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Discussion Starter #1
Hey guys,

one of the minor details on my car yet to conquer is: sitting in park or sitting at a stop light at idle the rpms vary from 4500 to 7500. It is almost as if the computer is searching for an idle. My younger brother has a 1989 vette and he says his does the same thing. He says he has taken it in 3 or 4 times (at least once to West Coast Corvettes) and it will behave for a while and then start it again. So, what do you think dc crew? is this just the nature of the beast? I have a hard time believing that.
Thanks in advance,
roger;)
 

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It shouldn't fluctuate at idle. Maybe a little higher when cold but it will come back to normal once it warms up.

Check your plugs. Fouled plugs can cause many little quirks.
 

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-=Jeff=- said:
I assume you mean 450-750.. not 4500-7500

I would check and Clean the IAC (Idle Air Control) on the Throttle body. that may help
I'd think it would stall at 450rpm... wouldn't it?
 

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-=Jeff=- said:
If it stayed at 450 yes, but it sounds like the ECM is trying to recover from 450 to keep it running
I agree..

I would check the IAC out first.. like Jeff said try cleaning it.. I would also clean the throttle plates.. maybe there is alot of carbon build up so at idle it's not getting enough air.

Say you take the IAC passage out of the picture the engine should still idle with no load, there is an adjustment on the throttle body (a set screw) to set min idle.

And maybe the ECM is searching for enough air flow due to the throttle plate being blocked with carbon build up..

Thats my guess.. I would try cleaning the IAC passage and throttle plates.. first then if that doesn't happen maybe it's time to check out the igntion system or fuel sytem..

-Rick
 

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Discussion Starter #7
Duh...

yes 450 to 750. pretty quickly up and down, up and down. First thing in the morning it idles on the high end but steady. Then it drops to about 650 steady as it continues warming up it starts with the up and down. The carboned up IAC sounds like a good place to start. It appears that the screws holding it on will allow that piece to swing, or pivot up and down. I'll take it off and clean it up. would level be a good position to start with? someone told me one time that the current should be 1/2 volt at the plug in. they said if you swing it one way or the other the voltage would increase so find the spot where it is 1/2 volt. does this sound correct? Don't know if this helps or is connected to the problem, but I'm getting lousy mileage, like 9 or 10 mpg.
I'll start with the cleaning and check back with you on this thread.
Thanks,
Roger
 

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first Im sure your refering to the throttle possision sensor NOT the IAC valve when you refer to

"It appears that the screws holding it on will allow that piece to swing, or pivot up and down. I'll take it off and clean it up. would level be a good position to start with? someone told me one time that the current should be 1/2 volt at the plug in. they said if you swing it one way or the other the voltage would increase so find the spot where it is 1/2 volt. does this sound correct? Don't know if this helps "

the IAC unscrews from the lower throttle body housing andneeds to be cleaned but also requires a certain installed height and is easily broken

buy a manual. and carefully read it, it will help a great deal
 
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