rough running 95 Cherokee sport 4.0L

Friend of mine is working on a 95 Cherokee Sport 4.0L, automatic. When he got the Jeep, it would not run. He replaced the crank sensor and it now runs, but lopes badly at idle and backfires thru the throttle body and exhaust when revved up. Very sluggish pick-up.

He has replaced the crank sensor with 2 new ones from O'reilly (BWD brand), the MAP sensor, the temp sensor, the coil, plugs and wires, the entire distributor with cam sensor, and the computer. No change in symptoms.

Once in about every 30 starts it will run and drive perfectly. The vacuum is good, and the fuel pressure is at 39 lbs. He has seen the timing change by 30 degrees when revving it.

I advised him to go and get a different brand of crank sensor, but other than that, i am out of ideas. Anyone care to chime in with an opinion? Any and all ideas appreciated.

He might check the distributor. Could be loose or bad cam position, maybe timing chain is loose, there is a way to test with the distributor cap off, I think it basically Turing the engine by hand and seeing if there is slack before the distributor turns.

But I'd make sure to get a mopar cps, the aftermarket brands can be very hit or miss far as quality.

One the extreme end the computer might be bad, CPS is more likely but I wouldn't dismiss it if all else fails.

PS I think somebody here had problems with The O'Reillys house brand CPS, I think he got a second one that worked but I could be mistaken he might have got a mopar for the second cps

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Thanks for the reply, your thinking is the same as mine, even tho he has tried at least 2 additional O'reilly crank sensors. What is curious is that once in a while the truck starts and runs all the way to redline without a stumble. it is NOT throwing any codes.

He has checked the timing chain and it is okay. He has also re-seated the new distributor several times and even put it off one tooth in either direction to see if it made any difference. No change.

We were talking last night and he said that he had found a water leak at the heater valve. Since that is right over the sensor grounds, i pointed him in that direction. i also told him to drill out the cps mounting holes and try to re-position it.

We will dig at it some more this afternoon.

The truck was built in Aug of 94 and all the wiring diagrams say that the crank position supply should be 8 volts. We have a 5V supply and a .02VAC output signal on a digital meter. Anybody seen an 8V supply? We did find a high resistance ground at the dip stick stud and fixed that but it still only runs good on occassion. Mostly it lopes, shudders, and backfires all the way from idle to full throttle.

Subscribed, I just picked up a 91 2 door with similar problems.

check both the cam and crank sensor orange wire to ground. it should be 8 volts. the 8volts feed the crank, cam sensor and the speed sensor. it could be a bad connection or a bad sensor. try checking the voltage at pin 7 of the ecu if you can get a meter in there.

Well, after ANOTHER crank position sensor change, the truck does the same thing. If you hook a timing light to a plug wire, and rev it up, you can see the ignition dropping out. Just to recap, we have changed 3 distributors, 4 MAT sensors, 4 crank position sensors, 2 TBS's, 4 Iac's, 2 air temp sensors, and tried 2 different cam sensors. Every so often, the truck will start and run perfectly. If you re-start it immediately, it still runs fine, but if you wait 5 minutes and start it, it runs like crap.

The wiring and the pins on the computer all look good. I think we are going to go to our friendly junkie and get another computer, but throwing parts and money at it is not working so far.

I remember a topic on the forum detailing a Renix repair for splices in the big harness that runs along the firewall. i wonder if the 95"s had the same set-up. My thinking is that somewhere we have an intermittent loss of signal or ground. I looked up the 95 wiring diagram on my computer, and they all show an 8V supply to the sensors and we have 7.75V on that line.

i gather there is no check engine light? do you have a vacuum leak? check around the intake manifold gasket. also try wiggling stuff as its running. could be a corroded connector to a injector.

No check engine light, no codes, no vacuum leaks, don't think an injector would make the ignition drop out, just a giant puzzle.

when the timing light shows the ignition dropping does the rpm gauge drop to 0? or slowly fall.

not all the cylinders drop out at the same time. there seems to be enough firing cylinders to keep it running, but it is popping and banging out the exhaust. Then, after several starts, she runs like a top. I am convinced it is an intermittent open somewhere in the ignition circuit harness, but we haven't been able to find it.

Wiggling wires and connectors, unplugging/re-plugging sensors, and just flat out cussing at it does not change the symptoms.....

I'd seriously try swapping the ecu. Other than that it's probably something in the wiring harness. Just my feel for all the troubleshooting results so far.


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Has this motor been out recently? I had one that had the wrong flex plate in it, ran just like this.

Swap in a known good upstream O2 sensor. If that fails to improve things, then ...

Check the O2 sensor wires. Follow the wires from the sensor itself ... all the way up to
where they enter the hard black plastic protector that is above the fuel injectors.

Look for any signs of burns, chaffing, etc.

If that fails to turn anything up, the disconnect the O2 sensor, and bring the wire
harness up from under the car and lay it out flat on the radiator support and open up
the split-plastic convoluted wire cover and expose the wire harness. Look for any signs
of a repair or splice(s) that may have been done in the past. If there are any repairs
that was done using butt-crimp connectors - get rid of them (cut them off) - and splice
the wires together properly by stripping some of the insulation off, twist and solder the
wires together, and use heat shrink to protect and insulate the splice. This is the method
outlined in the FSM.

Butt-crimp connectors should be banned from the planet.

Well........I think we have a winner!



I send this along for everyone's general fund of knowledge. After replacing everything we could imagine would affect the ignition, we have found that the alternator is intermittent. On the later model XJ's, the alternator is a part of the trigger for the coil. So, when it is not putting out, ignition is hit or miss. When it is operating normally, the truck runs great.



Another lesson learned about haphazardly replacing parts. Oh well, now there are several spares......

Good find! Intermittent issues are the worst to track down... I was going to recommend checking all the grounding terminals too. On old cars like that surface between the terminal lug and body gets oxidized and you get all sorts of weird electrical gremlins as a result. I would clean those up and add a couple more grounding points in either case if you haven't already.

thanks, been there and done that..........