Monday, August 11, 2008

Jeep XJ Fuel Pump Relay

I have been dealing with a couple of Jeep XJ Cherokees lately that have had intermittent fuel delivery problems. The first one was Jenny's 92 XJ. Occasionally on hot days her jeep would appear to vapor lock. While vapor lock was common on systems where the fuel pump was at the engine and pulled fuel from the tank, the pressurized system of the Jeep should be immune to vapor lock. Jenny's Jeep would also occasionally read low fuel pressure ont he fuel rail and then at other times read the correct fuel pressure. Bypassing the ballast resister would sometimes cure the low fule pressure problem.

Next my son's 91 XJ began havign trouble starting. It would sometimes fire then stumble an ddie only to start right back up. At others times it would simply spin but not fire at all. Testing the fuel pressure at the rail would show fuel and checking spark gave me quite a jolt. But it simply would not start. Sometimes it would seem to start while holding the key in start but as soon as the key was released to run his Jeep would die.

The most annoying part was that after he would call me or my dad to come rescue him, we would get there to check it out and it would fire right up. Pull it in the shop to test it and it would start and run fine every time.

Finally, one day it failed to start while I was there and i had a chance to keep experimenting with it. After quite a bit of head scratching, I found that the fuel pump relay was not always making contact. I swapped his cooling fan relay in to the slot to get him home while I stopped by the parts store to get him a new relay. I picked up one for Jenny's Jeep while I was there.

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