My 1991 Ford F150, when warmed up after about 10 minutes, the idle will increase to around 2500 rpm's. I can't figure the problem out

My 1991 Ford F150, when warmed up after about 10 minutes, the idle will increase to around 2500 rpm's. I can't figure the problem out - 1

Added (1). My 1991 Ford F150, when warmed up after about 10 minutes, the idle will increase to around 2500 rpm's and will not come down to normal idle. I can't figure the problem out?

Which engine and carburetor? That year was produced with more than one.

You should check the wires to the following sensors, and the output of each sensor in response to what it is measuring: coolant temperature; exhaust oxygen; inlet manifold air flow; crank angle; throttle position. Any or all of these could be sending wrong signals to the computer, causing it to boost the idle; it's probably NOT the computer itself, just the sensors or wiring. There could also be an idle air controller and/or cold start fuel injector malfunctioning. Also, check the basic mechanical accelerator pedal, carpets, and throttle control cable to make sure they are not sticking (preventing a return to idle when you take your foot off the gas pedal).

When was the last major tune up? Have you done any work or does a shop do it for you?

Vacuum leak.