I have a 1997 Bravada that is getting about 10MPG if I drive it nice around the city. I've been using AutoTap to read various PCM parameters and a few have my attention. Namely, these two along with their values because they are stuck at 99.2%:

Short Term Fuel Trim from O2 Bank 1 Sensor 2 = 99.2%
Short Term Fuel Trim from O2 Bank 1 Sensor 3 = 99.2%

Has anyone else ever seen this behavior with or without Autotap version 3.1? I'm wondering if this is an Autotap software problem because the voltage values for these O2 sensors do change.

 

B1S2 and B1S3 are used for monitoring the efficiency of the catalytic converter. They have nothing to do with engine performance. LTFT & STFT for B1S1 & B2S1 would be a more accurate indicator of a possible problem. If you're only getting 10mpg, there might be DTC's in memory, and they could help figure out what and where the problem is. Negative fuel trims indicate a rich condition, and the PCM is subtracting fuel to the engine. Positive fuel trims indicate a lean condition, and the PCM is adding fuel. Generally, fuel trims need to be single digits, with zero being ideal.

 

Unfortunately, I have no DTCs set or in memory. If I remember correctly B1S1 and B2S1 hover in the low single digit negative numbers and occasionally hit low double digit numbers. The engine runs fine. Just have terrible fuel economy. Maybe everything is just old and tired.

 

No DTC's is a good thing!

If you're saying fuel trims for B1S1 and B2S1 are single digits, that's fine. Actual oxygen sensor data for B1S1 and B2S1 needs to constantly switch above and below .450VDC. There are several other things, that are not engine related, that can cause poor fuel mileage. If there are no DTC's in memory, that's a pretty good indication that the engine is performing as designed.