bad connections, bad wiring, bad relay, mechanical pump failure, intermittent signals etc etc
lots of things.

 

Could bad fuel pressure regulator cause a considerable decrease in fuel milage? I just went camping with a friend and his cherokees setup is basically identical to mine, close enough I would imagine we should be getting close to same same mpg and we both fueled up at the same place. When we got to camp I had used half a talk and he only used a quarter of a tank

 

Could bad fuel pressure regulator cause a considerable decrease in fuel milage? I just went camping with a friend and his cherokees setup is basically identical to mine, close enough I would imagine we should be getting close to same same mpg and we both fueled up at the same place. When we got to camp I had used half a talk and he only used a quarter of a tank

 

Yes absolutely. Your engine runs more efficiently when the mixture is right. If it's off because of a bad fuel pressure problem you'll be less efficient and therefore worse mileage.

 

Is there a way to test the fuel pressure regulator to see if that's it's before I drop the tank and replace it?

 

Is the regulator in the tank on your XJ? It's on the fuel rail on mine. You actually don't need to drop the tank if it is. That access port on the front of the tank can be done with the tank in place. But a line pressure tester should be able to give you answers on that. I'm afraid I'm not experienced enough to tell you what numbers to look for in that regard though.

Have you also verified your injectors aren't all gummed up?

And your fuel filter is new I presume? I'd have changed that first but I don't know if you did or not.