I finally solved such a problem this week. Took about 2 years. Mine is a 1989 4 cylinder 16 valve 5 speed. First repair was a short in the wiring. Second repair was replace sending unit (using a NAPA replacement). My pressure gage would work when engine cold and idling, but was erratic (pressure would drop when accellerating, pressure return when coasting) until engine warmed up, then showed 0 pressure all the time when warm. Solution was to replace the wiring harness for the sending unit and a foot and a half or so of wire with a salvage harness from a mini van of the same vintage. I am delighted to have pressure reading again. Darn scary to show 0 pressure, and the sending unit is very difficult to get to on the 16 valve engine. The guy that figured it out is Jim Welch and/or Dan (last name unknown), (785) 286-2097 (shop number in Topeka, Kansas) if you want to ask him what he did.