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.
Assuming you have a tach.
Adjust the idle speed to 650 - 700.
The cold idle solenoid plunger should just be touching the throttle tab, see pic.
With a cold engine start, cycle the throttle WOT one time, that sets the cold idle solenoid plunger to idle speed 950 - 1000 RPM.
Adjust the cold idle plunger to achieve that. Adjust with a 1/4" wrench, screw plunger in or out.
When the pass side head reaches 120*F, the cold idle solenoid should de-energize and the idle should return to the 650-700 idle speed. The temp sensor is like under the alternator in the head. That sensor also controls the IP cold idle advance solenoid.