1989 3.0 fast idle

so when the engine is cold and the fast idle is working the engine acts like it has a vacuum leak but only when the fast idle is working otherwise everything is fine. Ive checked for vacuum leaks very thoroughly and came up with nothing so whats the deal?does what part is the high idler?

 

if its not vacuum, then its your coolant temp sensor that is faulty causing the engine to think its cold all the time

also you might have a leak from your throttle body from adjusting screw...it should have a gasket around it and it usually goes bad....lastly make sure you take all the air out of your coolant/radiator...

 

the fast idle shuts off when it should but theres a major hesitation when its on only, and my question was what would create that? its done this since I bought the truck and I have already replaced the valve cover gaskets so the plenum had to come off and when I hooked everything back up it still did the samething.