Good Old Random Cylinder Misfire Mystery

[Nicolai8775]

Just bought a 2001 Suburban 2500 with the 6.0 with 158,000miles!

The guy said he replaced the camshaft and crankshaft position sensors and it needed a crank relearn. I did that with my EFI Live cable.

I still had P0300 after the relearn. And the first crank won't start the truck. Sometimes it backfires on first crank. Always starts on second crank.

I got the idea that it was fuel related since the scanner was showing ltft running in the -12 range.

Replaced filter, fuel pressure test showed 48-50psi at idle and 58psi with the regulator unplugged. The regulator looked brand new and did not leak gas when unplugged. Replaced fuel pump cause the guy gave me a receipt with a recently purchased aftermarket pump listed on the receipt. Replaced with a Delphi.
It did slowly leak down after and hour.


Cleaned the MAF.


Torque graphs show occasional misfires in a couple cylinders on both banks. Must not be enough to trip p0300 but it's there. 5 shows up a little more frequently.

He said he replaced plugs and wires, going to verify that today. The coil packs look brand new.

I pulled the intake cause some of the grommets looked super squished and over tight. Gaskets were new, camshaft position sensor looked new.

Replaced with the metal felpro instead of reusing the plastic felpro gaskets.

I've even added an extra ground from frame to motor in case the one on the cylinder head is weak.

I've noticed that air either sucks in or blows out when I open my gas cap. Which I have never experienced on the last 5 gmt800s I've had.

Is their some sort of weird EVAP thing going on?

As of now I have bank 2 O2 sensor 2 circuit low code.
And a code for misfire in cylinder 5.
Weird thing is it hasn't turned on the check engine light yet but it shows as a fault in Torque.

perhaps a bad injector? Would one injector cause p0300 or just a specific cylinder misfire like my p0305?

I've done a ton of diagnosis and the post is getting long. Let me know what you think and I'll promptly respond to any questions.

 

[swathdiver]

Verify the type of plugs and that they are gapped to .040. How do they look?

Try to find out the exact camshaft used.

Pushrods and rockers tight with not too much play?

How are the long Term Fuel Trims?

 

[Nicolai8775]

That's at idle, I'm gonna check again at cruise speed. I think that's when it starts dropping to below -10

 

[mattbta]

Subbed. Mine is doing similar. Sometimes won't start on first crank and backfires. My LTFT bounce around quite a bit. Idle I saw -3 to -12 today on both banks, but went positive when cruising. New cam sensor. Relearn already done, too.

EDIT: mine has no DTC's and only 1 misfire on cyl 8 that i've seen.

 

[Fless]

When the engine is not running (key off or out of the ignition) there should be no fuel tank pressure or vacuum. Having that symptom indicates that the evap vent system (typically the canister vent valve) is not opening like it should, or the charcoal canister is plugged, etc. A purge and seal test should verify operation or indicate where to look for the cause.

 

[Nicolai8775]

Ltft jumps all over the place depending on how much throttle I give it at cruise. Any acceleration ltft jumps to +25% or higher

 

[Nicolai8775]

When the engine is not running (key off or out of the ignition) there should be no fuel tank pressure or vacuum. Having that symptom indicates that the evap vent system (typically the canister vent valve) is not opening like it should, or the charcoal canister is plugged, etc. A purge and seal test should verify operation or indicate where to look for the cause.

I've not ever had an EVAP issue like this. Would you educate me? Would this cause a fuel delivery issue or cause it to run rich?

 

[Nicolai8775]

Verify the type of plugs and that they are gapped to .040. How do they look?

Try to find out the exact camshaft used.

Pushrods and rockers tight with not too much play?

How are the long Term Fuel Trims?

About to tear into to find out what kind of plugs he used.

As far as I know this is a bone stock lq4 with no cam upgrade.

I posted about LTFT further up. It seems lean at idle and super rich with any acceleration. Ltft numbers change rapidly, which seems odd to me.. seeing as it's "long term" I assumed it was more of an average.

I don't have the motivation to pull of valve covers at the moment, but I will definitely check that. Top end seems quiet though, no chattering

 

[swathdiver]

Ltft jumps all over the place depending on how much throttle I give it at cruise. Any acceleration ltft jumps to +25% or higher

That's a clue that there's a massive vacuum leak. While graphing the LTFTs or the O2 sensors, have someone spray some water between the heads and intake. Put a ratchet on the intake bolts and see if they are loose. GMT900s torque theirs down in two sequences in inch pounds of torque.

 

[Nicolai8775]

That's a clue that there's a massive vacuum leak. While graphing the LTFTs or the O2 sensors, have someone spray some water between the heads and intake. Put a ratchet on the intake bolts and see if they are loose. GMT900s torque theirs down in two sequences in inch pounds of torque.

My favorite method is to go get a car wash to test for manifold leaks lol. I will try this and verify my gaskets didn't slip when I put the intake on. Ooooof that'd be such an amateur thing to do

But I did follow torque sequence... And kinda guessed at how much torque I was putting on it, which is less force than the one time I snapped an intake bolt off and had to drill it out and helicoil it to put a new bolt in, perhaps I was being to careful.

I will verify, but it was doing the long crank and backfire and crappy fuel trims before I did the intake gasket.