When I start the truck up it idles just fine but after driving when downshifting to neutral it will idle up to almost 2k rpm. Since I've bought it I've put new all new intake manifold gaskets. This engine has been refreshed times ten so I'm really lost on why it doing that. The idle air control valve is brand new along with the throttle position sensor. I do have the screw on the throttle all the way backed out I looked into that I don't know if that could be a reason but any help would be appreciated.

 

Welcome to the forum

Warm up the engine
Let it idle and unplug the IAC Valve, it should close and in most cases engine will die as idle drops, that is a good sign it means no vacuum leaks or issues with throttle body
If idle stays high then there is a leak
Press brake pedal a few times and see if RPMs go up, means booster has a leak
Pull PCV Valve hose off and plug it, if RPMs drop then PCV Valve is bad

If engine dies then you will need to adjust the Anti-Diesel screw, looks like an idle screw but fuel injection can't use an idle screw, lol, so its renamed as Anti-Diesel screw
Turn it about 1 full turn clock wise
Restart engine and see if it will idle, if not do another full turn, ect..............
Until it will idle at about 600-650rpm, barely running, doesn't have to be exact, just not stalling because you are going for a drive

With IAC Valve unplug computer has no control over engine RPMs, TPS is a sensor, just reports throttle position
CEL(check engine light) will be on because IAC Valve is unplugged

See what happens when you drive it around, RPMs should drop when shifting if foot is off the gas pedal
If engine is still staying at higher RPMs then maybe throttle spring is weak can't close the throttle if vacuum gets too high when decelerating
If engine RPMs are "acting normal" then problem will be in the computer OR new IAC Valve

3rd party IAC Valves are not true solenoid valves, Ford computer needs a true solenoid valve, Motorcraft or Hitachi brands are true solenoid valves
A used Motorcraft IAC Valve is better than new 3rd party valve

Computer will be hard to diagnose better to try a used one

 

Thank you very much for the advice I unplugged my iac and it changed nothing so I went to my brakes and I stepped on the pedal and it almost revved the truck. I think it's leaking out of the vacuum tree on the intake manifold. I will replace and hopefully it will be be the end