High Long Term Fuel Trim on Load

Hey guys, I have a 99 GMC Sierra with the 5.3l V8.

I've been trying to diagnose a fuel trim issue for some time now. Here's what's happening..

On idle, ST and LT seem okay (around 0% - 2%).

When revving to 2500RPM and holding, LT trim jumps to 10%+

When actually driving, and the engine is under a load, LT jumps to 15% . ST hovers between -1% and +3%.

Both banks are the same.

I did a fuel pressure test and the results are:

Ignition on, engine off: 55PSI
Engine on at Idle: 50PSI
Engine at 2000RPM: 49PSI

Quickly revving the engine, the pressure jumps to 55-60PSI. (seems like pressure regulator is working).

There are no stored codes.

What should I test next?

Thanks!

I'd suggest a visual on the MAF wires for any debris/contaminants. Remove the MAF from the intake tube to get a good look.

If you find any, clean the wires carefully with MAF cleaner. Q-tips are pretty foolproof. You can use a small pick or a pocket screwdriver, but it takes a steady hand.

Reinstall and recheck your trims.

Checked the MAF today and it looked clean but went ahead and cleaned it with MAF cleaner.

Didn't make a change though – LTFT still high above idle and under load.

Did you happen to see what the stft was doing after you cleaned it? Those blocks off idle will have to readapt if the cleaning fixed it and you'd see the stft going negative off idle until the ltft set a new base line. Of course that may not have been a fix, but that was the first thing that came to my mind too.

Nissan Technician