im having this wierd problem that i just cant figure out. car starts up and idles fine at around 750rpm. Warms up fine, i take it for a ride at highway speeds for anything more than 10 min and when i slow down and exit it off, the idle goes up to 1200-1500 rpm and stays that way till i shut it off and let it cool down completely. It only happens after highway drives. if i stay in the city/low speeds its fine.

 

Quick and dirty guess is that something is sticking on the intake. Start by looking there.

 

1st check to see if the throttle and cruise control cables have some slack in them to the throttle cams, there should be a very small amount of slack in them.

next feel the coolant lines to and from the thermowax, if one side is hot and one side is cold then you probably have a blockage in your thermowax or BAC junction which is keeping the thermowax cold while the engine warms up and doesn't need assistance from the thermowax to hold the idle up(ie it is always treating the car as cold and helping it idle).

 

+1 on the clogged BAC hoses, my last two FC's have had really badly clogged thermowax's, and hoses and BAC valves.