Thread: Stock l67 misfire help

I recently had a misfire and knock in the bottom end of my 2000 gtp, i replaced it with a 95,000 mile engine from junkyard that is stock with no mods, sounds great but still has a misfire on the same cylinder as the other did, drive it easy and stay out of boost and under half throttle its smooth with no misfires on scanner, and this is with the sensors, coils and everything that came on the jy engine, i have swapped coils, 3 different icms, swapped injectors from known good cylinder, plugs, wires, miss will not move, even tried smaller gap on that one plug and it did not help, plugs are ac delco iridium 41-101, no light on and no codes, car runs perfect with sc belt off, what do i do next? Pcm problem?

Which cylinder is misfiring?

I'd check the wiring to the injector

#2 and it looked ok around the plug to where it goes into the harness, its just strange that a bad wire would only miss when it has boost

unplug that injector then with a test light see if the plug flashes with the car running. or you can stick a side marker bulb in the plug from the headlights.

if you have a open cone filter and the pcm just laying there look for a rubbed through wire leading to the pcm. seen that one cause a misfire a time or two already.

It does flash with noid light looks same as others and runs smooth as can be until boost but i have no way off watching the light while in the car driving, pcm is in the stock box but i havent looked at it real close

you got a gremlin for sure. id think its a issue somewhere with the cars old harness, not the engine.

Yea its no fun i have only owned the car for a little over a month now

If you wiggle the wiring harness does anything change the next time you drive it?

After all the times i have messed with this thing unpluging and pluging things back in and moving the harness changing the engine it hasnt ran any different, i even had the pcm harness moved around because i had the airbox out, all i have gained is i dont have a rod knocking now lol

Reason I bring up the harness is on some cars as they age, the metal holder at the driverside dogbone can dig into the harness etc. Seems that when you give it boost..aka pull motor back toward firewall..that's when your issue happens. That one stretch of harness is usually the only thing that would be moved.

I will have to pull the tape off and check it out

If that metal holder is still in place..you should be able to get a decent idea w/o pulling the loom/tape off. If you see it digging in etc, that'll be your sign.

Took the metal peice off and all looks ok there

Hmmm ok. You said it's #2 right?

Thinking out loud here, #2 is the furthest from the intake, it's the closest to the vacuum port on the LIM. It's one of the two cylinders that would likely see a vacuum leak if you used garlock gaskest for the SC. Being in the back, it's possible for it's wire to lay on the metal and arch out. It's not typical for it to touch the upstream O2 wiring and get interference or cause it.

Yea its #2, i havent changed any gaskets but plan to because it still has the plastic intake ones. I did spray starting fluid around with no change, and the o2 wire is zip tied up to the bracket, also tried taking the metal boot off the wire

I know it would be way off tune but think i could throw the good pcm from my 01 gt non supercharged in it just to see if misfire under little boost goes away. Im not sure if it would even run

Cruising nice and easy ltft is pegged at 16.4, change fuel filter it was hard to blow thru but still no difference

If your ltfts are pegged I would say you have a vacuum leak.

Idle ltft stays right around 0, i cant find any vac leaks