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2003 gmc envoy_slt
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Discussion Starter · #1 ·
I have an 03' Envoy I6 w/109,000. Decided to change the spark plugs since i'm at +100,000. Began rough idling/misfiring (P0300 on generic scan tool).

I used OEM AC Delco Iridium's 41-103 as suggested by store and also saw on this site to stick w/ AC Delcos or NGK.

I have removed and replaced everything twice to make sure i'm getting connection. still same problem.

I replaced original spark plugs...same p0300 code/rough idling. Visual inspection on wires to coils look good...no individual cylinders have come up on code..just random misfire code.

Also disconnected battery wires as someone suggested to see if pcm would reset/recognize the new plugs...no change.

Any suggenstins/Advice will be much appreciated. Thanks.
 

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2003 chevy trailblazer_ls
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5,269 Posts
id say go back and make sure you didnt pull any electrical wiris a little too much...
 

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2003 gmc envoy_slt
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15 Posts
Discussion Starter · #3 ·
Thanks for fast reply..i'll go do that....should i just push in on the wires? or disconnect and reconnect? they seem to be a pain to pull out?
 

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2009 chevy
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6,513 Posts
The P0300 is a "general" mis-fire. All it means, is that the PCM has detected a mis-fire (or more), but has not nailed the cylinder that is mis-firing. As the mis-fire is tied to a cylinder, the codes P0301 thru P0306 will begin to show.
If yours hasn't lite one (or more) of the P0301 - 06 codes, it may mean there is more than one cylinder causing the problem.

The only difficult part of changing the plugs (other than getting the coil bolts out) is making sure the high voltage boot is properly installed when the coil is installed.

The other problem, is that there might be a carbon track (indicates shorting to ground) on the inside (or even outside) of the boot. Basically, look for anything that might cause the high voltage to arc to ground.

And, welcome to the site.
 

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2003 chevy trailblazer_ltz
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69 Posts
If the problem was after spark plugs changed.

1. The logical is that two or more misfiring could be present, please review again with a lot of calm, the carbon tracks on the boots, connectors, wires, I suggest to review one by one outside spark plug with each coil connected to the correspondig connector and see if you can see a blue spark, for that, you must put one old spark plug in the cilinder you are checking, and the coil with the connector connected and the spark plug to see the spark....You must be carefully.....but the most important is one by one in the order 1, 2,3 ,4,5,6 so, you can check if one connector could be the cause, coil or spark plug, or the PCM drivers.:tiphat

2, if the above procedure is ok, you must check, Clugged injectors, Too high fuel pump presssure.:duh:

3. If not, you must check the CKP system learning with a original scanner:ugh:

Excuseme for my english.

Cesar Gallo
Medellin, Colombia
 

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2003 gmc envoy_slt
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15 Posts
Discussion Starter · #6 ·
Thanks for the great info. Test driving seems to point to possible clogged injector(s). I remember leaving a vacuum port open beneath the resonator when i first test drove it after initially installing the new plugs. I'm thinking maybe dirt/debris got in during the short 1/4-1/2 mile test drive or by some other means.

I put some over the counter fuel injector cleaner in the tank and drove for about 60 miles...it is definitely getting better as i drive but still idles rough at times, though not as bad as before. Also, the CEL does not come one, but my obd scanner still picks it up.

I'm thinking of cleaning the throttle body as i've come across on this site in other posts regarding clogged fuel injectors. I've driven about 1/4 tank and getting better so i will wait at least til the tank is empty to re-evaluate and hope the rough idling/misfire goes away. Thanks for your help guys.

Alex
 

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2003 gmc envoy_slt
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Discussion Starter · #7 ·
Solved

Should of done this a lot sooner, but I wanted to close the loop on this as i never made it back to report on the outcome.

My 03 envoy was running/idling fine, but hadn't replaced the plugs, so i did and started idling rough and mis firing (PO300's codes) almost immidiately, cylinders 3&4 were misfirring. So i tried cleaning TB, helped a bit, but problem persisted.

Bought a used coil pack to use for diagnosing, replaced with cylinders 3, then 4 but still missing. Also swapped out coil packs from other non-misfiring cylinders, no change.

After a month of trying everything i knew and retracing my steps, I had an epiphany and realized it must be the spark plugs. So i go buy a new one and swap it for cylinder 3....no more misfiring.

I never thought spark plugs would be bad..probably just mis gapped, but never gave it a thought since they supposed to come pregapped. So i yanked out the plugs and exchanged them for another set, checked the gap this time, seemed ok...problem solved, been idling great ever since.

Lesson learned..if you start having rough idle and/or misfires soon after changing spark plugs, check gaps or swap out.

Hope this helps someone.
 
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