Help with Fuel Trims - 2002 Express 3500

[Clarkbug]

Hoping to get a little advice from those who have turned wrenches far longer than I.

We have a seldom used 2002 Chevy Express 3500 gifted to us by a family member. We only put a few thousand miles a year on it, and other than some maintenance stuff, it's been pretty good to us. It has the 5.7L engine and 4L80E transmission.

We noticed this spring that it smells like it's running very rich. The check engine light isn't on, doesn't feel like it's driving any differently. I plugged in my scan tool just to see what was going on, if anything. Did find one code (P0463), but was more wondering about these fuel trims.

I'm guessing the Bank 1 sensor 2 readings are just showing at 99% and not actually a problem. But the bank 1 values that are both over -10% have me concerned. I took these screenshots with the van in park, but giving some gas so it wasn't just at idle.

I know trims that are highly positive generally indicate a vacuum leak, but what about ones that are negative? Is that an injector problem?

I haven't pulled the doghouse to try and visually inspect anything (there previously had been some rodent damage from the van sitting for years before we got it) but wanted to ask if anyone here could give me a starting point.

 

[clinebarger]

A leaking Fuel Pressure Regulator will cause negative fuel trims

 

[clinebarger]

Poppet style Multec injectors hate to sit for any length of time

 

[BrendanC]

will echo the master above. i would also change the upstream O2 sensors with DENSO sensors. likely not in the best of health being 24 years old. change the fuel pressure regulator on the driver side fuel rail with either Denso or GM genuine parts. also snag the vacuum line for it as the boots like to crack. give it a dose of techron fuel injector cleaner and drive it rough for the rest of the tank.

plugs/wires wouldn’t hurt but you’re getting into spending money at that point.

 

[Lawnman23]

change the fuel pressure regulator on the driver side fuel rail with either Denso or GM genuine parts. also snag the vacuum line for it as the boots like to crack.

Being this is the 5.7, won’t the FPR be under the upper intake cap, attached to the injection assembly?

Or do the Express vans do something different?

 

[BrendanC]

Being this is the 5.7, won’t the FPR be under the upper intake cap, attached to the injection assembly?

Or do the Express vans do something different?

oops. i missed that 5.7 detail. you are correct.

 

[Rob_Roy]

I would start with a tank of fresh fuel and a stiff dose of injector cleaner. I have the same drivetrain in my suburban, which also sat a lot and had “burn your eyes” idle fumes when I first got it. A few tanks of treated fuel did wonders for it.

 

[Clarkbug]

Thank you for the advice everyone. It does sit for a good length of time with the same fuel in it. I'll get some good cleaner and will start there. I do have plugs and wires, a distributor cap and rotor all ready to go (GM Genuine stuff... I have heard GM stuff can get funky with aftermarket ignition bits), so that's on the list for later this summer.

I'll visually inspect the FPR, or will post back here about how to find it when I get there.

Any issues continuing to drive it until I get in there?

 

[Rob_Roy]

If it seems to run and drive normally, send it.

 

[zrxkawboy]

It does sit for a good length of time with the same fuel in it.

Personally, I'd treat the gas with a good conditioner/stabilizer every time you fill it, so that sitting will be less of an issue going forward. (Of course, this is after you get the current problem sorted.)

 

[zzyzzx]

Isn't -11.7% long term fuel trim barely out of spec and going to be difficult to troubleshoot? Perhaps waiting for it to get worse is going to make it easier to figure out.

 

[Lawnman23]

I'll visually inspect the FPR, or will post back here about how to find it when I get there.

Don't worry about the FPR, its not accessible directly. It's attached the the side of the injector assembly, which is enclosed under the plastic upper intake manifold cap. The entire assembly is under vacuum, there is no hose or anything.

 

[Lawnman23]

Also, if you have E0 gas in your area, its worth the added cost to use that full time instead of E10/E15.

I wanna say for 2002 GM was using the updated Multec 2 injector assembly across the board for Gen1 engines but if not, the original poppet injection assembly really did not like ethanol. If gas sat in the system for long periods of time, it would break down and cause the poppets to stick and cause issues. I don't think that's whats happening here, just a FYI. I have the same L31 engine in my truck, I always run E0 in the "off season" over the winter when it sits for 5+ months at a time.

I imagine @clinebarger can confirm which setup a 2002 Express has.

 

[Tman220]

Where is the evap purge solenoid located? When it's open it can easily favor one bank or the other and cause fuel trims to go -10% pretty easily on one bank.

 

[D60]

Poppet style Multec injectors hate to sit for any length of time

I was wondering about this, too (based upon what I've learned from you). Would a top end cleaning potentially free them up?