Good Old Random Cylinder Misfire Mystery

[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?

Replaced with a nearly new MAF and both long fuel trims are steady at -1.58 but everything is misfiring now lol

 

[Nicolai8775]

Replaced with a nearly new MAF and both long fuel trims are steady at -1.58 but everything is misfiring now lol

Ok since it's warming up a bit bank 1 is at +1.58 and bank 2 is at +7.6

 

[iamdub]

Any idea of the brand of crank and cam sensors?

 

[Fless]

Is it still misfiring after it warms up?

Have you watched or graphed the upstream O2 sensor activity? Should be between 0.1v and 0.9v and active.

 

[Nicolai8775]

Ok since it's warming up a bit bank 1 is at +1.58 and bank 2 is at +7.6

Test drove it. ltfts dropped considerably. Never going higher than +11.5 and for the most part stayed pretty even between both banks.

Graph showing a slight miss on 5 still though nowhere near as frequent

 

[Nicolai8775]

Is it still misfiring after it warms up?

Have you watched or graphed the upstream O2 sensor activity? Should be between 0.1v and 0.9v and active.

I will do this on the drive home!

 

[rockola1971]

Ive seen this before...On one of my 2003 Tahoe 5.3L's. Turned out the spark plug wires were trashed. I would inspect your wires. Pull the boots up and way from the spark plug and Ign Coil connectors(terminals) on the wires. See if any have a broken connection to the center conductor of the spark plug wires. Some numbskull could have pulled a wire or 2 or 3.... and broke the terminals off the wires or just broke them away and didnt even notice it since the boots at the spark plugs tend to hide this unless the terminal flat out just stays on the spark plug. Swap spark plug wires and see if the misfires move.

 

[Nicolai8775]

Ive seen this before...On one of my 2003 Tahoe 5.3L's. Turned out the spark plug wires were trashed. I would inspect your wires. Pull the boots up and way from the spark plug and Ign Coil connectors(terminals) on the wires. See if any have a broken connection to the center conductor of the spark plug wires. Some numbskull could have pulled a wire or 2 or 3.... and broke the terminals off the wires or just broke them away and didnt even notice it since the boots at the spark plugs tend to hide this unless the terminal flat out just stays on the spark plug. Swap spark plug wires and see if the misfires move.

I bought new wires just now cause when I was trying to gently pull the boot off the terminal stayed behind. Pulled one off my other truck just to get her going for the drive

Also bought an ECT for shitts and giggles cause that's about the only sensor that hasn't been replaced by previous owner besides knock sensors, TPS, and IAC lol

 

[Nicolai8775]

Upstream O2 sensors ran between .1 and .9 but the voltage jumped all over the place. Is this normal?

 

[Nicolai8775]

Is it still misfiring after it warms up?

Have you watched or graphed the upstream O2 sensor activity? Should be between 0.1v and 0.9v and active.

Ok I figured it out that the sensors are supposed to be active, as in they are supposed to jump around.

At idle Bank one is currently sitting at .6volts, hasn't moved and I've been idling for about 10 minutes.

Bank 2 is bouncing around as it should between .1-.9 though it did show it hitting 0volts occasionally.

Edit: bank1 starting bouncing around while cruising.

LTFTs are about +9%- 12%.

Misfires occasionally on multiple cylinders, but it's nowhere near as bad as it was.

Getting balls for gas mileage though.

I didn't like the plugs or wires, there was a random AC Delco in cylinder 4 that definitely wasn't gapped at .040. replaced all that.

Noticed that that canister on top of the intake was extra wobbly and realized it was missing the little rubber piece to keep it tight. Fixed that.

Things seem much better, still having a hard start long crank issue occasionally.