I have a 1988 F250 with a 351w, I just had recently purchased the truck. The first thing I noticed is that temp gauge did not work and the truck was high idling. I installed a tach so that I could monitor how many rpm's the truck was idling at. It idles at 1500 to 2000 rpm. I replaced the temp sender and temp switch. Now the temp gauge works but the truck still idles high. The weird part is I started the truck yesterday and the and the check engine light came on and i had keep my foot on the throttle to keep it running. once it warmed up the check engine light went off and it was idling the way it should so i thought that it was fixed. I started it this morning and it is back to the high idling with no check engine light. I can unplug the IAC and it will idle so low that it will die plug it back in and its back up to 1500-2000 rpm. i can hold the throttle all the way down and start it and when it starts i let off right away and it will idle normal with check engine light on and about 800rpm. Shut off the truck and start it with no throttle and it will start about 2000rpm and it will stay there with no check engine light.

Looking for any input at what it might be

 

You should have codes if the CE light was on so have a go at pulling them, there are a lot of possibilities for this problem so a code of some type would help. The fact the idle speed drops way down when the IAC is unplugged suggests the computer is getting bad throttle position info and codes could confirm this. You could also try unplugging just the TPS when the engine is high idling, if this makes no difference it's probably your problem.

 

Did you figure out what was causing this? I have the same truck and the exact same issue.

 

Did you find the problem? I bought the same truck last week and am having identical issue.
Any help is appreciated.

 

Check for vacuum leaks. They can lead to erratic idles.

 

Sorry for the repeat, I had gotten an error and didn't think it posted. I'm new here.