Assuming you have a tach.Mine running about 800RPM hot and cold, day and night. Don't see any difference of idle with the engine temp. Probably fast idle solenoid how do you diagnose that ? Truck is 1991.
There are times I am glad it doesn't raise the idle. Like when Diesel is over $3 a gallon!! LOLThat's because the fast idle solenoid isn't strong enough to push the throttle lever.
Ain't that what I said in the above post?750 to 800 when cold but I have to tap the throttle to get mine to kick in.
once its run for about a minute or 2, it drops back to 600-650.
With a cold engine start, cycle the throttle WOT one time, that sets the cold idle solenoid plunger to idle speed 950 - 1000 RPM.