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Rough Idle - Almost solved!!

Ed Hall

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I bought a compression tester so that I could diagnose the rough idle condition that this engine has always had since buying the car 30K miles ago and replacing the plugs, wires and cleaning the EGR tubes. Problem is the fitting on the compression gauge that threads into each spark plug hole is too shallow to be able to threads down so I wasn’t able to do a compression check.

I then proceeded by removing each spark plug wire while the engine was idling and was able to isolate the problem to one cylinder. I then removed that plug and found that it looked clean. So just for grins, I lowered the gap on that spark plug from .60 to .50 and started the car. The idle is now very smooth. My question is why is it that this one cylinder needs less spark plug gap? Anything wrong with leaving one spark plug in with less gap than the rest?

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Just a little theory stuff..

The larger the plug gap, the stronger the spark needed.

So, if that plug wire has a break or unusually high resistance, then there may not be enough spark to jump .060.

Also with a wider gap, the spark will find "other places" to go.

Like a crack in the coil, a crack/split in the wire.

The plug itself may be getting bad. However I suspect you have swapped it with another cyl to check that out.

I had a miss in mine when I bought it, and a compression test revealed very low compression on one cyl. It was a burned exhaust valve.


2008 STS V8
2016 Colorado Z71
1970 Corvette LT-1 Coupe

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If you have a bad wire or cracked coil or bad cap, that would cause your problem and it would not necessarily be a valve causing a compression problem

Do you have an ohm meter? I would test that wire along with a few others, I think bbobyski suggested that. Also if you cap is cracked, has carbon tracks either between cylinders or between that cylinder and the center post, you will get a mis fire. Look at the cap closely especially the contacts, for cracks and carbon tracking. Check your coil for cracks

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Sometimes the center of the ignition wires burn out, and make intermittant or a weak connection, if you look into the boot on both ends you can see this, in addition, a resistence test with an ohm meter will uncover it. You can buy an inexpensive ohm meter at Radio Shack or Home Depot these days.

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