high rev on start up - Ford Mustang Forum

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post #1 of 4 (permalink) Old 07-06-2006 Thread Starter
mustang86's Avatar
Joined: Jun 2006
Location: Dade City
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Angry high rev on start up

'86 mustang conv. Love my car, only a few problems. High rev on start up. Stays around 1200 rpm's then goes to about 900 and stays there. When I pull of the IAC valve the car smooths right out. When I plug it back in, it goes to the high idle. I have replaced the IAC and cleaned out my intake. I also changed the TPS and still nothing. Thinking of changing the throttle plate. Is there anyway to adjust the idle on the 5.0 HO with the MPI? I'm really not a ditzy chick, I work in auto parts store and have quit a bit of basic car knowledge, but this is driving me crazy. Can you help???

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post #2 of 4 (permalink) Old 07-06-2006
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Location: Bellingham
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The IAC is supposed to allow air to bypass the throttle body on startup when the car is cold. mine does the exact same thing and its normal. Because I dont like driving my car before I have let it warm up for a minute or two, I use that as my indicator. as soon as the idle drops down below 1000 RPM's, I start driving.
Now, if yours does that even with the car warmed up, that may be another issue, but it sounds normal to me.

89' GT Ragtop.
Ford Racing "C" springs
SLP Loudmouths
Summit o/r x-pipe
BBK equal length headers
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Chipped by DynoSources
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post #3 of 4 (permalink) Old 07-07-2006
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my car used to do that,and i mean it would idle high, like 2,500-3,000 rpm..i messed with it, tps, new iac,..still idled high, so i just unplugged the iac and left it like that..it runs perfectly now
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post #4 of 4 (permalink) Old 07-10-2006
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Maybe the coolant temp is reporting the engine is cold - so the IAC is bringing the idle up higher than normal. Test, measure, and if needed replace.

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