Short Term Fuel Trim numbers fluctuate

Hi, I have a 1999 996 3.4 and just upgraded a bunch of oil related parts for longevity, and now am using iCarsoft diag to check values and ensure everything looks solid. I have a few questions and am seeking advice on whether MAF may be bad or some other sensor (or is this normal)? Here is a table of two "runs" - Run 1 was in the driveway to warm up the car and check everything and Run 2 was a 40 minute drive with RPM's around 3K and up to redline twice. When I was checking the car over (Run 1), I noticed vacuum leak at Brake Booster connection near Bank 1 intake, and I taped it to seal it for the moment. I noticed Bank 2 TRA was - 4 (negative 4), which to me suggested a rich condition on Bank 2, maybe an injector issue, etc. I removed the MAF and cleaned it with MAF cleaner and also ran the engine and sprayed some carb cleaner into the intake as I had noticed the intake tracts in the block were oily when I replaced the AOS with a UAOS. Engine runs well and is smooth underway, with sporadic stumble at idle but not severe at all I went out for thorough exercise (Run 2) and recorded the same metrics to compare. I notice now Bank 2 is +4 TRA (positive 4) which to me now suggests a lean condition and extra fuel I have no warning lights or messages in the DME I also believe I can see the TRA values bouncing as I look at it - near zero then a value, etc. but I did not capture it on my camera. I notice the MAF Air Mass value also seems low... Am wondering if MAF may be worn out, or if there are other sensors that are involved which the DME relies upon which may be worn out? Does the DME use the O2 sensors for this calculation along with the MAF? Here is my table for your review. Was also surprised to see exhaust gas after the cat to reach over 960 Fahrenheit while I was driving. I thought the car was going to burn up until I read later this is normal operation temp <!--td {border: 1px solid #cccccc;}br {mso-data-placement:same-cell;}-->

I�m not an expert, but I�ve been looking into this since I plan to take some readings myself. It looks like the two tests were done under different conditions � one at idle and one while driving. That �4.0 TRA shows the ECU is pulling a lot of fuel at idle, so it�s probably rich on that bank. Have you changed your injectors? They�re known to leak and cause rich issues, and some people think that can lead to bore scoring by washing the cylinders. Made sense to me, so I replaced mine just to be safe.

I agree your low range trim of -4.0 on run one suggests a rich condition with ecu trying to decrease fuel. This is interesting because you found an air leak which should cause a lean condition although in a recent video Jake R and the PCA rep both said an air leak causes the car to run rich so now it's confusing. Maybe someone can clarify. In any case it's always good to fix vacuum leaks so smoke testing helps a ton.

a leaking diaphragm on the fuel pressure regulator would explain it all... pull the vacuum hose from the pressure regulator while running and see if there is fuel in the hose...

I will pull the line and check the presence of fuel tomorrow. I think I eliminated the injector today since the symptoms did not seem to remain consistent. LOL, I was hoping to at least get the scratches all healed from my last weekend's projects and was trying to avoid having to twist into pretzel shapes to get to vacuum lines, however it looks like the pressure regulator is right over cylinder 4 - so should be easy to get to. Thank you.

OK, so I tested the vacuum line off the pressure regulator and you are correct! There is a very very tiny amount of fuel it looks like (it evaporates too rapidly for me to get any on my finger) on the inlet to the vacuum line. I guess "any" amount is unacceptable...so I will order a new replacement and see how it affects outputs. I mean it is like almost just unnoticeable it is such a small amount. I guess the question I have is why would a leaking pressure regulator only affect Bank 2 and not both Bank 1 and Bank 2 short term fuel trims? Also it seems the vacuum line from the regulator goes to the cross over tube between the manifolds. This seemed strange to me, however I guess that works as it will provide a variable manifold vacuum to the diaphragm...

After all the work you have done, cleaning MAF sensor, and so on, I think it would be a good idea to reset the ECU/DME to base values. You do this by disconnecting the battery and wait a few minutes (30) to reconnect. Maybe even wait overnight. This resets the DME/ECU. Then do several drive cycles to let the DME/ECU recalculate what the engine is doing. Then run your software and observe the values.

Sharing my update. I did change the FPR and also the MAF sensor. The trims have settled down now and are near 1 and 0 on both banks. I think bank 2 (4-6) is 1 and 0 and Bank 1 (1-3) is 0,9something and 0,something extremely low. I will capture another iCarSoft output and update my spreadsheet and post for sharing.