97 Tahoe 492 cube BBC Vortec major fuel trim spread between banks

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.

Thats a pretty big cam to control with fuel trims at idle. What rpm do you see 35ish kpa, no load? What injectors are you running?

36 lb/hr injectors.

I forgot to mention this is in an off-road truck with some fairly tall & large tires. Not quite monster truck, but pretty close (bigger than anything else I've ever seen on anything on the street).

At 45mph, it takes 15% throttle to maintain speed and during these conditions the MAP is operating in the 60-70 kpa range most of the time.

The only time I see the MAP go below 45kpa is during deceleration above 1200 rpm. And the lowest I've ever seen it go is 33 kpa.

Care to post the tune?

What would be nice to see is open loop idle going to closed loop as the cam efficiency comes up.

A copy of the current tune is attached.

I don't see anywhere in the tune where an "open loop idle" can be commanded. But even if it could be, that doesn't help me out. This fuel trim spread between banks still occurs at part throttle.

Well, I would say you have an issue in one of the cylinders on the side that causes the issue. Injector, low compression..... Heat gun it and see what temps are, you may find a cold one.


If all that checks out you can use 02 switch points and closed loop mode table to define idle O2 switch points. Which will be very different due to the cam.