I have an 86, 4.9L with the single barrel Carter. I bought it with a nasty misfire that I fixed. While I was fixing it, I went over all vacuum lines and components. Additionally, I replaced the carb gaskets and E/I gasket (used the correct torque/tightening sequence). I've only put about 300 miles on it since I fixed the misfire (previous owner installed the wrong distributor on it) and it was running great until a few days ago.

Currently, the idle won't kick down and will sit at 1500 RPM when warmed up. I didn't run the codes yet because I need to get a test lamp (borrowed one before), but I will do that tonight. My first thought was vacuum leak, but I went back through all lines and components as well as the bolts and tightened anything that was loose. There were a couple manifold bolts that seemed a little loose, but it didn't make a difference.

Even though I doubt the idle screws are set wrong because the idle was perfect before, I tried to go through the idle set screw check procedure. When I retract the IAC Valve using a jumper, the plunger doesn't stay retracted when I start it back up unless I unplug it (unless I'm misreading the procedure and should unplug it after I retract it?). When I unplug it with the IAC retracted, the curb idle is around 500 RPM (good). My main question regarding this part is whether the fact that my curb idle is at 500 RPM can be used to out-rule a vacuum leak? Or does the fact that the IAC is unplugged kind of throw it off?

When the IAC Valve is plugged in, it looks to restrict the throttle lever plate from moving (i.e. there is a gap (maybe 1/4") between the end of the curb idle screw and where it rests on the carb during idle check). Additionally, the IAC valve looks to move pretty slow and barely retracts when I open the throttle. However, I don't know how fast a properly functioning IAC Valve moves. So does anyone know what how a properly functioning IAC Valve moves?

Like I said, I didn't pull the codes yet, but I will tonight. I just wanted to ask a couple questions to get the ball rolling since my wife thought it was a great idea to buy something that needs to be picked up by tomorrow despite me telling her the truck isn't running right .

 

After replacing idle air control valve, the electric stepper motor as well as the carburetor [over several months of trouble shooting], I found my problem with the help of a $.69 cigar. Hope this link will be of assistance: https://www.ford-trucks.com/forums/1...expensive.html

 

On the way back from getting a couple tools at the store, I thought to myself: wife and kid are out of town tonight; what a great night to sit on the deck and smoke that cigar my buddy gave me a while back lol.

I will definitely have to give that a try tonight; although I did spray around with a couple cans of carb cleaner last night as well. I didn't get any hits though, but some spots are hard to get to with that method. I was pretty meticulous though with replacing old hoses/caps when I was trying to fix my misfire, but I have high hopes it is something dumb/easy to fix. If I can't find anything vacuum-wise, I'll probably pull the IAC Valve and clean it pretty good (and check the voltage/resistance).

 

Had a couple KOEO (23, 51) and three KOER (21, 23, 41). Switched my efforts to the ECT sensor and problem solved (severed wire). I ran out of time and didn't re-run the tests after I fixed the wire, but it's back to running fine again.

 

Hmmm, the computer thought the engine was always cold.