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What can make my RPM's fluxuate when idle?

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_MustangMan_

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Discussion starter · #1 ·
OK so when my car is idle the RPMs will fluxuate between about 500 and 900 rpms, i thought it was my vacuum hoses, however after putting new ones in the problem still occurs. The guy at the shop said it might be a censor and to next time i get a full tank get Premium unleaded and put 2 cans of C-Foam in the tank and that should reset the censor. Does that sound like it will work?
NOTE: The previous owner always used premium unleaded gas and I have been using EThanol-free 100% pure gas, could that be part of the problem? Any thoughts will help thanks, and i have a 99' GT.
 
#2 ·
:dunno but mine does the same 03gt switched my iac and tps sensor and same ish
 
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#4 ·
mine only does it from 650 to 750, sometimes gets stuck at 1000rpm. try disconnecting the battery for 5 mins that should make it all reset, and i wouldnt put 2 bottles of seafoam into my gas
 
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#6 ·
Mine does that, sort of, but it's more of a hanging idle. Sometimes it will stick close to 1k at idle, and other times it's normal, and on occasion it bounces down to like 500 then back up to normal. Pretty sure I need to clean my IAC valve.
 
#11 ·
IAC needs to be cleaned ASAP, I just cleaned one out last week on my other mustang I let it soak in CARB cleaner, then PB blaster as well. It like brand new now. All that garbage builds up inside and doesnt allow it to function properly.
 
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#15 ·
The issue you are having has nothing to do with your fuel or needing seafoam. You'll just be wasting your money and time. It's the Idle Air control valve(IAC valve). Every ford I've had, has had this issue at one time or another.
On my f150, it would get stuck at a high rpm at idle.
On my mustang, it would fluxuate from 500 rpm to 1000 rpm.

I've also heard of other fords having similar issues.
 
#17 ·
Could be a throttle position sensor as well. When i would start my car, the idle would go to and hang at like 1500 rpms for about 5 seconds then fall and bounce between 500-800 until id put it in reverse or drive. I replaced my IAC and nothing. I took the car to a tuner and they ran a diagnostic with my diablosport and it was showing the TPS wasnt opening properly so i replaced it as well. I actually still had some issues after but i got a PA Performance alternator and never had a single idle or throttle issue since. Drives like a whole new car after that
 
Discussion starter · #18 ·
99_STROKED_GT said:
Could be a throttle position sensor as well. When i would start my car, the idle would go to and hang at like 1500 rpms for about 5 seconds then fall and bounce between 500-800 until id put it in reverse or drive. I replaced my IAC and nothing. I took the car to a tuner and they ran a diagnostic with my diablosport and it was showing the TPS wasnt opening properly so i replaced it as well. I actually still had some issues after but i got a PA Performance alternator and never had a single idle or throttle issue since. Drives like a whole new car after that
Click to expand...
yeah I'm going to try disconnecting the battery, and if that doesn't work ill clean the IAC

sent from THE FUTURE.
 
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_MustangMan_
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