I have a 1990 f 150 xlt 5.0 lt auto the truck idles fast

Here are a few things to try. You could be venting vacuum with a bad intake gasket. Use a spray bottle with water and go along the seams with the engine running to see if you can find any openings. The engine will change RPM if it draws in water.

Autozone or Oreillys will check the engine free with a scanner. Usually a TPS throttle position sensor will code a "check engine" light and a manifold leak won't. The TPS is like a carburetor and controls spark and fuel as it works. It is cheap and simple to replace.