tables, with plenty of internal references so that your path through the sections will be the shortest one possible.
Thanks for reading,
Great info! Here's a gm 3.8 K engine I just had this past week. This thing was running terrible from the moment i brought it off idle, Even with no load on it. I noticed egr codes right away and when I mentioned that to the shop they said that it has had egr faults for years with no driveability problem. That's where they went wrong. This car had 2 problems and the egr valve was one of them. I unplugged the valve and it ran great but was down on power. I grabbed my escan to do a full throttle take off from a stop and noticed a maximum ve of only 60g/s. Calculated was over 120g/s. I put the backpressure gauge on it and it pegged the needle and hit the stop on the gauge (15psi) then blew the hose off. Never a dull moment.
Thanks for posting that capture. Love the ATS EScan! I didn't include it in my short post, but we use it in ATG manuals. It's another great way to get a high level look at an entire category of possible faults.
Wow. Awesome info, just checked out your website, are the manuals downloadable? Want to purchase the one in the above example and maybe a couple more.
GREAT information, Tim. Always wondered why some techs don't get onboard with VE, every engine has to be able to INHALE and EXHALE. This valuable piece of the diagnostic puzzle is more important then ever due to active intake manifolds, Variable valve timing, etc. ATG's free VE calculator is a perfect example of your dedication to our industry. THANK YOU
Thanks Jim! We noticed that it was getting harder to find VE calculators online. That's what drove this project, and we're grateful to have found a champion in David Buckshaw, who really pushed to get the formulas and interface just right.
As for adoption rate, I think the issue with VE is the same as with a Lab Scope or any of the other neat tools and tricks we're evolving as an industry - they aren't ever mentioned in flow charts. Therefore, they never become a part of the diagnostic routine unless we play with them on known good vehicles on our own time. Only after this 'unbillable' investment is made are we comfortable enough to put the idea in our test inventory to wait for an appropriate time to use it. I'll admit that this was my experience with most of the tools I now regularly use.
So my advice to the skeptics is to run it on 2 or 3 vehicles to get the idea, and then wait for just the right drivability fault to try it out. The worst that can happen is that your VE is perfect, but even that's a 'win' because you just eliminated measurement faults, exhaust restrictions, jumped timing, and related breathing faults from your possible cause list!
Thanks again, T