Fuel trim diagnostics

[afpj]

Team TYF, a buddy has a 2003 Tahoe 4x4 LT WITH 190k-ish miles. He asked me for an opinion since I had the same issue a few months ago.
He has high LTFT’s about (+) 15-16 banks one and 2 At idle. STFT are WNL around + 3 % or less. At 2000 rpm, LFTF drop to about 5-7 % both banks. Sure looks like a vac leak but it cannot be found. Water, flammable spray, butane (yeah we had a fire extinguisher handy), smoke , cannot find it. MAF is less than 3 months old OE Delphi ( replaced as part of this troubleshooting). He noticed that if he idles with the no Engine air filter, the LTFT are worse at +22% . He didn’t say what it dropped to at higher rpm. My question is, if a vac leak, why would higher air flow (no filter) make the fuel trims go higher? Is this an indication it’s not a vac leak? Pre cat O2 sensors are maybe 1 year old, and look to be switching regularly. I’m trying to understand the physics of how less restricted air flow pre MAF would affect fuel trims.

Thank you all for any wisdom to impart to us.

 

[Fless]

When you remove the air filter, is that the only thing that's taken out, or is the MAF being bypassed, too? With everything in place, you could watch the MAF (g/s) in live data to see if it tracks with the engine displacement over RPM.

Has a regular old vacuum test been done with a gauge? If vacuum brakes, is the vacuum connection solid at the booster?

Any exhaust leaks?

Is this a Flex Fuel engine and, if so, what is the alcohol percentage reading? If running non e85 but it thinks that's what it's using, it's going to be dumping fuel into the system.

After that I'd say reset the LTFTs; they've been learned and may need to be re-learned over a bit of time, and monitored.

 

[afpj]

Fless, thank you for the info. With the air filter out he left the MAF in place so Air was going through MAF . Via torque app, MAF reading was about 5 g / s at idle, and went up from there with RPMs. No exhaust leaks that we can detect by listening. Vacuum test with a gauge has not been accomplished, I don't know where that gets attached to off the top of my head (brake booster line?). It has regular vacuum brakes not hydro boost. Connection seems pretty solid. I cannot remember if he had flex fuel or not, but on my torque app I do have alcohol percentage on my display but it's always blank even with my flex fuel vehicle.
I think I understand the science behind high FTs with a vac leak, bit not why it goes up with less restriction (no air filter) on the intake (but with MAF in place). Will see if I can get with him this weekend for vacuum test . thanks!

 

[Fless]

Same here with the Torque Pro app and not being able to read alcohol %. Wish someone had that PID!

Had another thought -- check to see what the MAP sensor is reporting.

 

[afpj]

Copy, thanks. Will add that to my list to check this weekend. Have to read up on what a MAP reading should be on these.

 

[nonickatall]

Did you check the fuel pressure regulator? if the fuel pressure isn't right, your engine control management will also get out of control.

That would match the changed values when idling because the pressure is lowered when idling

 

[afpj]

You know, didn't think to look at that. I couldn't remember if he had flex fuel or not. Thanks for the suggestion

 

[swathdiver]

Aftermarket parts play havoc with the computer system, verify the brands of what is on there, MAF, O2 Sensors, etc.

Can you graph the O2 sensors while spraying water around the intake gaskets?

 

[afpj]

Well, life got busy, was able to pop over for a couple of runs with torque pro. MAP reads about 5.3 psi at idle (565 rpm) in drive gear (and in neutral at 2000 rpm MAP reads about 4.5 psi - about the same neutral at idle) In drive on the freeway at 2000 rpm, steady throttle flat road MAP was about 10 psi. Not sure what all that means.

O2 sensors precat seem to not vary in wave form at idle (closed loop) with or without water spritz around intake gaskets.

Does any of this help the diagnosis? I cannot find MAP sensor "norms" for this engine. Perhaps my search terms suck.

I'll try to pop over with a vacuum gauge next weekend when I'm not supposed to be on duty.

Thanks!

 

[afpj]

Put a vacuum gauge on it, 21 in hg (green zone) at idle, and at 1500 rpm (no load) about same. It only drops when I goose the throttle, then goes back to 21. No needle jumping otherwise. From what I read, this is normal, less likely to have an intake leak with these finds?