jmdale1984

iTrader: (6)

I installed Mac headers and y a couple weeks ago, and went for a dyno tune at G-Force the next day. The car seemed to run fine after the headers were installed, but maybe felt a little weak. I shrugged it off, assuming I had a bad A/F ratio, and that my tune would take care of it.

On the dyno, it started misfiring badly at about 3500 RPMs. We checked the plugs, and changed the wires, didn't help. G-Force said I must have a bad coil pack, but that they couldn't pinpoint which one.

So I drove it back home (about 3 hours) and it seemed fine. Let it sit for about a week, and now it's misfiring so bad I can hardly drive in 6th gear.

The motor is a LPE 346 NA, PP LS6 heads, LPE GT-11 cam, only about 10k on the motor, so I think we can rule out compression as an issue. It ran very strong before I swapped the headers. Noteably, the headers are leaking at the collectors. Damn ball type flanges. The car (along with the coils and most other electronics) has 213,500 miles on it.

It's showing no DTCs, and no flashing SES for a misfire.

I'm about to replace all the coils and coil wiring, to be sure. If one coil is gone, I'm sure the rest will follow soon at this mileage.

Is there anything else that could be causing this misfire? It's definitely getting worse, and doing so quickly, and I don't wanna drop $600 on coils and not fix the problem. It's so bad at this point, I'm beginning to think it's more than one coil.

Deeavi

iTrader: (16)

You didn't mention Plug wires?

95GreenZ28

iTrader: (70)

Sounds like maybe your header burnt through a wire

jmdale1984

iTrader: (6)

I changed the wires on the dyno... didn't stop it from banging out above 3500. That may be why it's acting so bad now though. They put LS7 wires on... and it seemed like they want to touch the headers.

van383

iTrader: (1)

check 21A fuse under hood! had a customer have the same issue.

jmdale1984

iTrader: (6)

Not sure which one is 21A... but I pulled and checked every single underhood fuse, they were all good. I wish it had been that simple.

Good news, found out that the #7 spark plug wire wasn't snapped in correctly... pushed it on, and that got rid of a lot of the missing (it's driveable in 6th now)... but it's still missing enough to be a PITA. Ordered coils/coil harnesses today from the dealer, should have them tomorrow.

jmdale1984

iTrader: (6)

I do have one other thought...

When installing the driver's side header, my o2 wire was just slightly too short to plug in, so I unbolted the o2 plug from the block on that side the get the extra inch or so I needed. I assumed that it wasn't bolted up there for grounding purposes or anything, so I didn't think it would matter. Is it important that it's bolted to the block in that location?

95GreenZ28

iTrader: (70)

Well there is a ground behind that bracket that does need to be hooked up.

jmdale1984

iTrader: (6)

But not the bracket on the O2 plug itself? If you're talking about the ground strap, that's hooked up correctly.

nickate

iTrader: (5)

I would bet big jing you have a cracked one or two...

Please follow this advice as I experienced this first hand!

Texas 02 Z28

iTrader: (14)

When I was misfiring bad, it ended up being water in my 02 sensor harness. Dried it out and it ran fine. It was running stupid rich and misfiring.

Also, someone local was misfiring number 5 and it ended up being that the coil pack harness on that side wasn't plugged in all the way.

The 02 sensor should be fine without being on the transmission. Maybe it's a little loose though? Or something's in it?


I say make sure you're getting spark at all the wires (put wire up to chassis and see if it sparks consistently), check all the harnesses that affect afr, and double check the spark plugs. If your coil packs are stock with 213k miles, my money's on that though. Hope you find it.

jmdale1984

iTrader: (6)

Pulled all eight, and they all look perfect.

It would throw a code for sure if the CPS wires melted on the header, right?

jmdale1984

iTrader: (6)

Oh well... I guess it's off to the local performance shop in the morning... see if they can pinpoint this. I think I'll have them do a crank learn, then maybe the PCM can tell us which cylinder(s) it is.

Relapse

iTrader: (4)

I took my ride out to the track one night and ran it 3 times, Afterwards it started backfiring repulsively for around a week to the extent that i was about to take it to a mechanic, Finally i found my problem after i had it scanned, Cylinder 8 was misfiring slightly but not enough to throw a major Code, Turns out my spark plug wire on 8 had come loose and was not fully connected, sure enough i just plugged that bitch back in and she runs perfect now. You might want to go over those wires one more time, just my 2 cents.

jmdale1984

iTrader: (6)

So I go to my local tuner and they scan it this morning. It showed zero misfires throughout our 30 minutes or so of testing. Under load it is pinging now, but the timing tables were changed in such a way that it can only pull about 2* of timing.

The plugs and wires have been checked and checked again.

But the zero misfires seemed too perfect to me, so I called Mike @ G-Force and asked him about it. Apparently, when he tuned it, he deleted the misfire tables. Why? I don't know. The car has a baby cam (215/231, .641/.644, 118 LSA, similar to the factory LS7 cam), and had been driven for 10k miles on a completely stock tune without popping a misfire code.

So I have no misfire tables to help diagnose this. I'm gonna try to find someone with a stock 2000 model computer to see if it can pinpoint this.

TUFAS4U

iTrader: (5)

i think i'd start writing the country song here purty quick

Texas 02 Z28

iTrader: (14)

That sucks that the tables were deleted, lol. Would've made it all a lot easier. Good luck



And jeez that's some monster lift. Looks interesting though. Don't see those specs everyday.

jmdale1984

iTrader: (6)

Swapped in 8 fresh coil packs, and it didn't make a bit of difference. I know it's not a fuel issue... time to start looking at the valvetrain. If my Patriot Golds are already shot at 10k, I may be rethinking my cam choice for my dd.

DopeFedZ

iTrader: (34)

Kinda sounds like what my car has been doing after the Kooks went on. I'm going to MTI today to see if they can look over the tune. I've replaced the plug wires and the plugs twice since the install about two weeks ago and the car has been running like crap ever since. It turns on fine, idles fine I can even rev it up and seems fine, but once I start driving and try going WOT the car seems like its running on 4 cylinders or something. Hopefully MTI can figure it out for me.

jmdale1984

iTrader: (6)

Yep... that sounds about right. Let me know what you find out please!!