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Do you mean it will not start?or it will not turnover(Crank)???
You need fuel pressure,Spark,Air and Injector pulse for it to run.You need find out which you are not getting.I will give you the Fuel pressure specs below.To check injector pulse you will need a noid light(pictured below)Also check for spark at a plug wire.let us know what you come up with.Is there a check engilight lit? if so you need to retrieve codes from the computer.
For your 1988 FORD TRUCK F250 3/4 TON P/U 2WD:
FUEL SYSTEM PRESSURE 30-45 PSI(POUNDS PER SQUARE INCH)Key on Engine off .
The pressure regulator reduces fuel pressure to 39-40 psi (269-276 kPa) under normal operating conditions. At idle or high manifold vacuum condition, fuel pressure is reduced to approximately 30 psi (207 kPa). Check to make sure that the Vacuum hose to this is not full of fuel!It's located on the fuel rail.