Beating a dead horse, New IAC, high idle

Shouter

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Well last night while cruising, my car developed a high 1500rpm idle. It lasted for about 10 seconds, then went right back down to 500. Before that I've been noticing a hanging idle, all I took the iac off and noticed it was full of carbon. This morning I bought a new unit, put it on, cleaned my throttle body, and fired it up. It's maybe worse now. I push the clutch in and it raises a few hundred rpms, drops to 1500, stays there for awhile before settling back down to 500. Any ideas here?

I have an Accufab throttle body and plenum, bbk cold air intake, and exhaust on the car with an sct eliminator chip.

This is really annoying.
 

lwarrior1016

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That is really annoying, i was battling a high idle on my car. Have you checked for vacuum leaks? Also, have you made sure your throttle body is going all the way closed? I know sometimes you get some build up in just the right spot and it make the blade not want to close all the way.
 
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Shouter

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Don't remember, it's from oreillys. I had it on the scan tool and ran the diagnostics for iac, and it seems to perform normally. What is weird, when I'm in idle speed diagnostic mode, I can manually adjust the iac value(%). When I set it at 50%, it behaves perfectly, rev it up, it comes right back down. It is when I leave diagnostic mode and the iac values change per throttle position when it slowly decreases rpm and hangs at 1500 for a long time, then drops down to almost killing it (100rpms) then catches up to idle at 600. No codes thrown, tps performs as it should, etc. I just cleaned the throttle body, so I might of missed a spot. Also sprayed carb cleaner around the post MAF vacuum lines, no change.
 
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Shouter

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As stated, the tps voltage seemed to rise and fall steadily with throttle
 
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Shouter

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Also, when I disconnect the iac, the rpms noticeably drop. Plug it back in and it revs up, then after a few seconds drops back to idle
 

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In my experience it is Motorcraft of nothing on ISC motors. After market dont last. 15 years Ford tech. 20 year running my own shop.
 
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Shouter

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Okay this is a video of the car on the scan tool, in the Idle Speed Control test mode. I manually can adjust the IAC valve, and it responds to changes as it should. When I set the IAC to 50, it will idle exactly where I want it, and rev up nice, and come right back down. Now I don't know what this is an indication of, but I figured it might help someone come up with an idea.
 
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Shouter

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"Double Post" but here is a video of me driving. I let off the throttle, push the clutch in, and coast to a stop. You notice its symptoms immediately. It hangs, then drops below its desired idle before catching up and idling where it should be
 

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