Had an extremely fast idle on my 1990, 5.0. Could find no vacuum leaks, replaced and adjusted the throttle position sensor, checked butterfly valves in throttle body to make sure they closed all the way and replaced the air bypass valve. Checked PCV valve and still had a fast idle. Had my brother blow cigarette smoke through the hose that I pulled out of my brake booster and saw smoke rising from under the upper intake manifold. I removed it and found the gasket had a large leak. Replaced the gasket between the upper air intake and the lower air intake and all is good now.

 

Well... Thats one way to do a smoke test.

 

^lmao

 

I like it, well done

 

My truck idles very high but not all the time, if I had a bad gasket, would it do it all the time?

 

I would say maybe, but you could put a gauge on it and pull the codes to be sure. Sounds like an IAC issue though.