96 Cherokee cutting out, what's left to replace

When I bought my Cherokee 22k miles ago it was not revving over 3k rpms. Between the previous owner and I we replaced the CPS, CAS, MAP, TPS, IAC, Rotor, Cap, Plugs, Wires, Coil, Fuel Filter and ECU. The Cam Angle Sensor ended up helping it now rev to 4k instead of 3k which seemed sufficient and I went on my way. Two thousand miles into ownership it started bucking and sputtering down the interstate. It felt like spark cut so I swapped out the Blaster Coil that the PO had installed with an Advance Auto coil and it ran perfectly... until this morning. This morning it started acting like it did 20k ago, coughing and stumbling and showing random weird tach fluctuations. I limped it home and installed my spare coil hoping that it was the same issue again. Upon test driving it started way quicker than it has for months, which gave me hope. However, it still stumbled a few times in the mile long test drive.

What can I test? I've got a small toolkit and a multimeter and a 45 hour drive to my home garage.

Another weird quirk that I've wondered about, possibly related. The Jeep charges really low. With the engine running but everything electrical turned off I get 13.7v at idle. If I turn on every possible electronic device simultaneously the voltage drops to 13.2v at idle. I had the alternator checked at two different auto parts stores and they both said it was fine. My theory was that I wasn't getting enough voltage to the coil and I am slowly burning them out.

The only other thing I've not checked is fuel pressure. I haven't done that yet, only because the stumble feels like an ignition issue, especially because the tach jumps and bumps around when the car bucks. A friend told me to check the voltage at the fuel pump to rule out old ChryMoCo wiring, which is probably a good idea.

Disconnect the catalytic converter in case the exhaust is choked?

MAP sensor hookup? (As in, make sure it is... properly)

Sounds like the fuel pump is on its way out. Do the symptoms get worse as the Jeep warms up? Does it have a longer then normal crank time then kinda sputters to life? Will the Jeep cruise ok but will cut out more under load or throttle tip in?

Symptoms don't seem to change in my 20 mile commute. Just random. It was cranking longer before I put a new coil in yesterday, but with the new coil it cranks faster and fires up quicker. I'm not sure on your last point after the coil replacement. Up until my newest problem it was fine under full load but acted like it had a hard spark cut at 4k.

that voltage output is normal on a Jeep of that era.  If you haven't ever cleaned all the wiring grounds, particularly the ones around the computer, it can't hurt. On my ZJ there's one ON the coil mount that can cause those kind of issues, but it's a completely different system. Still - if remounting the coil had some effect it might be a grounding issue at that point.

I've had distributors cause similar issues. Any play in the rotor shaft? Also bad grounds.

Good luck

Remounting the coil and cleaning the grounds did nothing. It's good about 5-10 miles then starts acting up. I think I'll throw a fuel pump at it and hope. 

If your tach is moving faster than your engine rpm then you have a crank position sensor or associated wiring issue, or a main power/ground issue to the computer. 

I tried the fuel pump because it's easier than doing the CPS and it was the original pump. Didn't do jack.

I checked all the grounds and connections on the engine bay side and everything's fine so I guess that my 5k mile CPS went bad? It's not a fun thing to replace, especially because all my extensions and wobble sockets are 45hrs from here. That or it's my computer? Ugg

One thing about Jeeps and a lot of others - only use OEM sensors. The aftermarket ones never last.

I found the factory spec for the CPS and it tested fine. I don't know if that's good enough, but it's not a fun task to replace if I don't have to. 

Well, if it turns out to be the CPS you can then add up all the cost and labor of all the things it wasn't, and then decide which approach is actually easier in time and dollars. It will be harder to prove that it's a cps issue than it would be to replace it. Without some kind of oscilloscope or datalogging it's tough since you can't do the multimeter tests of a cps when it is plugged in and running at many many cycles per second. 

So far the cost has been low, other than today's new fuel pump, and the labor has only been my time. I might phone a friend who's a snowcat mechanic and see if he wants to do some side work. A lift and tools make this so much better of a job.

The factory test was to check that there was no continuity been two of the pins which I verified. Not sure how scientific that actually is.

I feel like I'm going to drop a parts cluster bomb instead of firing a well aimed parts cannon.

A Jeep friend said upstream O2 might cause this problem on the 4.0?  Doesn't sound right to me. 

Eh, unplug it and see if it drives right. But I sincerely doubt it!! Unplugging the coolant temp sensor will also dump it into open loop where it will ignore the o2 signal, so that's another option. 

Hooray

Bought a new CPS. On the way home from O'Reilly it tried to start. Feels like ignition cut. At least it has fresh oil? Now it won't turn on in the garage. 

New Crank Position Sensor is installed. It fired up after install but only drove about 10 feet and now it won't start again. Lucky it's a manual without a clutch switch so I could at least move it a short distance into my parking spot. I don't have a clue what to do next.

I worked from the spark plugs back today. No spark at the plugs, dizzy wire has good continuity, 12v to the coil but no trigger signal to the coil. This tells me:

New CPS was a crib death

PCM is bad

Some wiring somewhere is bad

96 should be obd2 right? What codes it it throwing? 

I don't have my reader here(d'oh) and can't drive to a shop to have codes read because she won't start.

I was also reading that a bad Cam Position Sensor on one of these will result in a no-start. I'm going to go test it next.

The cam position sensor failed the test as outlined in the FSM. That's a good sign. Apparently the computer won't turn on if the cam sensor isn't outputting on these.

That didn't help. Damn. All sensors are new. Still no start. I'm running out of ideas here.

Have you checked the Cherokee secton on Jeepforum? I know in the ZJ section they had a step by step checklist for no start conditions. Theres a lot that can cause it and your frustrating situation is pretty common.  I imagine it's pretty similar between the XJ and ZJ with the 4L.

New CPS was a crib death

PCM is bad

Some wiring somewhere is bad

Well, your thinking is on the right track. You may want to have someone crank it while you wiggle wiring. I have had to do a wiring repair on the cam sensor wiring on a 4.0 Cherokee before. A scanner would be very helpful here. Even just an obd2 bluetooth dongle from amazon and a phone app might be cheaper than some of these sensors. 

Recently a student called me up and said that his 96 Tahoe wouldnt start. I went out, checked the codes and got one for cam/crank correlation, checked engine rpm through datastream and saw that it was only picking up small rpm right as the engine was stopping but nothing while it was cranking. I went down there and verified power to the cps connector and followed the wire back up the engine moving it around to see if it would help, but it didnt. I took the sensor out, smacked it around a little and waved a socket in front of it and could hear the fuel pump running when i did so, stuck it back in and it started up. He drove it home and took his wifes car to go buy a replacement sensor. If i hadn't had the scantool that whole process would have been a lot longer.