Random misfires at idle. I'm stumped! - Pontiac GTO Forum
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post #1 of 6 (permalink) Old 02-28-2011, 02:16 PM Thread Starter
 
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Random misfires at idle. I'm stumped!

Hello all! Got a problem and I'm having a serious blond moment lately trying to figure this one out. I'm working on my buddy's 06 GTO, automatic transmission, only mods are a K&N Filter and a straight pipe exhaust.
When it's sitting at idle, warm or cold, the car "bumps," enough to be felt lightly through the whole car. I'd say it's a misfire as it certainly feels like it. It happens in an irregular cycle between every 2-9 seconds.
Thinking he had fouled spark plus, I had him go buy Bosch 4-tine Iridiums, I've had good luck with them. I check the wires upon removing them and they looked fine, the plugs looked in good shape with a very slight white frosting, which I'm guessing maybe it's running hot or lean. He has a new battery, I haven't checked his coils yet, and I'm thinking I'm seriously missing something. He says he hasn't noticed any loss of performance or anything.
I'm really thinking my fiancee clubbed me in the middle of the night and I'm sitting a few fries short of a happy meal, but I'm currently stumped and need help for my buddy. He's thinking his car is in full rebellion against him and he's about to use it as a mobile target. Any help would be appreciated, thanks greatly!
By the way, any jokes as far as my own brain misfiring at idle as this should be an easy fix, are always appreciated.
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post #2 of 6 (permalink) Old 02-28-2011, 02:57 PM Thread Starter
 
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Actually, sorry for being bleak. The idle is actually fine, sitting at a steady 500rpm when warmed up, the "bumps" cause it to jump slightly. His car has 42K on it. It doesn't seem to bog down at all and feels like it's running a little weak for a LS2, but not bad. His wires appear fine although I haven't checked the voltage, and the car isn't throwing the check engine light or overheating or whatnot. The misfire or "bump" only happens at idle, upon acceleration there isn't anything noticable. We're in California so we're stuck with the wonderful 91 octane gas, and he's been putting in 87 for a couple of weeks due to the rising gas prices (trying to save a buck by skimping), so that might be a cause. Thanks for any help Gents!
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post #3 of 6 (permalink) Old 02-28-2011, 03:36 PM
 
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These cars retard the timing to compensate for lower octane. 91 is the recommendation and 87 is the minimum requirement. Anything below 91 will result in reduced performance and even knocking or missing if you're too close to or below 87. Try a good quality 91 and see if the problem persisits.

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post #4 of 6 (permalink) Old 02-28-2011, 09:16 PM
 
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Not an expert here but it sounds like it could be a coil pack that isnt fully bad yet but is getting there...? Hope you find the problem
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post #5 of 6 (permalink) Old 02-28-2011, 11:03 PM
 
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If he had excessive misfires the CEL would come on. The shake you feel is normal from what you describe. The LS2s have a CAM that's on the aggressive side for a stock engine. The only reason it doesn't feel strong is the 87 fuel.
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post #6 of 6 (permalink) Old 03-01-2011, 07:10 PM Thread Starter
 
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Thanks guys! As said, it was a brain fart. He chalked up and admitted he had been running 87 for a little while, and that's when the problem started. Thanks for the help!
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