If your 1988 Ford F150 truck idles fast all the time what should you do?

Disconnect the throtle cable and see if it shows down. If so replace the cable. If not clean the idle control solonoid. If this doesn't work it might be a gasket leak between the upper and lower intake manifold. Try spraying WD 40 where they are bolted together, if the idle drops replace the gasket. Note, the inside bolt is reached from the top of the upper manifold, T-40 torx head. It is a long torx, Snap-on has one for 28 bucks.