So my 91 XJ 4.0L started to run like crap awhile ago and cant figure it out. When i first start it up it runs great. After about 5-10 miles it will start to hesitate and backfire. It seems not to want to go over 3000rpm. if i shut it off for about 1 minuite it runs fine for like 5 more miles then the same thing all over.

It has brand new plugs, cap, rotor, wires, TPS, coil, o2 sensor, fuel air filters. also has no CAT.

Right now im at a loss any help would be great.

 

Check your CPS wires. If they're close to your exhaust manifold they can cause it to malfunction once your jeep heats up. You can tell if they're being effected if the wires are bare and plastic coating is missing.

 

I just checked the wires they look good plastic wrap is still on the wires too.

 

Will it not go over 3000 rpm out of gear? Or just while driving?

 

Just while driving.

 

Check the easy stuff first. Plugs, wires, distributor, battery, air filter, then check your brakes to make sure you dont have a seized caliper.

 

All that has been replaced. I checked the brakes not too long ago they were good.

 

More the reason to check it then. Might have a plug wire come off, or got a random ****y distributor.

 

I should check them again but everything has been changed less than 500 miles ago.

 

Anyone else have any ideas?

 

What's your fuel pressure? Check when cold then after it heats up...... How many miles on your jeep?

 

Also is it hard starting at all??

 

maybe the exhaust is blocked? my FSJ lost all power once and just wouldn't rev or anything. wouldn't even make it up a small hill on my farm, turns out squirrels had filled my exhaust and muffler up with nuts.. lil bastards

 

LOL! That was actually going to be my first suggestion, same happened to my xj when i first got it, except mine was a clogged cat.. But like he said, his cat died so he had to remove it

 

THIS^^ needs to be done soon.

When the Rooster crows, you need to wake up and listen.