Persistent misfire under load only

[greiswig]

I’m on a trip, and running into a weird one. First CEL that came on was for fuel trim, bank 1. That’s happened before, very inconsistently. This trip, I’d clear it, and it would almost immediately come back. Until suddenly it didn’t.

Today, though, passing a vehicle I floored it and it immediately started misfiring. Plugged in the GAP tool again, and that fault wasn’t there. But three codes around misfiring were, only cylinder 5. (The opposite bank from the fuel trim Bank 1 issue, right?)

Once triggered, that misfire just continued on. At least with the GAP plugged in. As soon as you clear it, misfire goes away. And it stays away as long as I don’t floor it again.

Under normal load, at least, the MAF values and lambda values between banks seem to look good. And under those same conditions, the GAP doesn’t register even one misfire. Only if I floor it.

Any thoughts? Or any suggestions as to what parameters I should log in the GAP tool to help diagnose?

 

[powershift]

Is the fuel trim too lean or fat? Either way it could be the MAFs not reading right.

 

[greiswig]

Correction: the CODE indicates cylinder 5, but live values seem to point to #4. (Plugs were all replaced recently)

Any reason to think that either the code or the live values are reporting the wrong cylinder?

 

[greiswig]

Is the fuel trim too lean or fat? Either way it could be the MAFs not reading right.

Which fuel trims in there should I be reading? There are many of them, and some seem to just report NA.

 

[greiswig]

Fuel trim screen shots, but again, this code hasn't reappeared today. It’s the misfire I’m more worried about today.

 

[FRCI]

Personally, I'd be inclined to inspect the connection on the coil pack for the indicated misfiring cylinder. Secondly, I'd probably just replace that unit just to be sure.

I don't know the internal wiring forthese particular units, but certain designs use the mounting bolt for the ground. I don't beleive these do, but I would make sure its clean and torqued properly anyway.

 

[greiswig]

Personally, I'd be inclined to inspect the connection on the coil pack for the indicated misfiring cylinder. Secondly, I'd probably just replace that unit just to be sure.

I don't know the internal wiring forthese particular units, but certain designs use the mounting bolt for the ground. I don't beleive these do, but I would make sure its clean and torqued properly anyway.

Can do, but it’s really odd that it is so load-dependent. If it’s running below about 90% load, it never seems to misfire. And if I force it to stay in 8th gear and floor it from 50mph, it’s slow acceleration, but no misfire.

The other thing that may not be coincidence is that this is the first lengthy trip since changing the plugs, and the first time I’ve had to pass trucks at highway speed.