Starter is not engagine flexplate on 1990 Ford F-150. It will not line up with teeth.

Inspect the flexplate if its bent and has missing teeths if okay try shimming the starter close to the flexplate

A QUICK TEST WOULD BE REMOVE THE STARTER

INSERT "STICK" AGAINST FLYWHEEL

HOLD IT IN PLACE WITH YOUR HAND--TAPE IT ON--VISE GRIPS (THIS DOES NOT HAVE TO BE A STICK!)

REMOVE BATTERY CABLE FROM BATTERY, THEN HAVE A PAL ROTATE THE ENGINE MANUALLY (SEE MY PIC AT BOTTOM), WHILE YOU WATCH THE DISTANCES BETWEEN THE "STICK" AND THE FLYWHEEL

THE STICK SHOULD REMAIN TOUCHING AND EVENLY DRAGGING ALL THE WAY AROUND!

IF THE STICK GETS PUSHED FORWARD---OR SUDDENLY THE STICK HAS AN AIR GAP----YOU HAVE PROBLEMS WITH THE FLYWHEEL BEING LOOSE, WARPED, OR EVEN CRACKED/ BROKEN LOOSE AT THE ANCHOR BOLTS

RETURN WITH GOOD TIDINGS!

THE MEDIC

(PIC IS MY '77 JEEP CJ5)