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Fuel Pump or Relay Problem

ken johnson

NAXJA Forum User
Does the fuel pump prime cycle time decrease with rail pressure or is the pump prime controlled electrically?. Please help I have a stranded 89 xj 4.0. Thanks!
umm...when you turn the key on the ecu primes the pump for two seconds then turns it off. I'm not real sure what you're asking though.
On my '89 it seems to be a function of pressure in the rail. If I repeately cycle the ignition key, the pump doesn't run every time. Does your stranded XJ have spark? Dirty CPS connector contacts are the most common culprit for no-start.
The fuel pump prime before start is controlled by the computer. Pressure has no affect on this. If your turning the key on for "2 secs" then off, then emmediately on again, I don't think the pump will run. I think there is a time-out function in the ECM. I know '90s Chevy ECM has a 10 sec time-out before it will cycle the pump again. Not sure about XJ computer since I recently replaced mine with a GM model.