I have a 1990 F150 XLT Lariat equipped with a 5.0 V8, 5 sp manual. Idle speed will not stay steady, running between 700 rpm up to 2000, although it does not happen all the time. No consistency whatsoever. does not seem to matter if the engine is cold or hot, nor do weather conditions seem to matter. Driving through town, it may be fine for a few miles, then idle will quickly climb to the 1500 to 2000 range. it will do this sitting at a light or even run high between shifts. Sometimes I can shut it off, restart it, and it's ok for a while again.

This same problem happened about a year ago. I replaced the IAC, no help, and the TPS, which cured the problem at the time. Now it's back. Is the TPS likely to be bad again already? The check engine also came on for about 2 minutes while the idle was high, but went back out. What else should I be looking for?

 

Sound like a classic Vacuum leak if you know the IAC & TSP to be good.

You should run error code scans before replacing anything. There is a Sticky thread at the top of this section which tells you how to preform error code scans w/o a code reader, however I suggest getting one of these for under $30. Best tool you will own for your truck.

Equus 3145 Ford Code Reader

 

Thanks! I will try it tomorrow... and look for a code reader