I've been helping a friend out and working on this thing on and off for the past year. It's a 1997 Tahoe with a Vortec 7.4 bored and stroked to 492 cubes (using 97 K3500 7.4L 4L80E base tune). It has a mild cam (I don't have cam specs, sorry - but it does seem to generate 48kpa MAP at 825rpm idle no load). Long tube headers, very short exhaust with single chamber flowmaster mufflers mounted about 1' from the headers. True-dual exhaust with no crossover or X-pipe (transfer case and trans is in the way). The upstream o2 sensors are mounted in the header collectors and have been replaced with brand new delphi units.
Engine starts and runs fine until it drops into closed loop. Doesn't matter if you are sitting at idle or driving, the left bank STFT will start bottoming out - causing the LTFT to bottom out at -50% and the right bank STFT will start going high, causing the LTFT to max out at +50% and the engine runs very badly. If I reset fuel trims, it instantly runs better (as it should) until the LTFT's return to their 100% spread from bank-to-bank.
I disabled LTFT learning in the tune and limited STFT to +/- 10%. Engine runs good but, most of the time the left bank STFT will run around -7% while the right bank STFT will run at around +9%. The kicker is both o2 voltages still cycle high and low as you would expect to see on a stock engine except the fuel trims are still split from bank to bank. So it is like the PCM is still able to maintain stoich while having the fuel trims limited in the tune.
No exhaust leaks, replaced o2 sensors, verified o2 wiring and injector wiring. Everything is fine.
Could the aftermarket cam be causing the fuel trim spread between the two banks? This engine does have less than 500 miles on a fresh build.
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