quartercenturytruckin

I did go through and drive it to gather some more data. Had no misfires.

When I accelerate and the rpms get to ~2000 the MAF sensor shows 38-42 g/s and the long term fuel trim goes up to +25% and my short term rises to about 12-15%. My current theory is that the MAF sensor is faulty, showing artificially high airflow, and causing the engine to run richer than it really needs.

If this is the case, then towing a trailer at 2000-2500 rpms could foul the plugs enough to fail to ignite after about 20 minutes of driving.

If this theory is correct I should be able to clean or replace the MAF sensor and see a drop in readings when driving at 2000 rpms and a corresponding drop in short term fuel trim.
If the drop in MAF and fuel trim does appear as I think it will, I can probably go ahead and reset the long term fuel trims by disconnecting power and draining the capacitors.

mountainmanjoe

hmm so it leans out under load. Is the fuel pressure steady under these conditions?
Definitely do that smoke test ASAP and wouldn't hurt to run some injector cleaner through it.


What is your theory based on? The values coming from it are normal.

Keep moving through your checklist

quartercenturytruckin

It definitely thinks it's too lean under load and is adjusting the fuel trim up around 35% based on the code reader.

I based the theory on the fact that I read on a different forum that about 1 g/s per 1k rpms per cylinder was typical and the following search results into google· A failing or contaminated MAF sensor will often cause negative fuel trims at idle and progressively more positive fuel trims as engine speed and/or load increase.

· A positive fuel trim percentage indicates that the powertrain control module (PCM) is adding fuel to the engine to compensate for a perceived lean condition.

· When the air-fuel mixture in the combustion chamber has more fuel than is needed for efficient combustion, it's considered "running rich". This can lead to a buildup of carbon on the spark plug tips, which can cause misfires and poor engine performance.

It also lines up with the fact that I don't get a misfire until I've driven at a high fuel trim percentage for around 20-30 minutes. That implies to me that the truck is running too rich and fouling the spark plugs after a time.

Am I completely off base here?

mountainmanjoe

No, that's nonsense. generally accepted rule of thumb is 1 g/s per litre of displacement at idle. And that value can go up to 40x at WOT.
My 5.3L engine swallows 97 g/s @ 3,000 RPM
You can carefully clean the MAF with a suitable cleaner if you want, but I don't think it'll solve your issue.

A trim of +25% is kind of a nonsense value. It's maxed out which means the computer is trying its hardest to add fuel , but not succeeding in reaching a balanced mixture.
It makes me suspect fuel delivery.

so check them

quartercenturytruckin

You're making a lot of sense.

I went thru my check list. Found no vacuum leaks, cleaned the MAF sensor, the IAC, the throttle body, and the air filter. I unplugged the battery while I did this to clear the stft and ltft.

Couldn't get to the cam position sensor or spark plugs because things were hot.

Drove it and ltft was back to 0%. Stft was higher, as much as 28% at times. Ltft is rising as it learns.

Seems like I didn't address the issue at all.

You said you suspect fuel system, should I check fuel pressure next?

mountainmanjoe

yup. what I did once is put a pressure gauge on a 3ft hose and tucked it under my wiper so I could watch it while driving.
A high end diagnostic tool like a Tech2 can run an injector balance test to see if they're flowing properly.

quartercenturytruckin

Guess I'll go buy or rent one today and check it. my current scanner will not check injector balance. I'm not sure if the OBD2 system in the truck will provide that info anyway. I'll let you know the results when I get them.

Edit: ordered a gauge and it will be here tomorrow.

quartercenturytruckin

I rented a fuel pressure gauge and check it on my lunch.

Key on engine off = 58 psi when the pump runs. This drops immediately to 52 psi when pump stops. After 60 seconds psi had dropped to 48.
Turned on engine and the pressure "fluttered" 48-50 psi. pressure did not rise when revving the engine.

test repeat

Key on engine off = 58 psi when the pump runs. This drops immediately to 52 psi when pump stops. After 60 seconds psi had dropped to 50.
Turned on engine and the pressure "fluttered" 48-50 psi. pressure did not rise when revving the engine.

Looks to me like the fuel pump is good and there aren't any leaks in the system. Is the pressure supposed to rise when I rev the engine?

Also, I found this article that implies either a fuel delivery issue or an air metering issue.

https://gearsmagazine.com/magazine/p...e%20with%20RPM.

mountainmanjoe

Well you said it misfires under load, and the fuel trims shot up at 2,000 RPM, so what's the pressure if you hold the accelerator pedal down for a bit?

You can also try unplugging the MAF and see if the trims improve.

quartercenturytruckin

I will check tonight and report back.

What should happen when I hold it down for a bit?