Last night I started my truck up after it sitting approx 24 hours (not sure if that matters) - it idled too high, worked its way up to about 2000-2200. It did not respond to reving it up and letting off gas (stayed at levels noted above). I drove a few blocks and it wouldn't drop below these levels. Upon getting to the last stop sign before a more major road I quickly shut it off and immediately started it back up (had somebody behind me). It started immediately and immediately returned to proper idle level. I drove it several miles, shut it off and on a couple times, let it sit during a movie and drove it home..... the problem never returned (it worked properly).

Anyone have any suggestions as to what the problem might have been and/or what I should do (if anything) to look into it?? If I need to wrench on anything I'd like to do it before winter hits here.

THANKS.

 

PS: my signature is not appearing in my posts..... ???

my truck:

88 F150 300 six 4.9l 4sp stick 4x4 lariat reg cab

 

OK, found the show signature box!!!!

 

I first assume that no codes were thrown - no 'check engine' light. If so, the first move would be to have the codes pulled.

If you haven't already done so, pull and inspect the Idle Air Control solenoid there on the throttle body. These are notorious for crudding up, causing idle problems - high idle speeds, also idle speed swings.

While you're at it, check the intake throttle body butterflies for clear movement and ability to seal off.

Another possibility is a vacuum leak, but I don't see how a leak could be intermittent and self-correcting as you describe.

Just for thoroughness, inspect the PCV valve for proper operation, and related hose for leaks.

As a last resort, lowest probability, inspect the exhaust tubing to the EGR - had an occasion where the tubing split, allowing fresh air instead of exhaust gas into the intake manifold.