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Discussion Starter #1
Before I get flamed saying I haven’t searched and all the other typical forum drama crap, I have searched for this identical issue and nothing has came about exactly like my issue. First off car is a 2004 NSRT 80,000 miles with stock turbo with AGP waste gate, AGP CAI, AGP intercooler, AGP hardpipe with HKS SSQV BOV, AGP catchcan, Diablo intune 93 tune, and stock everything else. Had a hanging idle at 1500-2k when rolling to a stop (text book vacuum leak), ended up being a bad diaphragm in the IAC. Replaced the IAC with a brand new OEM one from Modern Performance. After replacing it I realized that the wire loom upgrade done by the previous owner wasn’t soldered or crimped with any type of connector. The wires looked like a rats nest and since I didn’t have time to restrip and solder the wires back I just cut the connector off to do later. Without the plug connected the car ran great once the engine was up to temp. Throttle response was amazing, RPM fall during decel was great as well. Fast forward a week when I had time to fix the wiring and now the car is right back to a hanging idle. Verified no boost leak and verified no vacuum leak... any ideas?
 

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Discussion Starter #5
It's not crazy, The PCM has "learning" adjustable parameters it changes, to control how the engine runs.
yea I’m tracking, I just figure it would constantly learn since everything changes every day from barometric pressure, relative humidity, ambient temp etc. I don’t have to unplug the battery daily to relearn current world dynamics. That’s why I said crazy...
 

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If you reconnected the idle air control (IAC) solenoid and the idle appears to hang and not behave correctly, resetting the computer is a good idea and if that doesn't make any changes there's a good chance the computer is commanding that poor idle quality. If you repaired the wiring to the IAC, did you also check and repair the throttle position sensor (TPS)? While the IAC is on the same sub-harness and could sometimes get damaged the TPS wiring breaking loose from the electrical pin inside the connector can causes sporadic idle issues and rpm hanging like you're describing. It can often take a long time for it to set a check engine light.
 

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Discussion Starter #10
Sorry it took me so long to get back but short answer is no, the reset did not work. I unhooked and immediately reconnected the battery on the 20th of December with no changes. This weekend I left the battery unhooked for 48 hours to allow for everything to discharge. Still no changes. I have thought about replacing the TPS but the issue is literally gone without the IAC plugged in and there is never a sporadic idle or anything else so I have not gone that route to avoid chasing my tail and spending money that I don’t need to.


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Discussion Starter #11
If you reconnected the idle air control (IAC) solenoid and the idle appears to hang and not behave correctly, resetting the computer is a good idea and if that doesn't make any changes there's a good chance the computer is commanding that poor idle quality. If you repaired the wiring to the IAC, did you also check and repair the throttle position sensor (TPS)? While the IAC is on the same sub-harness and could sometimes get damaged the TPS wiring breaking loose from the electrical pin inside the connector can causes sporadic idle issues and rpm hanging like you're describing. It can often take a long time for it to set a check engine light.


I haven’t checked the TPS wiring simply because the issue doesn’t exist with the IAC unhooked. But I guess a smart man should never assume so I’ll have to find a free 10-15 minutes to inspect the TPS wires.


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Discussion Starter #12
If you reconnected the idle air control (IAC) solenoid and the idle appears to hang and not behave correctly, resetting the computer is a good idea and if that doesn't make any changes there's a good chance the computer is commanding that poor idle quality. If you repaired the wiring to the IAC, did you also check and repair the throttle position sensor (TPS)? While the IAC is on the same sub-harness and could sometimes get damaged the TPS wiring breaking loose from the electrical pin inside the connector can causes sporadic idle issues and rpm hanging like you're describing. It can often take a long time for it to set a check engine light.


I didn’t see any wire issues but it is OEM wires still. I ordered a wire repair kit to see if hopefully that is still the issue.


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Discussion Starter #14
Go back as far as you can, eliminating some of the old wiring and solder and heat shrink your connections.


I will definitely solder and heat shrink, I don’t understand why people don’t. I’ll keep you guys posted.


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I realize this is an old thread but I've been searching for the same solution as well. Truck has same problem.. cleaned the TB, replaced gasket,.. kicked high idle code.. replaced iac valve and upon replacing noticed the wires where somewhat loose in the connector.. continued to reconnect just to give it a shot.. same problem.. have disconnected battery.. reset the iac ECT.. but problem doesn't seem to exist when I unplug the iac valve and run the truck.. just curious if you ever found a solution.. will be replacing the connector in near future (have checked for vaccum leaks as well)
 

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Without know which exact truck, and even then, it would probably be a guess. If you disconnect the idle air control (IAC) motor/pintle's electrical connector and get a solid, consistent idle, but then plug in and the idle is erratic, high or low, then I'd suspect either an electrical problem, a problem with the IAC itself, or something else that's controlling the IAC.

That something is likely the computer, so you'd need to determine how it controls the IAC. Does your model use direct feedback from the IAC on the position of the pintle (valve opening) or does it read changes in the manifold absolute pressure (MAP) and/or mass air-flow (MAF) sensor. If you have an older, pre-electronic throttle does the computer read engine rpms and adjust the IAC to increase or decrease idle speed. Or a problem reading throttle position that might affect the computer's calculation of how much IAC duty cycle to use.

If there's a problem with reading the amount of air coming into the engine or one of the other sensors, it's possible the computer is trying to adjust idle speed and the IAC when it shouldn't. The best troubleshooting guidance is to see if you can track down a copy of your factory service manual and powertrain diagnostics manual.
 
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