I have an 89 XJ with the 4.0. I bought a new fuel pump but it is bigger than the one it had. Fit in the tank but has a hesitation and sometimes backfire. Do I need to have the smaller walbro style pump or do I have to change something else?

 

Check the fuel pressure at the rail first to see if it is too high.

Backfire is usually caused by a burnt valve or raw fuel accumulating in the exhaust system. It can also be caused by incorrect timing, jumped timing, or distributor not clocked where it is supped to be. It can also be caused by plug wires not on the correct plug or terminal.

 

Listen. The correct pump should give you about 39# with the vacuum line OFF the regulator, (on the front of your fuel rail), and about 10 less with it on.

Unless you are in CA or HWI, it's free to borrow a tester from the auto parts store. If somehow the wrong, higher pressure pump is in there, the regulator diaphragm might brake. Idk, >> http://www.jeep4.0performance.4mg.com/tech_specs.html

 

Will check fuel pressure then.