misfire

My 04 is running rough. No code from scan, but it is misfiring. Checked two plugs, they seem good. 60,000k. Any ideas?
I hate to have to take it to the shop, but I may have to.
Cruise control also stopped working as well as the instant average mpg. All else seems to be working.

Thanks for any help.

Darrell, is it a random or specific cylinder misfire? I've seen several cases now of a misfire of cyl #4. (the 4th one back). Rain water leaks from the rubber cowl seal, pours onto the cam cover and gets into the #4 plug tower, shorting out the coil/plug. Might want to check that.

Joel

I will check #4. I checked 2 & 3 this afternoon. I do not know if it is random or not. I do not have a code reader, Auto Zone scanned it for me. Thank you for your thoughts. If it is shorted, I assume the coil will need to be replaced or just the plug?

If the coil got water logged enough, it will probably be shot. Some got away with just a good cleaning & light coat of silicone/dielectric grease.

Joel

The problem is #5 cylinder. I changed the spark plugs with AC Delco OEM plugs. Still no fire. How can I check the Coil? Does anyone know what resistance reading I should get and what leads to check.

Swapped the coil from #5 to #2, now 2 is not firing. Must be the coil.

Good work, this is the basic diagnostics I preach.

Thanks again for your help. I am going to pick up a new coil in a few hours. Sure hope this works. It seems as if it will.

Good work Darrell! Did the inop coil have signs of water damage/corrosion? I'm beginning to think keeping a spare on hand is not a bad idea. Spares can be had on eBay for decent prices at times. Replacements are probably pricey buggers at a dealer parts counter.:banghead: Keep in mind, 2002-2005 coils are interchangeable. 2006+ are of a different design, as is the cam cover, etc...

Joel

Yes, the fourth and fifth coils (front to back) did show signs of damage. That must be what caused #5 to fail. Thanks for your help. I am back on the road again. $65 at Auto Zone.

Good work Darrell! Did the inop coil have signs of water damage/corrosion? I'm beginning to think keeping a spare on hand is not a bad idea. Spares can be had on eBay for decent prices at times. Replacements are probably pricey buggers at a dealer parts counter.:banghead: Keep in mind, 2002-2005 coils are interchangeable. 2006+ are of a different design, as is the cam cover, etc...

Joel
Maybe there is a way to use silicon to better seal these two coil packs?

Maybe there is a way to use silicon to better seal these two coil packs?

Good thinking. Silicone sealer is what I recommend to owners that accidentally snap one of those POS coil hold-down bolts. The damn things thread way down into the cylinder head, so if you break one, you are SOL. Best to leave it & just secure the coil to the cam cover with silicone. 2006+ coils are such that this problem is no longer.

Joel

I think I am going to put a permatex seal around each coil pack. That should keep any water / moisture from getting in there. What harm could it do? It appears the provided gaskets do not seal water tight. The 4th and 5th (front to back, not firing order) coils both showed signs of moisture. The other four did not.

I never did fully understand the water leakage thing with these coils. The egg shaped recesses in the cam cover have kind of a raised flange. I don't undertand how water gets up/over the raised flanges? I'm wondering if water leaks down through the coils somehow, or maybe water gets through the hold-down bolt hole?

Joel

The signs of water / moisture damage were present on the brass part of the coil pack.

Could this account for random misfires also? Been experiencing this for some time now.

There is tsb on replacing the hood seal. I had a intermitant misfire. It never missed during rain. Replaced coil and plug (under warranty)

Rob

I had a similar problem and the stealer diagnosed it as carbon trails on the boots of the plugwires and replaced 2 boots not the wires for $50 a piece??!!

I had a similar problem and the stealer diagnosed it as carbon trails on the boots of the plugwires and replaced 2 boots not the wires for $50 a piece??!!

The coil-over assemblies (all 6) are an integrated coil and plug boot. No way to seperate that I am aware.

Joel

When I get home tonight I will check the ticket out again.

There are no wires on a 6 cly. Trailblazer. There are 6, one piece coil packs, that sit on top of each spark plug.

boot is $3 at checkers for coil pak.

Welcome to AF, we know you mean well here, but look at the dates on the threads first, this is dead.