Ok, I’m beating my head against a wall here. I have a 93 F150 with the 5.0 in it. It idles very high, around 1300-2000 in park and around 1200 in gear. When coming up to a stop it sounds like it’s gonna do fine, then downshifts and stays at that RPM. Here’s the kicker, with the engine running, if you slam the door or hood the rpm’s go up!!!! It also gets terrible gas mileage. I’ve changed the plugs, wires, distributor, rotor, Idle air controller, cleaned the throttle body and replaced the gasket. I’m at wits end guys, any help would be awesome!!
With that terrible gas mileage and high idle I'd consider the possibility of a faulty ECT sensor. If the computer thinks the engine temperature is really cold it's going to add a lot of fuel.((Poor gas mileage and high idle)).
Maybe consider investing in a scanner. Even with OBDI they can be helpful.
One good one for yout old F150 is the Actron CP9690 Elite Auto Scanner. It comes in a kit with OBDI cables.
It's good for your old pickup truck with OBDI and you can use it on newer cars with OBDII.
((This is OBDI data being displayed))
Have you checked for any OBD1 codes? Checked for leaks in those 30-year-old vacuum lines and gaskets?
