High Idle - anyone...? WMBurns??

Mike97gt · Dec 3, 2009

As WM said if the car is still running with the IAC unplugged this is a problem. It should die out right away..

Ok I have to rewrite this since I just saw you replaced the IAC..

You need to check your VMV.. you might have a VMV sticking open and this will effect the PCM's ability to control idle. try blocking off the vmv line to the intake and see if idle returns to normal.

GDawg · Dec 4, 2009

svttech76 said:
As WM said if the car is still running with the IAC unplugged this is a problem. It should die out right away..

Ok I have to rewrite this since I just saw you replaced the IAC..

You need to check your VMV.. you might have a VMV sticking open and this will effect the PCM's ability to control idle. try blocking off the vmv line to the intake and see if idle returns to normal.

Okay Mike, you seem to be the only one really listening to what I've said. Yes, I replaced the IAC with a new one. The problem is still there. I've been moving from one house to another all the month of November so I've really just left my car alone in the garage. To me, the whole thing seems to be a TPS problem cause the high idle does not flucuate (sp?) like a vacuum leak. I don't know what the TPS is supposed to put out but I really think it's that. I will try to look at that this weekend cause I'm tired of not having my car running. This is an odd problem for my car and my car has been perfect for 11 years but now I guess it's getting old like me.

GDawg · Dec 4, 2009

Btw... what is vmv and how do I test it?

GDawg · Dec 4, 2009

One other thing...

I was just notified that I passed my ASE A6 certification test!!!!

But I'm still a dumb ass... LOL!!!

wmburns · Dec 4, 2009

GDawg said:
Btw... what is vmv and how do I test it?

Vapor Management Valve.

OBTW, I had to look it up myself. I have always called it an Evaporative purge valve.

Really all you have to do is remove and plug the vacuum line going to the Evap charcoal canister. The line is attached to the base of the throttle body pointing to the rear of the firewall (on my 2000). The vacuum line runs into the fender well to the rear of the car.

What we are trying to determine if this is the source of the extra air. If the idle speed drops to more normal levels, this will tell us were to look next.

BTW, the 1996-1998's have a separate valve and flow sensor. They fail quite often. IMO, this is a long shot because I would have also expected other DTC's.

Congats on passing the exam! That is great news.

>>Testing the TPS for fast idle concerns:
Fast idle concerns:
Key on, engine off, monitor TP MODE PID while wiggling TP sensor circuits. TP MODE PID can also be monitored during vehicle drive. With throttle closed, TP MODE PID must be C/T (closed throttle).

TP Mode PID is not C/T with throttle closed:
Note: At vehicle start, the TPREL will begin at about 1.25 volts, and count down to the lowest TP V value seen since engine start. If the TP V value goes below the "normal" range, then increases again, TPREL will set to the lower voltage. If TP V is about 0.04 volts greater than the TPREL value at closed throttle, the PCM will go into part throttle mode.

Monitor TP V and TPREL PIDs for sudden changes while checking for intermittent TP circuit/connector concerns. Also check for loose/worn throttle plates. If no concern is found, GO to Z1 in Section 5.

Rickyll7 · Dec 4, 2009

The kb kit moves the iac valve enough to stretch the wire harness quite a bit. Maybe the harness connection to the iac valve is failing. Check the voltage at the wires going to the valve. Not sure what voltage you are supposed to see though. Anyone know?

wmburns · Dec 4, 2009

I think the IAC voltage is between 0-12 volts corresponding to a duty cycle. So a 50% duty cycle would equal 6 volts (assuming I understand the table in the service manual and what "DVC/%" means).

However, for what we are trying to see, if anything is measured this would indicate the wires are OK.
>>
IAC 83 VBAT/0 9/38 9/45 9/41 DCV/%
<<
As an option, use the old IAC and look at the needle valve when the electrical connector is connected. You should see some movement. The problem with this test is the old IAC is an unknown.

However, if it is a wiring problem, the old IAC is most likely good. At best, this test can only provide information not a positive diagnosis. IE, if movement is seen, this means the IAC is likley good. If no movement is seen, it proves nothing.

Included is the pinpoint tests from Ford:

>>
KE23 CHECK IAC CIRCUIT FOR SHORT TO GND IN HARNESS
Note: Refer to the PCM connector pin numbers in the beginning of this Pinpoint Test.

Disconnect scan tool from DLC.
Disconnect PCM.
Measure resistance between IAC circuit at the PCM harness connector and battery negative post.
Is each resistance greater than 10,000 ohms?

KE30 CHECK IAC SYSTEM FOR INTERMITTENT OPEN OR SHORT
Scan tool connected.
Key on, engine running.
Access IAC PID and RPM PIDs.
With engine at normal operating temperature, accessories off and at idle, the IAC duty cycle must be between 20% and 45%.
Observe the PIDs for an indication of a fault while completing the following at idle:
Lightly tap on IAC valve and wiggle harness connector to simulate road shock.
Grasp the vehicle harness closest to the IAC valve. Shake and bend a small section of the harness from the IAC to the dash panel and from the dash panel to the PCM.
Do the IAC or RPM PIDs suddenly change in value indicating a fault?

Mike97gt · Dec 4, 2009

GDawg said:
Okay Mike, you seem to be the only one really listening to what I've said. Yes, I replaced the IAC with a new one. The problem is still there. I've been moving from one house to another all the month of November so I've really just left my car alone in the garage. To me, the whole thing seems to be a TPS problem cause the high idle does not flucuate (sp?) like a vacuum leak. I don't know what the TPS is supposed to put out but I really think it's that. I will try to look at that this weekend cause I'm tired of not having my car running. This is an odd problem for my car and my car has been perfect for 11 years but now I guess it's getting old like me.

I think your on the right track.. I would block off the vmv line that goes to the intake and then monitor TP voltage.. the voltage should be steady at idle and within the limits posted.

If it's all over the place when the problem happens or if it does not transition smoothly to about 5 volts at WOT then I would replace the TP..

Since your car is supercharged it is also a possibility you could have interference in the IAC circuits or TP circuit.. The kit changes the location of some of the harness and you might have some rubbing of the harness to allow back feeding to happen.

wmburns · Feb 17, 2010

wmburns said:
There is a possibility that the TB set screw was adjusted slightly open to compensate for a malfunctioning IAC. Now that the weather has cooled, too much dense air is getting through and thus the high idle.

Shout out to GDawg! What do you think about updating this thread? Haven't heard from you in a while.

ikillrinst · Jul 1, 2016

I see this thread has been dead for 6 years now. Hopefully I am not out of line here.... I too am having a high idle issue.. I purchased a 2001 GT(yellow) 2 years ago and it sat for awhile.. I actually used some parts off of it for my other 2001 GT(Silver). Anyway, I finally decided to get this one running.. It ran great when I bought it and parked it... The parts I borrowed were the TPS and coils. I have since replaced the coils and TPS.. the idle isnt ridiculously high. but enough to make it feel like it' hard to stop at a stop sign or red light... It runs at about 1200 steady.. I have cleaned the throttle body and IAC.. If I pull the plug off the IAC nothing changes at all.. If I pull the vacuum line off the IAC it immediately stalls.. Not sure what else to try. I am more familiar with the 5.0 as I have an 86 Mecury Capri GS with 61,000 miles... Any and all help appreciated. Unless the only one to see this will be the ghosts... lol

wmburns · Jul 2, 2016

ikillrinst said:
If I pull the plug off the IAC nothing changes at all.

^^This "proves" that excess air is entering the throttle body from "somewhere" or the IAC isn't working at all. The trick is to find the somewhere.

Here's a link to some information to help you get started. Review the information from the beginning. Good luck.

http://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/

OBTW, a 1100-1200 RPM idle while the car is MOVING is normal.

Benny with a 98 dohc · Oct 20, 2019

wmburns said:
Vapor Management Valve.

OBTW, I had to look it up myself. I have always called it an Evaporative purge valve.

Really all you have to do is remove and plug the vacuum line going to the Evap charcoal canister. The line is attached to the base of the throttle body pointing to the rear of the firewall (on my 2000). The vacuum line runs into the fender well to the rear of the car.

What we are trying to determine if this is the source of the extra air. If the idle speed drops to more normal levels, this will tell us were to look next.

BTW, the 1996-1998's have a separate valve and flow sensor. They fail quite often. IMO, this is a long shot because I would have also expected other DTC's.

Congats on passing the exam! That is great news.

>>Testing the TPS for fast idle concerns:
Fast idle concerns:
Key on, engine off, monitor TP MODE PID while wiggling TP sensor circuits. TP MODE PID can also be monitored during vehicle drive. With throttle closed, TP MODE PID must be C/T (closed throttle).

TP Mode PID is not C/T with throttle closed:
Note: At vehicle start, the TPREL will begin at about 1.25 volts, and count down to the lowest TP V value seen since engine start. If the TP V value goes below the "normal" range, then increases again, TPREL will set to the lower voltage. If TP V is about 0.04 volts greater than the TPREL value at closed throttle, the PCM will go into part throttle mode.

Monitor TP V and TPREL PIDs for sudden changes while checking for intermittent TP circuit/connector concerns. Also check for loose/worn throttle plates. If no concern is found, GO to Z1 in Section 5.

Follow up with z1 in section 5

High Idle - anyone...? WMBurns??

Mike97gt

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