I have a 1987 jeep xj with the 4.0 I6 and when I bought it, it's been having a problem. It will start up and drive fine, but after it gets warm it will buck, stutter, hesitate, backfire through the exhaust, and misfire but it wont shut off. The plugs will gas foul after a while like that. I've replaced the fuel filter, fuel pump, injectors, cleaned the tank, changed oil twice, redid plugs and wires, checked engine and gas compression, both good. Found one bad 02sensor, fixed it but that didn't help. Redid all the exhaust from drop pipe on. Replaced the map sensor, cleaned the throttle body, have a feeling the thermostat is out and it may be overheating, but dont know for sure. Please help! This problem wont go away and would love any ideas

 

You can try changing the coil I've seen them go bad that way , they will start losing spark when hot .

 

Go to www.cruiser54.com and complete Tips 1 through 5 before doing anything else.

No cap and rotor replacement?

 

I took it to a shop, they said the distributor had oil in it, and that the harmonic balancer was off, would this cause my problem? How do you test a coil?

 

We have done cap but not rotor

 

When you get it back, do the Tips.
Harmonic balancer will not affect running.

Make sure the shop does tip 13 when installing the distributor.