351W that has longtube headers with EGR pipe in the #1 tube.

Is it possible, "theoretically." That the exhaust flow coming from the primary header through the throttle body causing the IAC to heat up to a point were it sticks?

This is my 2nd IAC... The 2nd one fixed it for about a day.

When the vehicle is at operating temperature ect...After about a 20 mile highway drive. I park the truck and it wants to idle at around 1200 RPM and will not go to normal idle RPM....

I pop the hood and tap on the IAC with what ever I have screwdriver, socket ect... It then idles back down to normal RPM.

I'm thinking about building a restriction plate for the EGR to Upper manifold to test this theory.

 

The Idle air control valve is really pretty simple internally and would think you'd have other under hood heat problems before it got hot enough to affect it. The fact that when you tap it it starts to work again makes me ask have you tried just wiggling the wire plug connection without tapping on it?

Edit, I just caught on to your EGR pipe set up, I wouldn't think The exhaust temperatures would be a whole lot different in that little of change of distance from the head.

 

Idle air control valves get their intake air before the EGR, generally from the intake in front of the valve body and shouldn't be sucking any hot air from anywhere from there.

 

The factory EGR setup is exactly the same as yours w/headers. Are these new IACs? OEM or Ford? Yes it does make a difference.

 

There both original OEM Factory IAC's. The first one was the original 5.8L that I cleaned up when I did my motor swap.... The 2nd one is off the original 5.0L that worked fine till after about a day of use.

I did wiggle wires.... I'll tinker with it today...I have a piece of sheet metal I can stick between the EGR and manifold to test this.

Update: Went outside to look at both motors and I have to say there is no way that the EGR would heat up the IAC....The EGR sits further then the IAC so that is not the problem. Not even going to bother with the sheet metal Idea. Just going to clean the IAC valve and go from there.

Update #2: I unplug the PVC vacuum line from the upper intake manifold and plug the hole with my thumb it idles lower.

Also, I noticed that the #1 and 2 spark plug wires are arcing so I taped the cracked areas up temporarily with some liquid tape. Also, noticed one of the new silicon vacuum lines was cracked, "not all the way through." Going to replace that with some spare line I have laying around. I also ordered a IAC, Plug wires, and a BBK fuel pressure gauge from summit. Will go from there.

 

Okay, put new Ford Racing plug wires on. Fixed the miss..

Put a new IAC valve on....No change.

Put the BBK fuel pressure gauge on....

Had no change on the 1200 RPM idle only when it warms up to operating temperature...

I ended up plugging the 2 holes in the Original throttle body with some silicone to bring down the idle....Lowered the idle down to about 1000 RPM...Looks like the previous owner must of cleaned the throttle body out with some heavy duty cleaner, white residue ect....I bet the coating is all screwed up on it. The TPS may be damaged also...Not gonna even bother testing this one..

I'm pretty sure its throttle body, sensor related and-or possibly a vacuum leak that is undetected at this time.

It has new gaskets on the engine I swapped in...

Ended up ordering a 56mm BBK throttle body, new Motorcraft TPS, ACT sensor and Temperature Sensor....

Might pick up a PVC valve see if that helps..

Fuel pressure is 35psi KOEO and about 32psi idle running. Seems okay.

This has been doing this since I put the new engine in....

No codes.....