Good Old Random Cylinder Misfire Mystery

[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.

Also, when I did the intake gaskets, I did TB gasket and EGR gasket going into the intake. To afraid to touch those rusty ones going into the manifold lol

 

[Nicolai8775]

Here's #5 plug that's misfiring the most.

 

[Fless]

Are you going to swap that with another cylinder's plug to see if the misfire follows?

 

[Nicolai8775]

Here's #5 plug that's misfiring the most.View attachment 365365

NGK TR5GP Platinum plugs with .040 gap

 

[Nicolai8775]

Are you going to swap that with another cylinder's plug to see if the misfire follows?

I'm not liking the condition of the wires, upon closer inspection.

Also I noticed the tiniest crack in the boot for the fuel pressure regulator vacuum hose

 

[Nicolai8775]

I'm not liking the condition of the wires, upon closer inspection.

Also I noticed the tiniest crack in the boot for the fuel pressure regulator vacuum hose

But yes, I am.

Oops replied to wrong post. Yes I'm gonna swap plugs around

 

[Fless]

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?

I wouldn't expect an evap issue like this to cause a fuel delivery problem, so you'll probably want to work on the fuel trims first.

A couple of comments about evap systems... since this truck is new to you have you tried to add fuel to the tank yet? If the vent system has a bad vent valve or is plugged you may experience issues with filling up. Be sure to turn the engine off when you are fueling; that way the vent is commanded open (whether or not it actually opens). On my '04 the canister vent valve was intermittent, so a single test might not identify it as such.

Wells has an informative video on how the system works, and the basic purge and seal test:

Also the Car Doctor has some good info:

There are many causes of lack of venting. Most common are the canister vent valve (or its vent filter plugged), a contaminated charcoal canister (likely due to over-filling the fuel tank), and evap lines plugged with charcoal pellets from a damaged canister.

Eric O at South Main Auto on YouTube has a few evap videos that can be helpful.

The onboard evap test requires some specific conditions to be met. It's run from a cold start, fuel tank level between 1/4 and 3/4 full, etc. If it fails it will set an evap code and light the money light.

 

[Nicolai8775]

Well I swapped plugs cylinder 5 to cylinder 3. 5 still missing the most on the graph. Occasional cylinder 4 and 1 miss.

Spraying intake with water didn't change idle or fuel trims

Ltft1 reading a steady -2.34 and ltft2 reading a steady -1.56 at idle

 

[Fless]

The trims are a bit weird in that usually with a vacuum leak the trims would be high positive at idle, and go lower (closer to zero) at higher rpms. I think you said the MAF was cleaned but do you know it's working properly? Might want to swap it out for a known good one.

Also, perhaps a standard vacuum test would be helpful, maybe just to rule it out as the cause. Any exhaust leaks?

 

[Nicolai8775]

The trims are a bit weird in that usually with a vacuum leak the trims would be high positive at idle, and go lower (closer to zero) at higher rpms. I think you said the MAF was cleaned but do you know it's working properly? Might want to swap it out for a known good one.

Also, perhaps a standard vacuum test would be helpful, maybe just to rule it out as the cause. Any exhaust leaks?

My shop buddy suggested this too. I'm gonna pull the one off my 2001 2500hd quick