Been a while since I've been on, hope everyone is well. I have a 1988 F150 Reg. Cab, Manual 4x4, 302 EFI, Manual 5 speed. Got it about 2 years ago for my wife, she wanted something with a stick. About 109,000 on it, very good shape over and well taken care of by previous 1 owner.

Lately it has been doing the following. Start it cold, idles up as it should and settles back down to around 750-800rpm. Once you get it up to operating temperature and come to a stop, it will idle up around 1800-2000 rpm. Once it sets overnight or until it cools down, it will start up and idle fine, until it gets to operating temps again. If I unplug the AIC when its idling high, the engine will drop down and die, unless I stick a feeler gauge between the idle stop and the lever.

I haven't tried to pull codes, but no check engine light is on. So is it likely AIC sticking open, bad TPS, or possibly bad ECT sensor. I am going on the thought process that if a vacuum leak was causing the high idle unplugging the AIC wouldn't cause much change.

 

Yes more than likely it sounds like either the TPS, ACT sensor or the ECT is bad.
Or as you know it could be the wiring to these three sensors.
But I have seen the Computer do this also.

 

Thanks Bill. At least I'm on the right track. I'll do some more diagnostics this weekend and reply with my findings.

 

Might want to adjust your idle air also. My manual doesn't like when it is too low. Should idle low but still idle with IAC unplugged. ECT or related wiring gets my vote.

 

The idle air is not adjustable in a 1988 truck, it is controlled by the IAC valve.

 

GGraves, did you ever get this worked out?

The spot where the feeler guage went is what I was refering to. On both of my trucks that is an adjustment screw. Mine like it best what that is set to 600 rpm or so.

 

No, I am actually going to work on it this weekend. Been covered up at work and out of town over last couple of weeks. I'll post an update next week. Thanks again for the help.