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What can cause engine to race after warmed up?

Tiny
FORD9FAN
  • MEMBER
  • 1988 FORD F-150
  • 140,000 MILES
What can cause engine to race after warmed up?
When first started it idles as it should, then slows as it warms, then after it is fully warmed the engine idle starts to increase until it reaches about 3000 rpm's without touching the throttle.
It has been checked for vacuum leaks and the Idle Air Control Valve was first cleaned then replaced, with no change.
It also has a new fuel pump and pressure regulator.
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Sunday, December 2nd, 2012 AT 6:13 PM

9 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
I would look for vacuum leaks. Common issue.

Did anyone check the control for the idle air control? If you disconnect it, does the idle return to normal?

Roy
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Sunday, December 2nd, 2012 AT 6:52 PM
Tiny
FORD9FAN
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  • 5 POSTS
The engine has no vacuum leaks.
The engine shuts off when the idle air control is disconnected.
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Sunday, December 2nd, 2012 AT 7:49 PM
Tiny
HOMER1967
  • MECHANIC
  • 875 POSTS
How did you check for vacum leaks?Was the engine hot or cold? What size motor? If you are sure that there is no vacum leaks then I wouls suspect the tps sensor.(Throttle position sensor)
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Sunday, December 2nd, 2012 AT 8:29 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Ok, good, then the situation is around the idle control motor.

The motor should idle at 500 rpm with it disconnected.

You need to see if it is being commanded on when the idle rises or the valve is bad.

I still need the motor size

Roy
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Sunday, December 2nd, 2012 AT 8:33 PM
Tiny
FORD9FAN
  • MEMBER
  • 5 POSTS
5.0
Checked hoses visually, starting fluid around gaskets and areas not easy to check.
Idle control was replaced after cleaning original failed to correct the problem.
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Sunday, December 2nd, 2012 AT 9:01 PM
Tiny
FORD9FAN
  • MEMBER
  • 5 POSTS
Engine was warm.
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Sunday, December 2nd, 2012 AT 9:02 PM
Tiny
HOMER1967
  • MECHANIC
  • 875 POSTS
I am going to have to suggest that you take it to a shop at this point, because to diagnose any further is going to take specific tooling and the ability to know what the information you gather means. You have covered thae basics and it is just cost to much to keep throwing parts at it. Thanks.
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Sunday, December 2nd, 2012 AT 9:21 PM
Tiny
FORD9FAN
  • MEMBER
  • 5 POSTS
Thanks for the try.

Craig
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Monday, December 3rd, 2012 AT 1:16 AM
Tiny
HOMER1967
  • MECHANIC
  • 875 POSTS
You are welcome. Thank you for coosing 2carpros
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Monday, December 3rd, 2012 AT 2:51 AM

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