GeekDIYMechanic - 2006 Corolla Dirty MAF - P0171 ScannerDanner - Dirty MAF Sensor Case Study Schrodingers Box - Diagnosis and Understanding: MAF Sensor Pt1 Analog, Digital, Hotwire, P0101 P0102...A quick and successful diagnosis and repair of a dirty Mass Air Flow (MAF) causing a constant lean condition throughout the RPM range.
A friend of mine owns a pizza restaurant near my parent’s house. While waiting for my pizza to cook, we were talking, and he mentioned that one of his delivery cars had a check engine light illuminated.
Using his non-graphing Snap-On scan tool, he was able to pull a P0171 code, Fuel System Too Lean Bank 1. He then told me that he had already performed two smoke tests on the intake manifold in hopes of finding a leak. He found none. He also indicated that he cleaned the Mass Air Flow meter (MAF).
Changing scan tool modes to display engine parameters, I noticed that the Long Term Fuel Trim (LTFT) was around 18% and the Short Term Fuel Trim (STFT) was around 12% at idle. At 2500 RPM and 3500 RPM, the LTFT and STFT stayed at around 30% combined. In addition, the freeze frame data indicated the error was triggered at 2625 RPM.
Being that the LTFT and STFT trims did not change throughout the RPM range clearly demonstrated that it was not an intake manifold leak, which matched his smoke test results. But, I was puzzled as to why the car was experiencing a lean condition after cleaning the MAF.
I also noticed that the car hesitated while snapping the throttle off idle, but once RPM pass 2000, the engine accelerated without hesitation in park.
At this point, I’m thinking it is a dirty MAF (but he cleaned it) and/or fuel delivery issue. But, for me to determine this, I needed to run more tests. Unfortunately, all my tools were home, so I had to return 5 hours later.
Upon my return, he had gone home for the day, so I had to work by myself. Although he is a friend, he is not a super close friend, so I felt nervous about working on his car without him being present. This caused me to make some mistake that will be discussed in the What Could Have Gone Better section of this Case Study.
Since it was a Toyota, running a fuel pressure test was not a quick test, due to the fact that a Schrader valve is not present on the fuel rail. So, I chose to examine the MAF. After removing the MAF, it was very easy to see that it was very dirty. But how could that be? This MAF fixture actually houses two sensors. The outside, easily visible, sensor is the Intake Air Temperature (IAT) sensor, and the inside, less visible, sensor is a two resistor MAF sensor. So that meant, my friend cleaned the IAT instead of the MAF. After cleaning the MAF and reinstalling, I ran the car to see if it helped at all.
The cleaned MAF made an immediate difference. I then back probed the MAF and performed several snap throttle tests from idle to WOT and noticed that the MAF would got up to 4v. I then turned to my graphing scan tool to monitor the LTFT and STFT. The STFT went to -13%, but the LTFT was holding at 14% at all RPM ranges. In my mind this was a decent fix, so I went home and set up an appointment for the next morning with my friend. I wasn’t totally satisfied with the fix due to the off idle hesitation, but my friend only asked for help on the P0171 trouble code.
Arriving at this restaurant the next morning, the first thing he told me was that after doing 40 miles of deliveries the Check Engine Light turned off. But, I just didn’t feel like it was totally fixed. Hooking up my graphing scan tool, we briefly drove around. We noticed that the combined LTFT and STFT hovered close to 10% at times, which I wasn’t happy with. For some reason, the LTFT just wouldn't drop to low single digits for many parts of the RPM range.
We also performed some short WOT acceleration runs to ensure the fuel delivery was ok. To me, the car hesitated under quick acceleration, yet the A/F waveform looked as expected. Since the catalytic converter was not warmed up enought, I ignored the downstream O2 sensor’s waveform. I also demonstrated the off idle hesitation to him.
Although I suggested further tests, he felt the car was fixed since the Check Engine Light was off. So, I wrapped up my work for now.