So searched high and low and can't find a strait answer. My Cherokee runs great when you start it up. After about 4-5 mins it starts running very rough to the point it almost dies but doesn't. I check for vac leaks, replaced IAC, map, plugs, wires, and nothing. What's weird is when it starts to run rough I can shut it off and start it up immediately and it runs just fine again for another few minutes. Plugs and exhaust seem to be a bit sooty. And my cat was messed up so I replaced that also. I am getting zero codes or check engine lite. Called all my buddies and no help, any ideas?

 

"And my cat was messed up so I replaced that also"
How was this determined?

No pending codes? What's the fuel pressure in psi?

 

I assume my fuel pressure is fine because if I give it some gas it runs fine problem is just at idle. My cat was bad because it was rattling and had broken bits inside

 

May want to do some diagnostic work before you sling anymore cash at it.
(You can borrow tools from Vatozone.)

 

Like what kind of diagnostic tools? I have done almost everything I can think of. It a brand new head also. Compression is good on all cyclinders what else do you recommend?

 

Just a hunch.... take a careful look at your exhaust system. Look for leaks when it's warm.

 

Sounds like when it warms up it's going in to closed loop and when it does, the oxygen sensors are coming in to play and the PCM doesn't like what it sees.


Inspect the upstream O2S wiring going across the intake manifold for chafing.


Inspect the downstream O2S wiring for chafing.


Check the O2S Heaters fuse in the PDC.



.

 

I would look at the O2 sensors too. I changed mine not to long ago and I made a big difference in the way that it was running. If you had to replace the cat you may have been running rich and dumping extra gas in the cat will cause it to break down. This could all be from crappy O2 sensors. Try to find a scan tool that will show the O2 sensor operation.

 

Assuming fuel pressure is good and knowing fuel pressure is good are two things. But given that it runs fine after you restart it I too believe it's likely your o2 sensors.

 

So i replaced both 02 sensors and got new champion copper plus plugs, also found that my vacuum line that goes to the vacuum reservoir over by the cruise control assemble was cut. That explains why my cruise control didn't work or the fans in my dash above the radio didnt work either. I am sorry to say I am not sure which fixed it but one of those things did.

 

That vacuum line being cut would not have caused your original problem, but good fix anyway.


The new O2S' probably fixed your problem.