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2002 chevy trailblazer_lt
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Discussion Starter · #1 ·
Thanks to all for the wealth of info in the forums.Problem: I have a cyl 1 misfire on 02 T.B.Pulled coil off and checked it to ground with screwdriver while running as i should,no spark.Bad coil,right?:undecided Problem is it is intermittant. Put it all back together and after driving to get my kids at school on 5 cyls it started working again.My question is: is it possible that the coil may be bad,even though it worked again after i saw no spark? I checked the connection at the coil and looks good.Where is the other end of it though so i can check that?$62 i really don't have at this moment to just throw at a coil that may not be bad, Santa needs to come you know.Thanks in advance. :confused: [email protected]
 

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2002 chevy trailblazer_ls
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I would probably swap #1 with one of the other coils and see if the misfire follows the swap. The other end of the wires attached to the coils connect to the PCM.
 

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2009 chevy
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6,431 Posts
Very possible. At least 3 things jump to mind,

Coils have wire inside, sometimes a broken wire can be open when the coil is hot and make contact when the coil cools (might even work in reverse)

You could have a broken wire, or a broken connector, or a loose connector where the primary wire connect to the coil.

But, the most likely cause is carbon burns on the rubber boot.

BTW, the old methods of checking for spark will burn out a new style coil. Better to stick a plug in that circuit, or use a tester (looks like a plug) for the job.
 

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2002 chevy trailblazer_lt
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Discussion Starter · #4 ·
Problem Solved(I think)

Thanks RayVoy and tblazed for the input. I did swap the coil with #2 and problem went to #2.Bought new coil and runs like a champ(so far!):D I sure wish i'd have found this site a long time ago.Thanks again!:thx
 

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FYI, our coils are not like the wire wound ones of old. Our coils are driven by solid state devices (transistors). The intermittent misfire is becoming more common on this site.
 

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Moved to the 4.2 engine forum.
 

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2009 chevy
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Well... Mine's not intermittant. #4.
Lots of good info on this site.
The #4 problem is usually traced to a water leak. The hood seal lets water into the engine compartment and it drips onto #4.
To ensure the problem is fixed, the hood seal needs to be replaced.
 

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2004 chevy trailblazer_lt
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62 Posts
The #4 problem is usually traced to a water leak. The hood seal lets water into the engine compartment and it drips onto #4.
To ensure the problem is fixed, the hood seal needs to be replaced.
Great! :thx
Does it usually kill the coil? Or is it going to be something that can be cleaned up with electrical contact cleaner?
If I can fix this thing without having to buy parts, it'll make me even happier.

The diagnosis makes sense. The TB sits outside, facing downhill. Perfect opportunity to let water run in past the seal.
 

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2004 chevy trailblazer_lt
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RayVoy,
You are da' man!
After I removed the #4 coil, poured all the water out of it, dried it out, and sopped up the water out of the spark plug hole, it runs like new. :D

Thanks!
 

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2002 chevy trailblazer_lt
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Same problem, but still not fixed

I also have the seemingl common problem of misfiring.
Started with mild miss - showed code P0304 (cylinder 4 misfire).

I replaced the plug in number 4 cylinder, then swapped the coil pack between 2 and 4. I detected no moisture problem nor residue in the number 4 spark plug cavity. I then got P0300 (multiple random misfires).

I swapped coil packs back to original positions, still code P0300.

Since I'd bought the plugs, I changed them all out. I could detect no carbon tracking on any of the coil boots. The only thing I did notice was that the coil packs seemed to have a bit more "spring" when I removed them. That is, they rose a bit when I loosed the bolt - much more than when I replaced them. The plugs are the same size, though they are NGK platinums rather than AC Delco. I was careful to ensure that the boots were landing on the plugs. Still very noticeable misfiring both at idle and under load and reading code P0300.

I pulled the throttle body and cleaned it with TB cleaner. I disconnected the battery overnight. Still getting a noticeable miss, but not as bad as before. Took it for a drive and it seemed to perform much better when under load, but still has a noticeable misfire at idle. Still getting P0300 code.

It seems to run fine at start up, but begins to miss after 30 seconds or so.

I've been reading everything I can find online about this issue, but so far, no luck. I'm not sure what to try next. All I can find at this point is possible bad coil pack(s), dirty injectors (though this seems unlikely at this point), and PCM re-flash. I

know that it is generally considered rude to post the same thing in more than one thread, but I do not have a high enough post count to start a new thread and the last thread seems to have gone cold.

Any suggestions would be greatly appreciated.

Thanks,
Eric
 

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2004 chevy trailblazer_lt
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Interesting about the misfires.
I don't know about the 4.2, but many GM engines DO NOT like platinum plugs. Specifically the ones that use DIS with "wasted spark" systems.
Since ours use coil on plug ignition, I'm suspecting that they are not subject to the same issues, but I thought it was worth mentioning.
 

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2005 chevy trailblazer_ls_ext
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Interesting about the misfires.
I don't know about the 4.2, but many GM engines DO NOT like platinum plugs. Specifically the ones that use DIS with "wasted spark" systems.
Since ours use coil on plug ignition, I'm suspecting that they are not subject to the same issues, but I thought it was worth mentioning.
My 05 EXT came from the factory with double platinum plugs. I replaced them with Autolite iridium plugs. Made a big difference. The original plugs had about 46K miles on them.
 

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2002 chevy trailblazer_lt
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My 05 EXT came from the factory with double platinum plugs. I replaced them with Autolite iridium plugs. Made a big difference. The original plugs had about 46K miles on them.
The factory AC Delco's are platinum. My local NAPA only had the NGK's as a replacement for the AC's.

After some more searching and reading, I'm going to buy a spark testing tool and check every one of the coil packs.
 
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