Stumped (intermitant misfire issue)

[GhostRunner]

So twice now in the past month I've gotten a check engine light with the random/multiple missfire detected code.

Both times it was after I had been driving the car for extended periods (60+miles) out past town. Parumph once and out towards Snow cap, or snow peak or whatever it is north of town on 95.

The next day the car runs fine again and the check engine light goes off after it runs through all the diagnostics.

While I was at work after the second time, I decided to hook up to my shop's OBDII scanner, which also has the nifty real time data ability..

What I got was cylinders 3 and 6 misfiring above 2000 rpm, right up to red line. Intermittant, but still there, the rest of the cylinders were quiet.

So, fine, coil pack right? Ordered a new coil and put it on, started the car. Same thing. Let the car cool down (125F water temp) and tried it again, thinking it might be the engine heat affecting the plugs/wires. Nope, same thing. So I sent back the coil and put the old one back on.

Then I decide to pull the wires off and check out the plugs. I figured the previous owner had changed their plugs out since they had done a few mods to the engine, and I wasn't wrong. Bosch Platinum 4 plugs.. The gap was off between the electrodes, but the platinum was still on the plug, and the plugs themselves looked fine. I only looked at the number 3 and 6 plugs though.

Then I inspected the wires.. Made sure they were plugged firmly into the coil packs on all the cylinders, then took a good look into the wires on the plug side, and everything looked ok. Inspected the length of wire and found a crease in the #3 wire, but it wasn't cutting into the silicone, so I think they're okay.. Nothing burnt or bad looking.

PCV? Checked it. It's fine. The plugs themselves looked to be about 20K old or so, (The car has exactly 27,500 miles on it right now)

So what gives? I don't like the Bosch 4 platinums, and I really want to just give this car a tune up, replace the aftermarket wires and plugs with stock equipment and go from there. But in my experience that'll be a 50% chance of fixing the problem. I'm certainly hoping I'll be proven wrong, and theres's still things I want to check out. (saw that thread by mustangeater with the link to insulating plug wires and I'll be doing that with the new set of wires, seems like a really good idea to be doing out here in Las Vegas, where it's about oh 110F outside.)

I'll also add, when I had the coil pack off, I checked out where the coils attach to the plate, and nothing looked corroded, and even though it had rained the day the second missfire happened (It was last Saturday) there was no moisture under or around the coil packs.

The engine mods on the car that I know of are the free cold air mod, a descreened MAF which I want to fix quickly, accel 8mm wires, flowmaster muffler, K&N filter, deribbed air lid, I think there's been some PCM work done because there is no top speed limiter, and of course the bosch 4 plugs. That's all I can think of off the top of my head, and I'll list more if I can remember them.

Anyway, any suggestions would be appreciated.. I'm seriously thinking of dropping off the car at a local GM specialist who's absolutely a brain when it comes to this kind of stuff, but they're expensive and I *really* hate letting other people touch my car if I don't have to.

Also I hope this hasn't been specifically addressed before, I've searched through all the threads listing missfire or coil packs.. If so I appologise and I'll dig further into the forums here.

I'd also like to add that this sucks since I've only owned the car about 3 months now and have put just under 6k on it. >.>

I'll shut up now, since this is really long and I'm sure I've wasted someones time reading this, thank you for any help you folks'll give me.

[pontiac nino]

HI

Have you thought the possibility of you're problem being the O2 sensors? [img]smile.gif[/img]

[Speed Freak]

Its your cat starting to go .To mutch back preashure causing random misfire under heavy load .
If left alone it will get worse.
The dealership told me to keep driving it till it threw a code that would naro it down.MORONS!!!
With my hot tune the back preashure snaped a bolt off clean in my manafold.They had to pull the top end to get clearence to retap for a new bolt and tried to charge me $700,I had a few choice words with the service man then the manager then i had to call my Unc who playes golf with the farther of the bastard tha ownes the dealership .Free fix. Normaly at dealer just tell them your cat is goin.
free till 70k unless it causes something else to break.
My car was eating factory cats about evert 10-15k mi with the hot tune.
They must have loved me .I was on my 3rd free factory cat($850a pop) before i got my high flow ($50 ebay). no problems since.
I also replaced all my coilpacks wires and plugs before i found out it was the cat.
By the way we bought a 99c5,99z28,99youcon denali and my 2000 camaro from theas bastards well over 100grand you would think they would be more helpfull.

[Rob Bell]

Try checking the ignition module. Does your tach jump around when these misfires are happening? My previous car (cavalier) had a misfire and it ended up being the ignition module. It would be ok until it got warmed up, and then would misfire randomly. If I'd hit the gas it would go nuts, especially above 3000 RPM, and the tach would bounce all over. If I was sitting at idle, it would normally be at 750 RPM, and you'd see the needle flick down once in a while when it would misfire. This was all intermittent for a while, and gradually got worse.

[Ian]

i was gonna say the ICM also

[GhostRunner]

The engine doesn't change revs at all abmormally. Honestly I haven't felt the misfires at all, I'm just going on what I read from the computer scanners. Both times the ses light has come on, the entire time I drove the car there wasn't a change in drivability or in the way the engine sounded and acted..

O2 sensors.. I haven't checked them out.. Bank 1 O2 sensor voltage was all over the place. going from .10 to .650 then to .440, down to .01... you get the idea, while Bank 2 was steady. AF ratio was at 14.7 the whole time.

Cat: The cat temp is rock steady at 570 degrees F. It hasn't gotten any worse since the day I bought it. Though I'm not sure if that's the normal temp for the cat, abnormal backpressure would cause the cat to overheat.. I'll certainly look into that though since the thought never crossed my mind with this low a mileage on the car.

More stuff to check out on the car now, thanks guys.

[MysticCamaro97]

the exact same thing happened to my car. i went through hell tryin to find out what was wrong. just clean the MAF and it should run fine. just spray some carb cleaner or somethin on it.

[Jason McCallister]

Bank 1 O2 sensor voltage was all over the place. <snip> while Bank 2 was steady

FYI: Both bank 1 (B1S1) and bank 2 (B2S1) O2 sensors should be bouncing all over the place unless WOT.

I would replace the plug wires and plugs before taking it somewhere. The wire(s) could be broken internally. I've had two Taylor wires fail internally and you couldn't tell visually but resistance checks proved otherwise. Besides, it's cheap insurance and if it's not the wires then you have an extra set! ;)

[hou-tex-six]

Hmm, I had a similar problem. Only difference is I knew my cat was bad because it rattled like crazy. I kept throwing the light and it would go away, I assumed it was because of the bad cat. I got it scanned just to see, and the code was random misfire. I removed the wires and put some electric grease on them just to be sure that wasnt it (all plugs/wires looked good). It still didnt go away, then I replaced the Cat, even though I didnt think the two were related. After that all codes went away. So maybe it is the Cat... Good luck figuring it out.

Cp

[smcruikshank]

ignition module could be it, as said before. It's underneath your coil packs and when that goes it will throw a multiple cylinder misfire. If you take it to Advanced or Autozone they can test it for you, but those suckers are like $150

[F1GT]

I agree with Jason up there. I would change the wires and plugs back to stock. Cheap too.

[GhostRunner]

I'll be giving the car a tuneup on sunday, going to buy the plugs and wires tomorrow through work.

If nothing changes, tuesday (only day off) I'm going to the Chevy dealership to see about a diagnostic and possibly replacing the cat.

I'm not a mechanic enough to deal with all this "new" computer techie crap.. If this was my Chevelle I'd have this problem whooped weeks ago.

[GhostRunner]

Update:

Camaro's at the chevy dealership.

I Swapped out plugs and wires for OE AC Delco parts at work today. (Only took me about 15 minutes, btw.)

Started car: Cat goes *FART* whole shop smells like rotten eggs. Starts backfiring and missing horribly.

Turn off car, go into the employee bathroom to change my underwear, come back out and try starting the car again, same thing. Laughing at this point. Check to make sure plugs and wires are tight and nothing was put on wrong. Everythings okay... Cat is smoking hot. Guess it was time for it to go. >.>

Limp it the 5 miles to Bill Heard Chevrolet. They can't look at it until monday, so it's staying there until then.

Service guy tells me the severe misfire wouldn't be coming from a clogged cat. I said [img]graemlins/bs.gif[/img] and just check the cat first. If it's bad, replace it, and don't do another thing until you get ahold of me.

Anyway, what a way to start the weekend. [img]tongue.gif[/img]

[smcruikshank]

Well hopefully it will be a cheap and quick fix!

[kramer2125]

Ghostrunner,

Had similar problems with my 1999 Bird, i was very close to replacing the ignition coils (33 dollars a piece ((ac delco)) on rockauto.com) when i fealt that i was making the problem worse with every hour i worked on her. I gave her to the dealership (who wanted 350 dollars for new plug wires installed) and they told me i needed the coil pack!!!!! I concur with the "bad catalytic converter" diagnosis, and most places replace your 02 sensors during a tune up. Hope she's up and running soon. Cheers!