I have a 1989 ford f150 5.0 automatic 147,000

For the last several months I've had an idle problem where my truck will constantly high idle and I need to hold the brakes down at a red light so it doesn't slam into the car in front of me, and when I release the brakes the car shoots forward.

Also after putting it in drive, when I put it in park the idle shoots way up, until i turn the truck off.

This has happened off and on for months, and now it constantly does it.

Any ideas?

Thanks.

 

Could be a bad IAC but sounds a lot like a vacuum leak or binding throttle linkage.

 

Is there a way to diagnosis those problems?

 

Sounds like the IAC. Next time the truck idles high flip the ignition off and back on without letting the truck stall and see if the idle drops back to normal. That resets the IAC.

 

Thanks! I'll try that next time.