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Discussion Starter #1 (Edited)
I just bought a 2002 Envoy with the 4.2 engine.

The previuos owner has replaced 2 of the spark plugs and coils since they were the ones causing check engine codes and rough running.

His mechanic says if you replace a spark plug you are going to cause a problem as the coils are apparently fragile when disturbed. That how it was explained to me.

The truck has 133,500 miles and 4/6 are original plugs and coils.

I’m getting the truck ready for a trip and wondering if changing the plugs would be a good idea.

Right now everything is running great. So leave it alone?

I’ve never heard of causing a problem by changing plugs before.

Does anyone know what the mechanic is referring to or have an example of causing a misfire when changing plugs?

Thanks for the advice.

Coop.
 

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2004 gmc envoy_slt
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2,329 Posts
At 100K miles the plug should have been changed, and AC Delco 41-103 Iridiums should be used. There just needs to be care taken when removing coils when changing plugs.
 

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2008 gmc envoy_sle
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At 100K miles the plug should have been changed, and AC Delco 41-103 Iridiums should be used. There just needs to be care taken when removing coils when changing plugs.
i agree 100%

having 4 of 6 plugs original is odd, doesn't make sense in the least. 4 of 6 is past its due date.



usually its change them all or none.

and as for disturbing the fragile coils, that's also bunk. just need to take basic care in removal and re installation.
 

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2002 chevy trailblazer_ls
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20 Posts
I have never had a problem with coils going bad from changing plugs. I had coils go bad on their own though. I had the #1 coil replaced at Pep Boys years ago, and a month or so later it went bad again, and they had to replace it for free. I had the #3 go bad, and I replaced it. I went on a trip from Southern California to Oregon, and my alternator went out 60 miles from my sister's house in Oregon, and that killed my #4 coil, which I got replaced at the same time as the alternator. I have since replaced the #2, #5, and #6, and kept the old ones as spares. I changed my plugs at the same time I replaced those last 3 coils, and no problem with the other three coils that had already previously been replaced.
 

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2003 chevy trailblazer_lt
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4 Posts
FYI...at 222K miles I had my first coil failure, #2 cyl, no spark, 100% misfires, ran rough, SES light on. Replaced #2 coil. At 223K miles, transmission won't stay in overdrive. While I'm never hard on the truck and I figure with the mileage, my transmission must be wearing out the valve body. I take it to a local trans shop I used decades ago. I get a call saying the #1 cyl coil needed replacement. Trans is fine. Conclusion - a bad coil can yield different symptoms depending on if it's a misfire under load (my #1) or a complete failure (my #2). I've now ordered an AC Delco spare from Amazon. $50 for an AC Delco coil online. $360 for a shop to diagnose, replace and road test vehicle with 2 yr 24K warranty.
 

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2002 chevy trailblazer_ls
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hpgrisco damn $360 for diagnostics. You need to buy a good code reader to do more yourself. I bought a decent one that reads engine and ABS codes for only about $70.
 

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2003 chevy trailblazer_lt
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vansmack $360 was the full repair. This is the first time I ever took the vehicle for repair since I was sure it was the transmission. I felt stupid. It was a $360 lesson. I'm sharing my lesson here. I do all my other repairs/maint. Just not transmissions.
 

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2002 chevy trailblazer_ls
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Well that is better, because it sounded like that was for the diagnostics. I know some shops do charge a lot just to tell you what they think is wrong.
 

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2007 chevy trailblazer_ls
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340 Posts
I agree on dealerships being outrageous on diagnostics. My local dealer charges $95 and almost always just goes by the codes that were thrown. $95 for 2 minutes is robbery. When I go there because I just don't want the hassle of doing the work (I just let them replace a water pump in 4 hours) I go in wearing the hat I got from a used car dealer and tell them what I want. They are very pleasant and seem to know I won't fall for the framus oil needing flushing. Then I go sit in the waiting area and listen to the old ladies telling me how framus oil should need flushing every 6 months on a vehicle this expensive.

This dealer didn't change my plugs so they always recommend it for $225 so it sounds like your diagnostics were expensive. I ALWAYS read the codes (Walmart has a $20 reader that reads most) before going to a dealer even when I know exactly what the problem is.
 

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2005 chevy trailblazer_ls
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I've had 1 failed coil #3 and when they fail they can foul your spark plug, you want all plugs to have the same resistance so they should all be replaced after a coil fails imo. I've replaced my plugs way more than necessary so I would say the coils are not sensitive.
 
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