1991 XJ WILL NOT RUN

hey guys so my buddy has a '91 xj that will not stay running. some history 4.0L/AW4/231 has 55k on it. he's second owner-bought it from the navy. it was an on base emergency vehicle maintained perfectly with all the records.

so you start it and the jeep runs for a few seconds then dies. if you stay on it 3k or more rpms sometimes it will keep running (this is very rare though). brand new fuel tank and pump you can hear functioning, new crank position, new IAC, new TPS, checked relays-ASD is good etc.
ALL PARTS ARE BRAND NEW OEM MOPAR-he manages a chrysler dealer parts department. he also tried to swap a computer from another good running 91 xj federal emissions at my suggestion and no difference.

so i know it gets fuel pressure at the rail consistently, throttle body functions-it gets air, that leaves spark. obviously distributor is good because it momentarily runs.
-i need to try swapping the coil
-brand new CPS could be faulty?

any ideas you have are appreciated. this is everything i could think of

I had the same issue with mine the other day. Mine ended up being a loose wire on the ballast resistor. Don't quote me but if the connections are good I THINK it's safe to run a jumper wire between the two orange wires to test if it's the culprit.



The ballast resistor is what the two orange wires go into next to the airbox.

Post #2, yes you can run a jump wire, bypass the resistor.

OP, no mention of checking spark plugs, wires, rotor or cap?

Fuel pressure test with a gauge?

totally forgot to check ballast resistor. not the first vehicle that i've worked on with that little guy as the culprit.

Ecomike said:

Post #2, yes you can run a jump wire, bypass the resistor.

OP, no mention of checking spark plugs, wires, rotor or cap?

Fuel pressure test with a gauge?

Click to expand...

forgot to mention-plugs, wires, cap, rotor were all replaced a few months prior to the issue. double checked wires-no arcs, pulled plugs-all still clean as they should be.

i'm going to poke around with a test light tonight and try to jump the ballast wires

Haven't seen any actual fuel pressure numbers. Gotta rule it in or out early in the process regardless of whether the fuel pump is new; it isn't unusual to have a new part fail prematurely. For a 91, it should maintain 31 psi at idle. Must get actual pressure numbers with a fuel pressure gauge.

Also check the "quality" of spark. You are looking for a strong, BLUE, snapping spark. Yellow/orange/white indicates a weak spark which may not be strong enough for the engine to run properly. A weak ignition coil is the usual cause of weak spark.