Jeep XJ Fuel Pump Relay
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.