Persistent misfire under load only

OK, here is the test, run with no load on the engine. You can see I futzed around trying to get it closer to 1500RPM before just leaving it at 1800 where it seemed to want to sit.

I'm not sure what it tells me, TBH:


Same graph with all the axes showing at left:

Combined short and long term fuel trim looks consistently positive at all RPMs tested, so it's running somewhat lean. Both banks are tracking the same, so the cause of the lean condition shouldn't be something bank/cylinder specific.

Looks like at idle combined fuel trims are sitting at about 10%, then at 1800 it climbs to combined 15-18%. Normally I'd think fuel delivery issue, but then it seems to drop again when you go up to 2800, which you would not expect with a fuel delivery issue - should continue to get worse as there is more stress on the pumps. Not sure what would cause this kind of trend - maybe MAFs. Maybe not really anything more than dirty injectors as the uptick at 1800 RPMs is only by about 5-7%.

In any case, I don't really think this level of fuel trim (10-18%) points to a cause for misfire or even the code. It's outside normal range and maybe something to address, but I'm still thinking the misfire is caused by something else, and might be causing a spike in the lean condition and causing the fuel trim control code.

I guess I'd swap coil packs to see if the misfire moves. If it stays the same, then do the same with the plugs. Check the condition/connection of the plug wires. You have an SCV6 right? (too bad we can't see the pulse-width modulation the individual fuel injectors with this model - I think you can on the V8).

You could also track demand and actual fuel rail pressures at idle and at elevated RPMs (no load), then driving conditions (though it's kind of hard to read them when the throttle is bouncing around much), just to make sure fuel delivery is not an issue under load. Maybe run MAFS at the same time.

OK, I’m home and starting to delve into this a bit more. To try to demystify the results I’m getting from the GAP tool, I decided to disconnect some of the ignition coils one at a time from their respective plugs and record live values in GAP to see if I could spot a pattern.

Reminder:
can’t.

greiswig said:

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?

Click to expand...

I’m using this as my pattern for cylinder numbering, so when I say I unplugged cylinder #1, it’s the passenger side front.

I can’t spot a good pattern. 1, 4, and 5 are the coils I’m able to access easily, which should give me some kind of a pattern even if the GAP tool is numbering according to the V-8 standard, I’d think:

Unplug cylinder #1, and the GAP records misfires on #1
Unplug cylinder #4, and the GAP records misfires on #2
Unplug cylinder #5, and the GAP records misfires on #4

Oh, and NO trouble codes at all. Maybe because not up to operating temps, but it surprised me.

Wait…what? Anybody have a good guess?

Is the V8 cylinder numbering odd/even by bank? So driver side bank would be 2,4,6,8 and passenger side would be 1,3,5,7?