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After I rev or even touch the throttle it will stay at that rpm for several seconds before it drops back down. If i disconnect the vacuum line going to the IAC from the intake it (just on the intake side) it will act almost normal, just idles high. Already tried replacing the IAC. The car has comp cams 262s, 75mm throttle body and plenum, and headers. From what I've read none of these require a tune though. Did all of these at once and had the engine out so im not sure if these are causing it, something has gone bad, if i didn't put something back on right or there's a vacuum leak. Any ideas?
 

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If this is one of the first start ups the IAC will do things like higher idle and slower ramp down until parameters are fully learned i found. Happens to me after a tune change or battery dieing.
 

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Discussion Starter #3
Its been running for quite awhile like this, at least an hour
 

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Ive had it take a couple days driving back and forth to work. The car runs for about an hour a day with 4 drive cycles and last time it took 3 days. It needs a few drive cycles and cycling the A/C on for a while and off for a while with brakes applied to fully relearn it seems over a few drive cycles i think.
 

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Discussion Starter #5
Ive had it take a couple days driving back and forth to work. The car runs for about an hour a day with 4 drive cycles and last time it took 3 days. It needs a few drive cycles and cycling the A/C on for a while and off for a while with brakes applied to fully relearn it seems over a few drive cycles i think.

Its really difficult to drive like this. The if I rev it to 4k rpm it will stay there for about 5 seconds before going back down. I feel like I've driven it around a fair amount like this but it hasnt gotten any better
 

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Have you checked the throttle position sensor voltage with the aftermarket TB? The TPS might not be lined up exactly how the oem one was.
 

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Discussion Starter #7
Have you checked the throttle position sensor voltage with the aftermarket TB? The TPS might not be lined up exactly how the oem one was.
I have not. Its the stock tps, what voltage should I be looking to see and when?
 

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I have not. Its the stock tps, what voltage should I be looking to see and when?
I鈥檓 gonna have to google the numbers but it鈥檚 supposed to be within a range.
If memory serves .9 volts is the high end. I haven鈥檛 done it in awhile, so I can鈥檛 remember which wire to probe.
Try this one.
Here鈥檚 another link. Setting the Mustang TPS Voltage
I haven鈥檛 found the minimum voltage, but don鈥檛 get caught up on that it HAS to be at .99V or 1.0v like something.

All else fails put your old TB on and see what happens.
 

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I was told if its 99-04 the computer will learn the TPS low voltage, it doesnt have to be adjusted, unless for some wierd reason it cant go through a full sweep under 1v.

Edit: i mean full sweep under 5v.
 

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I was told if its 99-04 the computer will learn the TPS low voltage, it doesnt have to be set adjusted it, for some wierd reason cant go through a full sweep under 1v.
For kicks put the stock throttle body back and see if the problem persists. You won鈥檛 be missing out on much if any with the stock TB.
 

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Interesting thread, but it ended without a resolution.
 

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Discussion Starter #14
It was set at about .89 volts. Brought it up to .98 and that just made it idle at 3k rpms. Brought it down to .85 and it was back how it was before. That article says anywhere between .75 and .99 is good so unfortunately that doesn't seem to be it. Guess I'll switch to the old throttle body and see if that fixes it.


Here's a video showing exactly what its doing. Service engine light is on for a bad temp sensor, and yes there is an exhaust leak
 

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The issue i have is that it does eventually settle out. Sounds like the computer is commanding more air through the IAC. If you disconnect the plug, it should either stall or barely run at idle and should not hold the high RPMs. If it still idles high with the IAC disconnected, pull the hose off it (block unmetered air from entering the hose) and see if the iac is still sucking air, i dont beleive it should be and if the iac is stuck partially open it might explain a bit. If it still idles high and the iac isnt drawing air, Spray intake cleaner or something all around the intake area and see if there is a vac leak. Past all those possible issues id still lean towards an issue learning tune.
 

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Discussion Starter #17
The issue i have is that it does eventually settle out. Sounds like the computer is commanding more air through the IAC. If you disconnect the plug, it should either stall or barely run at idle and should not hold the high RPMs. If it still idles high with the IAC disconnected, pull the hose off it (block unmetered air from entering the hose) and see if the iac is still sucking air, i dont beleive it should be and if the iac is stuck partially open it might explain a bit. If it still idles high and the iac isnt drawing air, Spray intake cleaner or something all around the intake area and see if there is a vac leak. Past all those possible issues id still lean towards an issue learning tune.

It only will act normal ish if I first start the car, then disconnect the iac line on the intake side. If i pull it off the iac it will idle like poorly and stumble, or if i pull it off before starting (even the intake side) it will do the same. The IAC is always sucking in air. Thought that was normal though.
P
Sorry for the double post not sure if anyone cares


EDIT: And I've sprayed brake clean all over and couldn't find anything.
 

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It only will act normal ish if I first start the car, then disconnect the iac line on the intake side. If i pull it off the iac it will idle like ass and stumble, or if i pull it off before starting (even the intake side) it will do the same. The IAC is always sucking in air. Thought that was normal though.

Sorry for the double post not sure if anyone cares


EDIT: And I've sprayed brake clean all over and couldn't find anything.
Have you tried leaving the hose hooked up and disconnecting the plug though?
 

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It sounds like too much air is going through the IAC or maybe the TB when the TB is supposed to be all the way closed. Have you tried to lower the idle via the TB?
 

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Looking a the video it almost looks like a sluggish IAC... but he says its been replaced. I still stand by unplugging the plug from the iac and seeing what happens.
 
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