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High idle on startup and while driving?

Tiny
CHAPMDC89
  • MEMBER
  • 1999 FORD MUSTANG
  • 4.6L
  • V8
  • 2WD
  • MANUAL
  • 188,000 MILES
On start up the motor will idle up very high about 3,000 RPMs and stay for about 2 minutes then drop. Also, while driving, the idle will stay up. When you apply the clutch, the engine will rev up. When you come to a stop, sometime the idle will drop, sometimes it will stay above 2,000 RPMs or higher.
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Monday, October 31st, 2022 AT 1:24 PM

9 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,300 POSTS
Hi,

We could have a few different things causing this. The first and easiest thing to check is the throttle cable linkage. A cable goes from the throttle pedal to the throttle body. Check to make sure there isn't anything preventing the throttle body from fully closing when the throttle is released. See pic 1

If that is good, then I would recommend servicing both the IAC and the throttle body. The IAC is the idle air control valve. This is what is responsible for maintaining engine idle speed when your foot is off the pedal. It, too, is located on the throttle body. See pic 2.

Here are two links. One explains how to service the throttle body and the other is for the IAC.

https://www.2carpros.com/articles/throttle-actuator-service

https://www.2carpros.com/articles/idle-air-control-valve-service

Note that when you service the IAC, make sure the port where it mounts is free of corrosion and carbon deposits.

Let me know if this helps or if you have other questions.

Take care,

Joe

See pics below.
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Monday, October 31st, 2022 AT 6:56 PM
Tiny
CHAPMDC89
  • MEMBER
  • 5 POSTS
Hello Sir,
The vehicle is a 1999 Cobra, 4.6.
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Tuesday, November 1st, 2022 AT 7:35 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,300 POSTS
I went back to check the Cobra, and it's the same. Have you been able to check anything yet?

Let me know.

Joe
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Tuesday, November 1st, 2022 AT 6:59 PM
Tiny
CHAPMDC89
  • MEMBER
  • 5 POSTS
Yes, I have replaced the Throttle Position Sensor, Mass Air Flow Sensor, Idle Air Control Valve, the Positive Crankcase Valve, and removed the K&N air filter and replaced it with a regular air filter. The vehicle was running great until I took it to a shop for a routine tune up. Oil was found in the plugs, so the head gaskets were replaced. When I got it back it started idling high.
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Wednesday, November 2nd, 2022 AT 7:52 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,300 POSTS
Hi,

I'm not sure why the head gaskets were replaced. Since this started after the work, check for engine vacuum leaks. Something may have been damaged or left disconnected when they worked on it. Also, make sure to check the PCV and the intake manifold for leaks.

If by chance you have a live data scan tool, let me know what the short-term fuel trims are.

Here is a link you may find helpful:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know what you find.

Joe
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Wednesday, November 2nd, 2022 AT 8:27 PM
Tiny
CHAPMDC89
  • MEMBER
  • 5 POSTS
I checked for vacuum leaks and could not find any, but I will check again. Also, the PCV was replaced. I will need to check the procedures for checking the fuel trim. I am thinking that I may just go ahead and replace the throttle body and the computer since both are over 20 years old. What are your thoughts?
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Thursday, November 3rd, 2022 AT 7:12 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,300 POSTS
Hi,

Let me know if you are able to find anything. Also, that is an expensive repair, especially if it doesn't fix the issue. However, that's your call.

Let me know what I can do to help.

Joe
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Thursday, November 3rd, 2022 AT 8:12 PM
Tiny
CHAPMDC89
  • MEMBER
  • 5 POSTS
I am going to check the fuel trim, but I am looking for the procedures. I am also going to check for vacuum leaks again. I really think that it is a leak, but I have not been able to find one.
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Friday, November 4th, 2022 AT 12:29 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,300 POSTS
Hi,

To get fuel trims, you need a scan tool that can provide live data. Basically, you go to the live data setting and there will be several things to check. Locate the STFT and LTFT. Let me know what they are.

Let me know.

Joe
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Friday, November 4th, 2022 AT 8:19 PM

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