MAF testing using fuel trim
1999 Olds Bravada poor power. Power = 4.3 V6, Automatic transmission & AWD transfer case. The complaint is: “the vehicle has low power when accelerating or when climbing hills”. First test was with a scan tool, fuel trim long term and short term on both banks. The two front oxygen sensor voltages, engine load, throttle position and engine RPM were also gathered. The engine has a very hard time making it to 60 MPH, but once there, it will cruise fine, unless you need to climb a hill. Seat of the pants says it’s a fuel supply issue, let’s take a look at the scan tool and let it tell us where the problem is.
The first capture is a parade of the total test drive, which is one mile out and one mile back. The operating conditions started at idle, short drive, then WOT up through two shift points. Some things I noticed are; max engine load = 67%, Long term trim bounced between -3 to +18%, short term trim bounced between -10 to +50 and the front o2 sensor voltage went to 0 at wide open throttle.
The second capture is a zoom to the WOT data. Here the calculated engine low is low at 67% max, the long term trim are locked at 9%, which is normal for this platform &amp;amp; year, but the oxygen sensor voltage is laying on the floor. Is this low o2 sensor voltage caused by a lack of fuel supply, or the MAF sensor not reporting enough air being inhaled by the engine? I did test the fuel pressure, and I didn’t see any problems there. The MAF only reported 107 G/S of air; this seems low, although if there were an exhaust restriction, the MAF reading would be low. Don’t jump to conclusions; let the scan tool tell us where the problem is.
The next capture is of the vehicle at cruise. Notice the oxygen sensor is back switching normal, but pay close attention to the fuel trim numbers and how they are trending. How the trims trend are as important as what the numbers are. The short term trend from a -30 to a +35 or so, with throttle movement; the long term trend from a -3 to a +10, then after the WOT throttle, they move on up to 16% or so. This is not normal operation of the fuel control of this engine. Examining the captured data, I can see the MAF is reporting ok at idle, but as the demand for power increases, the trims start climbing, which is giving a clue of the MAF under reporting the air flow to the engine. When it comes to the WOT porting on the test drive, the system goes to open loop and fuel enrichment, as seen in the long term trims, although, the oxygen sensors are waving a red flag of insufficient fuel supply. By using all the data, the problem is pointing to an under-reporting MAF sensor, which is causing all the problems found. I have taken the captured data and plugged it into a VE calculator. The calculation was 53% VE, which is way low for this engine.
If perchance the low MAF reading was caused by an intake or exhaust flow problem, the engine load would be low, the MAF reading would be low, but the fuel trim numbers and trends would be normal. Since the fuel injection pulse width is largely determined by the input from the MAF sensor, a restricted airflow would not cause any trim shift, only a low engine load pid value.
With a new MAF installed, the data looks way different. The first thing that catches my eye is the load PID value of 95%. That is normal for this engine family &amp;amp; year. Notice how the fuel trim pids trend. This is way different than on the first captures.
The WOT/VE portion of the test drive looks a lot different too. The zoomed data is a little easier to understand and notice again how the trim data is trending smoothly. You don’t see the huge mood swings as were seen in the first data capture. The long term trims are fixed at 9%, which is normal for fuel enrichment for this engine platform. Take a close look at the oxygen sensor voltage at WOT. The voltage comes up above .9V, but as the throttle is held open and the engine is eating up the fuel, the oxygen sensor voltage starts to drop a little. This is the electronic fuel pressure gauge built into the scan tool. The oxygen sensor voltage is showing a weak fuel pump, which is on the ragged edge of failing. This also could be caused by a restricted fuel filter that is not allowing the needed fuel volume for the engine.
As the throttle is backed off to cruise, the engine goes back into closed loop and the o2 voltage and trims return to normal operation.
The final VE test was done from the captured data and it shows a VE of 77%. Can I use the VE calculator to condemn a MAF sensor all by its self? I don’t trust it enough to hang my hat on the data, but it sure helps out along with the fuel trim data.
Would you happen to have a copy of that excel worksheet? If so attach it to this post.
I tried uploading the excel file, but I get a message the file type is not supported.
Soon we’ll have support for other file types. If you have the sheet shared via Google Drive or One Drive, you can post the link here. If that won’t work for you, email me the file and I’ll set it up for you. …
Hi Albin, In de google play store you can download VE-calculators for youre labtop or handy.
Thanks Albin i always get something out of your post keep up the good work.
Great writeup and very informative. The scan tool is very powerful tool way above just reading codes. I hope that others will take a piece of this and put it into their routine on their test drives. It can tell you a lot with very little effort besides driving the vehicle in several states like cruise and full throttle acceleration. Thank you so much for taking the time to share all this information.
I agree scan data is great way to reason metered MAF error through interpretation. Known good MAF/VE's #'s are nice to have, but can be variable. There are also physical airflow issues like you stated making interpretation necessary. I also would not hang my hat on a generic VE calculation.
I would like to add that while I agree that a trending downwards H02S under maximum flow demand could be a marginal fuel pump, it could also be calibration within the software or restricted injectors. This data to me would be interpreted as marginal fuel flow entering the engine and not only fuel volume/pressure to them.(If I hadn't confirmed by rail pressure measurement)
I am seeing approx +40 total correction in the fuel trims in the area I am seeing as cruising. I would like some elaboration on "The short term trend from a -30 to a +35 or so, with throttle movement; the long term trend from a -3 to a +10" .
Great Case study, Thank you.
This might help a little with your questions. This compares the data from the old and new sensors.
Not all MAF sensors fail the same way. The short term swings you see are caused by the reported air flow not keeping up with the actual air flow. The PCM is going to inject the fuel in accordance with the amount of air that is reported by the MAF. In the case of a -30% short term, the airflow has been over reported, which caused too much fuel. The oxygen sensors reported the excess fuel, so the PCM has cut back the fuel. When a MAF gets dirty, they start to report slow and on throttle transissions, their data reporting is a little on the slow side. This is what causes the huge trim swings in both short term & long term.
I wonder if the short term swings are due to table transitions within the software? When cruising and under changing airflow transitions the program will transition between authority of the speed density table and transition into MAF authority. The program would command (+)short term fuel trims because of the unmetered air entering the engine from the under reporting MAF sensor and then command (-) short term correction as the program shifts to the speed density table during the times of changing airflow. This is a theory and could be tested by turning off the speed density table within the software next time a bad MAF is experienced that swings on transition or.. Mimic an under reporting MAF with calibration and then switch off the speed density authority to see if the ST fuel trim swings are still present.
Can you share the before and after data at 3,600 RPM? This is where peak toque (and peak VE) should occur, right?
Here is the pics of the scan data, both old and new MAF, & the VE calculations at the same speed.
Bad MAF @ 3500 RPM
VE Bad MAF @ 3500 RPM
New MAF @ 3500 RPM
VE @ 3500 RPM
There is one more piece of data I would like to post up about this problem. I have taken the data at idle, 1500 RPM, WOT & 2500 RPM at cruise to compare the old MAF & the new MAF. this gives a good picture of how this MAF has failed.
The Cyl/Air before and after are as follows:
- New MAF: 3594 RPM 121.85 g/s = 0.68 Grams/Cyl (80.5%)
- Old MAF: 3527 RPM 82.27 g/s = 0.47 Grams/Cyl (55.6%)
100% Cyl fill = 0.844 grams.
I like to do the quick math on Cylinder Air to see if I’m in the ballpark and move on. Besides, this is what the ECM is using to calculate fuel.
The VE Calculator has its place but this method doesn’t slow you down as much. It’s too bad that most scan tools don’t offer math channels so the end user could create his or her own.