Misfire on cylinder 7 and 8 - need advice

[2000_SS]

I've got a 2000 Z28 with ported heads, cam, and a FAST intake on the dyno. Deatschwerks 42# flowmatched injectors, 4L60 with 2800 stall. It's misfiring on cylinders 7 and 8. I can feel it plainly while it's running. Misfire detection is disabled at idle and mid range, not that it matters since its truly misfiring. So far, I have done the following:

I did a crank relearn. minimal improvement, virtually no change.
I've swapped the coils from cylinders 7 & 8 to cylinders 1 & 2. no change.
I've swapped injectors from cylinders 7 & 8 to cylinders 1 & 2. no change.
I've removed plug wires and verified they click twice on the coil and positively click on the plug. no change
Installed fresh NGK TR6 spark plugs, no change.

Any ideas what else could be causing this condition?

[2000_SS]

attached log and tune, MAF open loop. Wideband is in the passenger side and cylinder 8 has historically been misfiring the most, so i don't trust my readings at all. I've tried doing a little VE tuning but i don't consider the results valid and they had no impact on the misfire condition regardless.

[kingtal0n]

what kind of misfires. what does the wideband do when it misfires. A true misfire will show a lean spike for an instant, you feel it 'pop' then the transient delay (0.2-0.5 seconds usually) then the wideband shows it.

If you get misfires but no lean pops/reads then it isn't a failure to combust misfire.

[2000_SS]

there's no lean pop but the engine isn't running smoothly. no other cylinders are misfiring, except a very very random few here and there that i would call typical of an engine with these mods. The wideband doesn't show any spikes or dips, greater than 1-2%, that correlate with the misfire events but neither the MAF nor VE are dialed in yet, so it's hard to say it isn't fueling related, but no changes to the MAF or VE seem to reduce the misfires. I would think if the misfires were "in the tune" then it wouldn't be just on cylinders 7 & 8.

[2000_SS]

could the torque converter and/or flexplate have issues that could cause misfires in the back 2 cylinders?

[kingtal0n]

Why do you think that the misfires are on 7 and 8

If its specifically 7 and 8 for sure, and it isnt in the wiring, injectors, tune, fuel rail/supply to those cylinders, then the only thing really left is the leakdown (piston ring seal) / compression.

Engines only need
fuel air spark

recap:
If you swapped plugs, injectors, Coils. That leaves wiring to those entities and the computer signal to them to still rule out.
Then the tune itself.
Finally engine compression, or should I say firstly (the first thing you do when a particular cylinder causes a problem is compression test). The fact its the rear two (if you are sure) is kind of odd, like the injector wiring for those two was mixed up or something for one was put in the other kind of thing.Or the intake system somehow deters air from entering them or something weird because they are in the back, vacuum leak/seal around the back of the motor kind of things.

[2000_SS]

Why do you think that the misfires are on 7 and 8

..... because that's what my scanner shows

[kingtal0n]

Well that doens't mean anything really. I dont know how accurate the scanner is but you can't depend on it. No sign of lean misfire on the wideband correlates to there being no misfires. Misfire codes on the other hand can be due to changes in the rate of change of the flywheel or something. 1872 the firing order for 7 and 8 are right next to each other right? so if the rate of change is slow through 78 the scanner might throw a misfire code. When in fact it isn't a misfire, its just a failure to get power from the combustion event, which also points to low compression, poor output from those cylinders during the time of firing. When you drive the vehicle does it seem to run normally? does it only seem to misfire at idle? That points to plugs also.

[2000_SS]

Makes sense. I guess you can't definitively prove which cylinder(s) is misfiring but it is obvious that at least 1 if not 2 or 3 aren't hitting right. What bothers me is the hand ported and assembled heads by some guy I've never heard of and the fact that this is the owner's first LS engine build after a lifetime of sbc stuff.

I told him i wasn't comfortable continuing the tuning process. The more i worked on it, it started to miss a little harder and eventually started developing a subtle pop in the exhaust around 2500rpm during open loop MAF tuning. I really feel like it's a mechanical issue at this point and advised him to do a compression and leakdown on it. Wiring all seems to check out good. I've gone all over the tune and the miss never improves or worsens with any changes besides, seemingly, runtime on the engine itself.

[2000_SS]

It's misfiring all over, not just at idle. The cam almost masks it at idle but it still shows up in the logs. It gets worse with load/rpm.

[kingtal0n]

alright so follow sop
1 compression test
2 inspections as necessary
3 boost/crankcase pressure test
4 timing test (if applicable- maybe not in this instance)

5 diagnostics (no order):
-Turn each spark/ cylinder off one at a time in the scanner to audibly detect differences
-Do the same but for injectors
-use the scanner to hold timing and fuel steady
-check all pressures (fuel, oil, etc)
-change spark plugs, wires / adjust and inspect gaps
-try changing key parts (cas, cps, all the position sensors)
-you said something about a flywheel or flexplate, Its worth looking into as out of balance parts might be triggering something in the computer
-try disabling codes or features related to the misfires events (guess)
-add your favorite diag here (others can chime in this is just to get you started)


I was helping a friend with a 4.8L mercedes engine v8 once and it felt like it had a misfire. I told him before we put the wideband on it to tune it, we needed to figure out where/why the misfire was because it felt like it could be mechanical or fuel system related. So we spent months pulling apart the engine a little at a time (those engines are hard to access) had to unbolt all kinds of stuff just to change the plugs etc... Finally we ran out of ideas. Im like lets just put the wideband on it and see.
Well the wideband sure enough shows no misfire. No lean spike/pop, no indication whatsoever, just a smooth running stream on the gauge. I'll be 'dammmed' it was the torque converter the whole time, just out of balance or something. Torque converter is the heart of a build and he used a cheapo-rebuilt unit that he never told me about till that moment I was like "must be something in the drivetrain spinning with the engine for it to feel this way with the engine running fine"
So anyways, keep an open mind I guess is what I am saying

[[email protected]]

Were you able to figure this out? my truck is doing the exact same thing.

[Joe_B_1973]

Compression test to isolate a problem cylinder, then leak down that cylinder. Also a new set of plugs and driving it under load for a couple of pulls should show up on the plugs. Quality plug wires are a must if dealing with a misfire issue and the wires are cheap store brand, or older than 2 years.