Help! Fast Idle Issue Is Driving Me Nuts...

[Unthrottled]

I'm having trouble diagnosing an intermittent fast idle problem on my stock 1994 Bronco XLT, 5.8L. I'd sure appreciate any ideas you might share.

The problem occurs sporadically both in Park and in gear while I'm driving. It's worse when I first start the car up. After a hour or two of driving, it usually disappears. The more the car sits, the more frequent the problem. The more I drive the car, the less often I get a fast idle.

I can sometimes get it to go away if I put the vehicle in neutral and rev the engine. But this doesn't always work.

I can also sometimes get it to go away if I turn off the car and restart it. But this also doesn't always work either.

I replaced the Throttle Position Sensor, but this did not fix the issue. Also replaced the Idle Control Valve, but no improvement.

My mechanic friend wants me to keep throwing parts at it. I want to diagnose the problem first.

Oh, two last things. The problem first started after the car had sat unused for a few months while I was moving my family across the country.

Also, I occasionally get a rough idle at start-up.

I don't know if that might help someone with more mechanical knowledge than me narrow down a diagnosis.

I'd be very grateful for any ideas or suggestions. What could be causing my fast idle?

Thanks!

 

[Tiha]

There is always checking for vacuum leaks and that is a great place to start, but since you said it happens more often when it sits for a while.

I am wondering if you have a dirty, sticky throttle body. Or even throttle body linkage or accelerator cable.

Fire the truck up, open the hood, manually the throttle linkage. See if it will stick, see if you can manually close it farther when it is idling high.

 

[Unthrottled]

Great idea, thank you for offering to help. I appreciate the suggestion and I'll give it a try.

Again, thanks.

 

[johnnyreb]

Great idea, thank you for offering to help. I appreciate the suggestion and I'll give it a try.

Again, thanks.

Try oiling the gas linkage. Even behind the gas pedal

 

[L\Bronco]

Hey unthrottled, I think Tiha is on the right path, could be a dirty throttle body or an incorrect base idle setting AKA hard stop setting.
Try unplugging the idle speed control solenoid in neutral warmed up. The idle should be about 500 to 550rpm.
Let us know what you find

 

[johnnyreb]

Since it will go away--WHEN YOU REV IT UP. It is WARMING THE MOTOR UP MORE----which COULD CAUSE THE METAL AND GASKET TO SWELL UP----WHICH COULD SEAL OFF A VACUUM LEAK MORE. ALSO--think about the temperature outside---different OUTSIDE WEATHER CONDITION can make alot of difference. Go back and check for a vacuum leak. A little hose off ---off or bad hose is very easy to over look. Look also is hard to area locations---like behind the motor.