Fuel trims become more negative as rpm increase

I’ve got a 2012 Silverado 5.3 flex fuel truck. Trying to figure out why my fuel trims are so negative. No codes and it runs fine, maybe a little sluggish. High mileage truck close to 300k miles. Really doesn’t seem to burn much oil, and don’t seem to have much blow by. Has a new maf and map sensor. The fuel trims at idle are usually slightly negative like long terms around -4 to -8 but they become more negative with higher rpm. Driving down the road their usually -10 to -15. Holding the rpm to 4K and their around -18 to -20. The alcohol content seems to be reading about accurate. The maf seems to be reading right around 5-6 g/s at idle the map seems to be accurate and using a vacuum gauge on the engine the engine vacuum seemed fine. Stock tune in the Ecm and stock intake. The fuel pressure is fine steady around 44psi. Just looking for ideas on what I should check? Injector balance test maybe?

With fuel gauge installed key on engine off does it stay at 44 psi or does it drop in pressure while waiting 2 minutes to 5 minutes?

Also does your o2s stay rich or do they move up and down in mv readings?

The fuel pressure initially rises after shutting off the engine to about 60. I assume that’s from the heat of the engine. It doesn’t seem to bleed of within 5 minutes. However letting it sit overnight it does bleed down to about 0. That’s what makes me think maybe a leaky or worn out injectors. I’m going to try and do a balance test on them if I get time tomorrow. I just don’t get why it becomes more rich at higher rpm. Most rich conditions I can think of would be the other way around. Maybe it’s just a worn out engine not sucking in as much air as it should. But it makes me pretty curious.

The 02 sensors do switch from lean to rich. They seem to be working fine. Both bank 1 and 2 have the rich condition. Bank 2 seems to be running slightly richer than bank 1.

Its normal for fuel pressure to drop to 0 overnight. The rising to 60 psi has me concerned. Fuel pressure should stay about 44-50 psi then drop down over night. Seams like fuel pressure regulator is sticking closed or your return line has a restriction. Causing injectors to spray more fuel than needed.

Hardtopdr2 wrote: Its normal for fuel pressure to drop to 0 overnight. The rising to 60 psi has me concerned. Fuel pressure should stay about 44-50 psi then drop down over night. Seams like fuel pressure regulator is sticking closed or your return line has a restriction. Causing injectors to spray more fuel than needed.

Pretty sure that truck doesn't have a return line. If there is a fuel pressure regulator it'll be in the tank, though it's more likely fuel pressure is controlled electronically.

Thanks for the reply’s. I’m still going to check the injectors today. It is a return-less fuel system. Fuel pressure is controlled by a module. Their might be a in tank pressure regulator but I’m not sure. I think the pressure rise was just from heat soak though because the engine was hot when I checked the fuel pressure. I will try checking it with the engine cold thought to compare

Does flexfuel have a readout for % of alcohol?
If so shouldn't be more than 10%,

Sorry see you'v checked that.

Yeah the alcohol content seems about right. Today it’s reading 10% alcohol. Did the injector balance test and they were all within 1%. So idk, these fuel trims still don’t seem right. I’ve had the y pipe off before and the cats looked fine. So I don’t think it’s a back pressure issue. I probably just won’t worry about it too much unless it gets worse and starts causing problems. No check engine light and no driveability issues yet. I think these gm’s throw rich or lean codes at 23% fuel trims. So it gets kind of close but hasn’t gotten that far yet. I work at a car lot. I might try and compare to one of our lot trucks. But still don’t think that should be normal

Maybe it is normal and those gm’s just have trouble picking up the alcohol content right. They don’t have a sensor it’s just computer calculations based off of the fuel trims after filling up.

Well so far we ruled out leaky or stuck injector.
So what about injector on time how many milliseconds are they on for each back? Could be injector driver maybe......

Other wise over reporting map or maf sensor? Don't know for sure but can you get a live data capture of map, maf, tps, rpm desired, rpm actual, o2's, injector on time, IAC, intake air temp, and engine coolant temp. Start capture when first starting up engine cold and save it in chunks when it gets to 4000 ms and do this till engine is up to temp and in closed loop then drive it to 50-60 mph so we can see the drive cycle. I have a nagging feeling this might be a intake air temp or a coolant temp issue just need to see if it stands out.

Please has the OP solve the issue?

I facing the same problem.

Help

Sounds like a dirty maf most likely.

First is that fuel trim on one bank or both I'm assuming both by how it was worded  key would be to make sure the 44 psi fuel pressure is what that particular engine is supposed to be at or is that low if it's low symptoms could be caused by dirty fuel filter or weak pump and the voltage on the maf is correct for that rpm or is that high for that maf sensor if it's higher the symptoms described could indicate a dirty maf sensor if both are correct I would suspect the fuel injectors.

Totally sounds like a bad fuel pressure regulator to me.