Still messing with my 1989 5.8L FI f250. Cleaned IAC seems to be ok.
Idle good when cold as soon as it warms up rough idle,new plugs and wires.
If i pull vacumm hose of of the MAP it gets real bad, but if I put the hose back on and pull the electrical plug the check engine light comes on and the idle is so good it is scary smooth as silk. I assume the computer goes back to defalt codes. Dont have tack to check map frequency output. Does anyone know what should happen to the idle when you pull the plug on the sensor. Can you tell by doing that if the MAP is bad. I hate to buy a 50 dollar part which they wont take back if the old one is good.
Thanks Mike

 

I had the same problem with my 300-6, it was the problem.

 

Put in a new MAP ran it for awhile still idles bad. Lucky I was able to take the part back where I bought it and get the money back. ALso installed new coil, no fix. Replaced the new icm with the old one no fix, new one is back in now. Where do I go from here oxygen sensor.
Funny thing the exhaust does not smell right.
Going crazy. Mike

 

Try cleaning the throttle body bores and flappers with toothbrush, solvent and wipe with rag. Hold flaps open to clean there edges.
Phil

 

If the exahust smells funny maybe your cat is starting to plug.
Just a thought.