Recently my Cherokee has been misfiring when started from cold. Once warm it won't skip a beat. It was not a gradual thing, I parked it one day and the next it was misfiring. I did some searching on this and found that the upstream o2 sensor is a common culprit. I decided since they are relatively cheap and I'm sure it could use it anyway I just replaced it instead of testing anything. Still misfiring. I've checked the fuses that seemed to be related to the o2 sensor and they all looked fine. I have a check engine light for evap leak (been there for months) and I've gotten Cylinder 1, Cylinder 3, Cylinder 4, and Random misfire codes. No rhyme or reason to the codes, I've reset the codes a couple times and get different cylinders so I don't think its injectors, spark plugs or anything like that. Seems to be something in the control of fueling. I can smell raw fuel in the exhaust when it is misfiring, but runs fine when not misfiring. Any suggestions or ideas?

 

Possible vacuum leak. Seals when warm.

But something else is niggling at the back of my head. When the engine is cold, the fuel adjustment is in a different mode than when it's warm. Closed-loop vs. open loop adjustment, IIRC.

Does your vintage have two temp senders or one? I seem to recall that earlier Cherokees had two, one fed the dash gauge, and one was used for engine control. Then somebody figured out, "Duh, they are both telling the computer temperature, and the computer does two completely different things with that info. Why not just have one sensor and let the computer do both things with the same info?"

If yours has two, and the one that controls engine function is kaflooey, you could read correctly on your dash but have incorrect engine control that is temperature sensitive.

 

I have a new coolant temp sensor, and when it was bad it didn't cause any running issues, just annoying that it was wrong. If there was a vacuum leak I would imagine it would be running lean not rich since there would be more air getting in than the computer senses.

 

Vacuum leak fools the O2 sensor, so the computer dumps in more fuel.

 

what ign system? mine did the same thing when the coil was on the fritz.

 

Factory coil, but the cap, rotor, wires and plugs are fairly new and still seem to be in good shape.