Misfiring: 95 Z26 w/ 3400

[heavywoody]

Getting my car ready for Bfest is becoming a huge chore. I have developed a misfire from my engine, and I'm not exactly sure where to look. I've already checked plug gap (0.060), tested the ICM (good; my spare one is good too), tested each coil, made sure wires are going to the proper locations, and checked fuel PSI, which is at 42 PSI.

So yesterday I was bleeding the air from my coolant, and shut the car off after the 2nd fan cycle. Went to start it back up, and it was misfiring. Odd; so I checked the wires to make sure they were seated properly... good. I ended up changing the wires due to me ripping a boot when I took it off #4 cylinder. I checked the new wires prior to putting them on, all checked out ok. Checked resistance across the coils, all are between 5-6k ohms.

Last night (the engine was still pretty warm), I got a spark tester, and noticed that cylinders #4 and #5 had intermitten spark. I double-checked the wires and plugs, all is well except the plugs are solid black from running so rich. After racking my brain for a couple hours, and going over the service manual, I decided to go to the bar.

What's funny is that I started the car up this afternoon, and it was running good. After I put the new O2 sensor in, the misfire reappeared. I swapped in a new O2 sensor today, and that didn't help a bit. What's funny is that I started the car up this afternoon, and it was running good. After I put the new O2 sensor in, the misfire reappeared.

The fact that I have intermitten spark on cylinders #4 and #5 is pretty odd. I am going to try swapping my 3/6 coil with either 4/1 or 5/2 to see if the intermitten spark is a coil problem that the resistance check isn't seeing; I'll have to wait until Monday night to do that though since I need it to be dark so I can see the spark tester light.

Any other ideas? I know that the valvetrain is working on cylinder #4... it was suggested to me to check compression to see if I had a stuck valve or something of that nature. I will check for spark by grounding a plug w/ each wire probably on Sunday, but the spark tester pretty much took care of figuring that out. Another thought I had was possibly a plugged fuel injector, but that wouldn't explain why the plugs on those cylinders are coated black w/ rich deposits.

If I could take the car to a dealership to have them hookup a scanner, this would probably be a bit easier to figure out.

[geoffinbc]

Have you replaced you spark plugs yet? 90% of drivability problems are due to poor spark plugs. This includes poor quality plugs so throw your Bosch plugs in the trash if you are using them.

[heavywoody]

They are AC Delco plugs, #6's I believe. I don't remember the exact plug that is in there, but they are brand new (aka no miles, new when I reassembled the engine after replacing every gasket). I suppose it's possible that the insulator is cracked, but it just doesn't seem right. It's not like I'm a rookie at changing plugs or doing my own work (even though I'm a "newb" on this forum, according to my post count anyways).

[heavywoody]

Here's another couple things I forgot to mention.

On cylinders 3 and 6, I have a nice, strong spark. Very bright on the tester.
On cylinders 1 and 2, I have a weak spark... not as bright as 3 and 6, but steady.
On cylinders 4 and 5, I have this intermitten spark. The tester only lights up sporadically, with no apparent pattern.

Is it possible for half of a coil pack to go bad? I would think that if the coil was bad, that both associated cylinders would have bad spark... then again, since the 1/4 and the 2/5 have the weak/intermitten spark issue, maybe that's the solution? I'll try to swap my good 3/6 coil pack to another spot, and see if that makes a difference, and if the strong spark condition follows to those cylinders.

Another thought I had was this. If the plug wires weren't put on the correct coil/cylinder order initially, and the car started/ran/missed/backfired, would that damage the plugs?

[heavywoody]

Update: Replaced all coil packs, after swapping my strong coil to a different location (which did nothing). Didn't help.

Replaced all spark plugs. Found out that for some reason, I had AC Delco #14's in. I'll have to blame my room mate for that, as he picked up plugs for me at AZ. Put AC Delco Rapidfire #6's in, started it up, and it ran nice and smooth. Checked spark, all seemed well (still have stronger spark on #3 and #6 cylinders, but the intermitten spark disappeared on #4 and #5). Everything sounded fine, it revved normal, etc. Ran it for a while to warm up.

Shut the car off, started it up again, and what do you know... it was MISSING again! This is really starting to piss me off. I'm going to let the car cool down, and check the wiring from the rear crank sensor. Then I'll check all my grounds, and maybe even replace the rear crank sensor (again; I replaced that sensor on my old 3100 in November of 04, and reused it on the 3400).

Next step after that if THAT doesn't work will be pulling the plenum and checking out my injector wiring and the injectors themselves to make sure they are firing. Anyone know what the injectors are supposed to OHM out at?

[Rhedalert]

11.6-12.6 IIRC

[IsaacHayes]

Well, coils, ICM, and CPS about covers the ignition system. Unless there is a bad wire going to the ICM or something. I've had problems before, but it turns out each time to be the CPS, and not the ICM. But usualy it's not 2 cyls, but all of them. The last time being I had no spark at all. Really sucked.

[SappySE107]

There is a guy on mymonte with pretty much teh same issue (rough on a restart when its warm). I don't think he has found the solution for it either. Intermittent problems suck, but since it does it when warm and not cold and not warm/hot while still driving, its even tougher. Id check your fuel pressure after you turn the key off. If it bleeds down quickly, you might have a leaking injector or 2 which will cause it to puddle on a few cylinders and run rough on a restart. That wouldn't have anything to do with weak spark though.

Did you do the compression check? I don't think thats gonna lead to anything but who knows. It could be a ground wire too. So many possibilities and you have already checked the ICM, coils, wires, and plugs. Too bad its 95 as I don't think you have any real scanning options other than a tech 1.

[EspanolaGuitarista]

Running rich would not cause spark misfire since fuel decreases resistance between the gaps. Lean misfires go the other way. I have encountered this problem and it is alot of head scratching. Coil packs are one thing but the ignition module that they sit on is another. Call the dealer and ask if they have a ignition module tester. If it was a cam sensor problem you would most likely have misfires at random unless there is a physical defect in the sensor ring. That is my .02 cents.

[geldartb]

i had this same problem on my 95 z26. maybe its a 95 things. i never found a solution. spent the whole summer last year chasing it.

[tractorman]

I dealt with a similar problem on my 01 lumina. After checking and rechecking I eventually found a bad plug... only just bad enough to miss when the engine was cold. But you've eliminated that option.

One thing that helped me find the problem and might help you is cutting out individual cylinders. This tells you more than a spark-tester. Put a paper clip in each coil boot so that it sticks up, clip a jumper wire to a good ground, hold it with a thick glove and cut out cylinders. Knowing exactly which cylinder is missing can go a long way toward solving the problem.

[Superdave]

which injectors are you using? If it's the Multec 2's give them all a good smack with a screwdriver... common problem