Recently I had a MAF sensor issue on a vehicle and I wanted to assess the accuracy of a testing method that has served me well my entire career. It is pretty common place to have a dirty MAF sensor with driveability or transmission slipping issues on GM trucks.
The truck I was working on was a 1999 Silverado 2WD with a 4.3 VIN W. Not my run of the mill truck, as I am used to 5.3/6.2/6.6 4X4’s. The customer complaint was that the truck would start to rev high going up hills. I was not able to duplicate the customers concern after finding the nearest hill (Not easy to come by where I live). I have seen many times a slipping transmission due to an inaccurate or incorrectly scaled MAF on the GM trucks in general.
I moved straight into my old “tricks”. I brought the revs up and checked the MAF maximum reading before the shift at WOT. It’s low I think. (because I don’t actually have specs on this truck). I also observed H02S data and notice it is dropping near 0mv. Knowing that this truck goes into an open loop fuel enrichment strategy that uses a MAF measurement fuel table, it must be an unmetered air issue or a lack of fuel delivery. Fuel enrichment PID shows 12.3 AFR on this truck so I should be well over 450mv. Cruising fuel trims are hovering near +28. I don’t have a fuel pressure gauge on the thing to measure delivery to injectors, but a manual fuel pressure gauge won’t tell me if I have plugged injectors anyway. I go to my old roadside stand by and unplug the MAF to throw the fueling system into a default speed density table mode. Now my H02S data Stays above 800mv and I am making some power. I don’t have a wide band in the exhaust pipe, and I am not going over this with a fine tooth comb. More or less, I am done here. The scan tool has given me as much information as I feel is required to make a call on the MAF at this point.
I install a MAF and recheck my readings. My WOT MAF at the shift point is up higher around where I would expect, H02S data is up over 800mv in fuel enrichment and cruising fuel trims are near +8. Quick and dirty for the most part has got me where I wanted to be.
I got thinking though. I have really been diving into understanding about airflow in an engine. I have always liked my MAX MAF WOT before the shift test. If I were to use a VE calculator I would only be guessing at what VE should be. At what point should I measure VE? Does VE change enough to skew my findings depending on RPM? Yes in fact it does and I would rather use the scaling/programming built within the software to infer airflow metering accuracy.
I always assumed that the biggest measurement error % would be in the area of the most airflow. Many times the issues are not so obvious as it was in this case, so if I can push that % of error as far into the red I have a better chance of finding my issue. I decided to make measurements in the area of max cylinder air mass and compare to the error at max airflow over the MAF. I used predefined maths in HPtuners to measure cylinder air mass (I know there are other ways within the program as well as other tooling) to determine where the engines maximum VE occurs.
Here are the results…
Max airflow old MAF = 101.90 g/s @ 4941 RPM 52.61% ATG calculator at 93kpa 23C IAT
Max airflow new MAF = 143.1 g/s @ 4942 RPM 73.86%
Max cylinder air mass old MAF = 93.60 g/s @ 4239 RPM 56.33%
Max cylinder air mass new MAF = 135.8 g/s @ 4239 RPM 81.72%
Here are the numbers I get when I compare new MAF to old MAF
Max air flow gives me a 30.25% MAF error and a 21.25% VE error
Max air mass or the best VE gives me a 31.08% MAF error and a 25.39% VE error
What are your thoughts, tricks, findings etc. I would like to start a discussion on airflow.
Hello Bill, I'd like to reply to these parts of your post: "It’s low I think...I don’t..have specs on this truck.." and "What are your thought, tricks, findings...". The specs you find for MAF g/s in service info or on Identifix are usually much to wide of a range to use for making a judgement call. What I like to do is record known good values for the types of vehicles that I work on, and for
Thats a great idea! I noticed there is no RPM in the spec. That would be handy to have as a required field to fill.
Are you talking about in the app? If so, when you click on a record you will get a pop-up with all the details of the data that was put into the calculator like this
That screenshot reminds me of another thing. You'll notice that the data shown is for a turbo engine. Sometimes people ask what is a known good VE for a turbo engine. Really there is no rule of thumb for any engine whether it is naturally aspirated or boosted. You can kind of say there is a range of like 76% to 100% (or a little more) for a naturally aspirated engine, but that is a very wide
The screenshot "Known good/bad" data you have shown does not show RPM. That is part of the discussion I would like to have. If you share "known good/bad" of a VE reading then you would need to determine the RPM range you are in. I had a VE 82% on mine in the ATG calculator. I will edit my original post a bit.
Ok, yes the known good VE value will be different depending on what rpm you grabbed your data to put into the calculator. The instructions in the app describes the method of driving WOT close to redline through at least one shift point. Since all engines have a built in rev limit, then that is the rpm where the data is collected. In other words, if everyone is performing the test the same way…
Bill, a few days ago, I put up a post here about this subject. diag.net/msg/m6dd30cz5z… Over the years, I have found the most accurate way to find a problem like the one you were working on is to test the whole vehicle dynamically from the front seat of the vehicle, with the hood closed and in comfort. Let the vehicle tell you want is wrong with it instead of you going out and
I saw that Albin. I would definately appreciate any input on this. I really would like the focus of this to be both on actual/physical (not metered) airflow and (metered)MAF funtion/testing/software strategy/algorithm (If I worded that correctly).
Any time you are working on a driveability problem on a vehicle, the vehicle needs to be tested as a complete vehicle. One problem I have with using a VE calculator to check for (lets say ) a misreporting MAF is; there are many other things that can cause a VE error besides the MAF. Several years ago I was doing some testing like this on two Ford vehicles, One was a late 90's crown Vic Ford…
Great points Albin. Absolutely agree.
Hi Bill, Great subject and one that peaks my interest. A good question for anyone looking to perform a VE test should ask, "What RPM (or range) should I choose to test?" The answer in my opinion is that you should select where max torque occurs. For this engine (I have a shop truck that's an 01 which is basically the same configuration) Wikipedia lists this engine's peak torque occurring at
Scott I would say the specs that you pulled from the speed density table would be correct for this engine. I saw the Wiki specs and questioned them as well. The engine, by the CYL AIR math graph I choose to run for my part in this discussion, shows maximum cylinder fill occuring between … RPM which is in line with what the engineers calibrating the software determined for your particular
Thanks Bill, Regarding your question about how the fuel is being controlled, it’s a blend between the VE table and MAF. If the math is correct then a wide band sensor would report commanded AF in PE mode. Have you ever configured a VE table? As for your numbers, looks like this engine is breathing better than what the VE table displays.
This has been a good discussion Scott. I have configured a VE table and set up the PE with a wideband. It was over a year ago and has long since fallen out my left ear. I had purchased some training that was process based. I have since purchased some books by Greg Banish and they are more my speed. Likely I will not find as much time as I would like to learn the program. I have always looked at
Yes Bill, thanks for kicking off this discussion. I’ve attended Greg’s classes along with Ben Strader at EFI University, efi101.com Digging in deeper IMO, helps with developing a deeper understanding of engine controls and goes a long ways when analyzing engine performance issues.
Nice, thank you Scott. I was wondering. How is it that you can tell my engine breaths better than what my VE table displays. In which way specifically? I thought I saw a trend that was a little late on airflow timing, That would increase my late power and airflow numbers. This engine has 60K miles on it and has not been modified or repaired as far as I know. I also noticed the CMP retard was in
On the “breathing better” comment, @3,600 in your table display I see that they’re predicting a fall off. If we were to look at this graph and mark out 200 RPM measurements across the cyl/Air the curve should closely represent your VE table at 90-100kPa. In my initial reply I see the % of fill coming in higher than 3,600 hence my reasoning for the ”flow better”comment.
Agreed Scott, thats what I observed as well. Breathing better not referring to the volume of air, but the later timing of cylinder fill. I understand that if I can keep an intake valve open longer/later that I can increase cylinder ramming effect at the higher RPM and continue to effectively fill the cylinder while the piston is on the upstroke. There are all sorts of tricks engineers have
Yes Bill, VE and Cyl Air give us a way to measure against what’s possible. On the exhaust scavanenging, I think of it as creating inertial pull downstream where other exhaust pulses converge. If the pull is strong during overlap, this should promote a rapid inrush as the intake valve opens further. I’m not familiar with plex tuning, can you elaborat? Also, can you email me your before and
Plex tuning is a company that sells tuning and combustion analysis tools. I will send E mails immediately. I think of scavenging the same way. I am a visual learner though and would like to see the effects of that within the combustion chamber and I feel I am at the limits of my tooling.
Hi Bill, I took your log files and displayed the Before and After 3D Charts using RPM and MAP Axis'. I prefer this view since you can filter to show the avg value and establish a minimum sample count before the value can be displayed. You'll see an example in the Post run MAF g/s test at 3,600 RPM. - Cylinder Air values: Before (note: average values shown) After (note: average values
Yes. Would be nice to have VE values ready to rock in those tables as well. I get lazy and don't get things set up to go. I would add that you had very little to work with here and I could have scanned longer under more conditions to get a better table. This table could then be copied and pasted and used with a compare function to the VE table as well I believe. I am by no means competent within
Good call on the late breathing Scott. I looked at the CMP retard PID at about (-16) to (-17) and after reading vague service info it seems (-5) to (-17) makes it happy.I got approval to check into it. I don't have a known good for one of these, but I feel I have fairly accurately reasoned where it finds its reference from. I did an actual measurement on the crank pulley of 12 degrees of slack
Following up on this Bill, did you make any repairs on this vehicle and if so do you have any more data to share?
No Scott. Nothing to do with airflow. This engine had chunks coming out of it during an oil change and lowish oil pressure. I don't really want to "tie the knot" with much in the way of internal repairs. The customer will drive it until its completely worn and go for rebuilt if they feel its worth it. I did develop a misfire on cylinder number 4 under very light throttle and did an ignition
Thanks for the follow up, wow that gear is quite worn. Fyi, I linked up your image. If you're interested in how that was done, you can click on the edit button and see how I linked it. Let me know if you have any questions.
Thanks Scott, I wish now that I had done a chain slack measurement differently. I should have turned the rotor and taken out the gear slop first. I am wondering how stretched my chain is now, but my time is done with this truck. There is a light rattle noise on light acceleration and a bulletin for timing chain and tensioner. Are you aware of any place to find GM camshaft specs? by the running