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I have an 04 impala ss, I'm getting a misfire on cylinder number 4. runs rough, MIL light will come on steady if driven slowly or around town, but if I drive it hard, the miss goes away temporally and it will drive low speeds fine for a while. (I have a scanner and can see the missfires when they happen.) I replaced plug even though it looked good and gap was .060, also replaced plug wire because it broke pulling it off. Ohm'ed all injectors, (11.5-11.7), put a noid light on the injector leads and all looked normal, swapped coil packs and still have cylinder 4 misfire, I sprayed starter fluid around intake, no change in idle, swapped out the ignition module with a known good one, then even swapped out the injector with a known good one. Same f$#king thing, cylinder 4 misfire, unless I beat on it. I do notice a slight exhaust leak on the tube for the egr, but its almost undeteckable until you rev it. Any thoughts????
 

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Maybe try to gap the plug down to .045 and see if it will help.
 

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1) I ran Autolite race plugs and had misfires with threee sets. I switched to Autolite platinum and have them gapped at .040.(nitrous) Never a problem. NGK is also a great plug. I do not like gapping at .060. Especially with a lot of cylinder boost.

2) Have the intake gaskets ever been changed? How many miles on engine?
The intaske gaskets deteriorate, and changing them out is considered normal maintenance. You may still have an intake leak. if bad enought, anti-freeze can cause a seized engine....best of luck
 
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