Have you tried resetting the FIPL? Sounds like it may be time for a replacement FIPL. You may want to also try to pull codes just in case.
Click on the Diesel Files link in my sig and navigate to the 1992 folder, and then the DTC code prcedures link and DTC code definitions. This will let you pull the codes. Be sure to follow the directions to the tee for best results. Post back with code results, and maybe we can point you in the right direction.
As far as resetting the FIPL (some of you may recognize this):
Hook up the negative lead of the volt meter to the battery ground and the positiove lead to the center GY/W ( grey/white wire which is the FIPL signal wire, I use a paper clip up the side of the wire in the connector. The FIPL must be attached to the wiring harness. Engine low idle speed must be set to specs prior to adjusting the FIPL.
Engine off key on........
With a warm engine, key on, engine off measure the voltage at the GY/W wire, if adjustment is required loosen either the bracket with bolts or just use the slack in the Torx screw holes. Now gently rotate the FIPL until you get some where between 1.05V and 1.1V. then tighten the torx screws or bolts which ever you loosened ensuring no movement of volt setting . Rotate the throttle lever to WOT see if you get 3.8 V but not more than 4.3 V.
The FIPL mounting bracket can be moved but ensure it is centered for a good rotation. Any adjustment should be made by only rotating the FIPL. If a new FIPL is required, check tang engagement as you take off the old FIPL and then make sure that the tangs in the new FIPL engage the throttle lever properly.
The closed throttle voltage: 1.1 V, .........wide open throttle voltage: minimum of 3.8 V but not more than 4.3 V.
DIESEL FIPL SENSOR INFO
DATE 1992
REASON
Early Black FIPL has High Failure Rate
RECOMMENDATION
Always replace the black colored early FIPL or any colored FIPL having 60 thousand or more miles, with the late gray colored FIPL.
PART NUMBER
F2TZ-9B989-C