I have a '97 Jeep Cherokee Sport 4.0l 4x4 w/ 160k. Jeep has run fine forever until recently. It misfired at idle originally but now it has turned into a misfire up to about 2500-3000 rpm then it seems to go away. The strange thing is a check engine light is NOT coming on. I have replaced the dist. cap, rotor, plugs, wires, camshaft positioning sensor and also ran Chevron fuel treatment through the car.....no help.

When it is cold it is really noticeable and it will "chug" along on acceleration until about 2000-3000 rpm and then it seems to smooth out. The only thing I haven't done is a compression check but that is next. I am afraid I may have a bad valve. Any advice would be greatly appreciated.

 

You may need a new coolant temperature sensor. Thats what I needed one time with a somewhat similar problem of missfire. Only mine misfired during idle real bad. But if you have fuel and spark than it is something that controls the mixture of the two. Such as an O2 sensor or IAC or coolant temp sensor. When I say CTS, i mean the one in the thermostat housing that links into the ECU. Not the sensor for the gauge.

 

That's actually funny that you said that because on the way home I noticed that my temp. gauge on the dash was not reading any temp. at all, it was pegged at cold. When i turned the heat on the gauge went up to normal temp range, then it would go back to 0 again. Another new issue......Jeep will not start unless I floor the accelerator...? Any thoughts........

 

changed the coolant temp. sensor and it did not fix the problem. Still runs real rough at cold idle and misses through all rpm's. Still no check engine light.......HELP

 

Hi,

I have a 97 cherokee and it is doing something very similar to yours. Between 1 and 2000 revs it is rough - feels like it off at least one cylinder. Once you accelerate above 2000 it smooths out.

I've replaced spark plugs, rotor arm, distributor cap, leads, compression tested okay......still having problems. The idle is really lumpy too.


This is the second engine i've had in this jeep. The first cracked a piston (it was cheeper to replace the engine than the piston - UK prices suck) the low compression had similar symptoms to this but this time I know it's not compression.


Any advice greatly appreciated. I'm sure it's a sensor somewhere.

 

Do a leak down test...you might have something up with your valve train...or head.

My jeep had a miss at idle i couldn't ever get rid of even after a FULL tune up.

I removed the head to do lifters....and found the head cracked in 2 spots on the exhaust side.

 

It turned out to be an O2 (lambda) sensor failure. In fact both sensors had crapped themselves.

As it turns out the ECU adjusts, not only the mixture but, the timing based on info from the O2 sensors. The Jeep was running constantly in open loop mode. Which was fine on the open road but lumpy as hell round town.

Open loop over-injects fuel so the fuel economy was in the toilet as well. About 50% of what it should be.....really, really bad instead of just ****ing terrible.

I worked it out during a long decent where the engine was doing next to no work. At the bottom of the slope the engine was particularly bad....it could only have been one thing.

I've since changed the O2 sensors, the CAT and the exhaust ..... massive difference.

 

coil pack alignment. Make sure your at TDC and align the hole in coil pack and distributor. My 97 was doing the same thing. It fixed it. Do you have spark knock like it's out of time?

 

You realize you're offering advice and asking a question in an eight year old thread? I suspect it's no longer an issue one way or another.