The poor thing can't catch a break.

The problems started by purchasing the vehicle, naturally.

It's a 1995 XJ 4.0 Automatic.

I've replaced just about everything related to the engine, including the engine. And yet I have some funky problems still. One day it idles nicely for me, and the next it's very shaky and I can hear all my door panels rattle. One time it will start up fine. Then randomly it'll struggle to exist on startup, then eventually it'll catch back up to a normal idle.

Here's what I've replaced:
- Spark plugs x3
- Ignition Coil
- Plug Wires
- Battery
- Distributor cap and rotor
- IAC Valve
- TPS
- Thermostat
- Engine temp sender
- Radiator
- Engine (first one blew up, replacement was at 210k miles)
- Fuel Pump
- One bad injector
- Fuel filter
- Fuel Injector Screens
- Engine mounts
- O2 sensor
- Catalytic converter

Here's what I've checked:
- Fuel Injectors click normally
- Strong spark at each plug
- Compression at about 140 across the board, no more than 5 psi variation
- Fuel rail pressure okay
- Can't find a vacuum leak
- Throttle body is clean

The engine runs damn near identical to the old one before it melted a piston. Mildly rough, with random instances of a struggling idle upon startup that smoothes out.

Some of the roughness could be attributes to the Rusty's off-road engine mounts but I didn't get the poly ones. Something just feels off about the idle. And the jeep trying to die in the middle of the woods doesn't make me feel very nice.

What am I missing? I'm about ready to sell this thing.

I appreciate any help. If there's any good Cherokee techs around Portland Oregon you could recommend as well Id appreciate it.

Or I'll buy someone a case of beer if they want to swing by and tell me if the idle is normal.

Or just bring your jeep by and drink beer. Whatever floats your rubber ducky.

 

Did you replace the crank sensor? It is on top of the bellhousing. Two bolts and a plug.

 

Not yet, but I think I've read that it basically only has two options. Jeep not starting at all, or running fine.

I'll look to see if there's any good ways to test it.

 

I hesitate to agree with the CPS (crankshaft positioning sensor), I did notice it is not on the replaced parts list. It is a reasonable part to replace, and it may solve all the idle/ running issues.

Side note, if you are saying you went with rubber mounts, you would feel less vibration over the poly. I prefer rubber. 🤷‍♂️👍

 

I'll replace it and report back. Thankfully that's an easy task

 

your replacement engine, what year is it?

 

Hey Bruce,

One year back, a 1994.

 

this I got off of co-pilot...some may apply, some may not...could help you diagnosing your problem
Yes, there are some subtle but important differences between the Powertrain Control Modules (PCMs) in the 1994 and 1995 Jeep Cherokee 4.0L models.

🔧 Key Differences Between 1994 and 1995 PCMs

• OBD Transition:
  
  • 1994: Uses OBD-I (On-Board Diagnostics version 1), which is simpler and less standardized.
  • 1995: Transitional year for some Chrysler vehicles. While still technically OBD-I, some 1995 models began incorporating OBD-II-style connectors and partial readiness for OBD-II protocols. This can affect scan tool compatibility and sensor behavior.
• Wiring and Connector Changes:
  
  • The PCM connectors and pin-outs may differ slightly between the two years. Even though the engine and transmission are the same, the wiring harness and PCM interface might not be plug-and-play interchangeable.
• Calibration and Programming:
  
  • The PCM software calibration may differ due to emissions tuning, sensor input handling, and idle control strategies. Swapping a 1995 PCM into a 1994 may result in drivability issues unless the harness and sensors match.
• Sensor Compatibility:
  
  • While most sensors are physically similar, the way the PCM interprets signals (especially from the crankshaft position sensor and throttle position sensor) may vary slightly.

 

AAA should be mandatory to purchase when you own a jeep.

Anyways, the Jeep decided it had enough freeway driving and stopped running on the interstate while I was on my way to visit my parents. Since I was closer to them than my place I had it dropped off there and did some digging around. I replaced the crankshaft sensor thinking it was that. Turns out it was the engine control 1 fuse. However it does seem to run a little smoother now lmao.

I've replaced the engine control fuse a few times now. It blew four times in a row a month ago, and so I replaced the fuel pump, and it ran fine for awhile. And now it blew on the highway, and when I put a new fuse in, I made it 10 miles and then it popped again. So, the fuel pump wasn't the problem after all I suppose.

It seems like this fuse is joined to the Guage Cluster, the O2 sensor, the coil and the fuel pump. I can't imagine it being the Guage Cluster or I'd probably smell it pulling 30 amps, so that leaves the coil and O2 sensors I suppose.

Has anyone ever tried throwing some fuses in line with each component to see which one blows first? Maybe a 20 amp fuse to each?

Tracing down intermittent electrical issues is never fun.

 

There is a third option.
Running fine...then running like crap before dieing. It would re start and run if I gave it 20 minutes to let the CPS cool.

 

sometimes the O2 wire insulation melts near the exhaust manifold leaving only bare wire and a possible electrical short. Check the entire length for any issues and correct. May be your gremlin.

 

Alright I checked a few things and I'm lost again.

O2 sensor wires look good to me. I checked the length back to the plastic shielding on top of the engine.

I replaced the coil again and gave it a test drive. It made it about three miles, and then when I floored it after four or five seconds it blew the fuse. I moved my happy butt over to the side of the road and swapped the fuse with the spare I brought. Engine started right up and I barely touched the throttle and pop, fuse is gone again. I waited five minutes, replaced the fuse again with the ABS fuse, and then let it idle for a minute before I barely eased into the throttle and managed to drive it the three miles home.

Once I was home I tried playing with the throttle to see if it would blow at a high rpm again without any luck. Maybe it has to be under load... Or when I'm driving somewhere inconvenient.

The temptation to replace it with a 50 amp fuse and then follow the smoke is real. Or fuse every component off the engine control fuse.

How would one normally start trying to eliminate variables with an issue like this? I feel like if it were a shorted wire it would happen at idle. If it were a bad component then replacing the fuel pump, coil and 02 sensor should have done it.

I'll have to find a wiring diagram and start pulling this apart like spaghetti.

 

You should replace the fuse and while the jeep is idling, get the hood open and start wiggling/pulling/manipulated the wire harness on the passenger side of the engine. This seems like a similar issue i had years ago where I would lose spark intermittently and it wasnt the cps. I did this very thing and found and plit wire in the harness to the coil that under rest it made enough connection to still provide power for the coil but under some wiggling I narrowed it down to the small section and after replacing that front corner section of harness I was over the issue completely.

It also hasn't been mentioned in your troubleshooting about inspecting quality of grounds. IIRC there are 2 ground wires together on the block where the dip stick is secured via stud, there's the 2 ground wires at the base of the coil and then also the ground wire from the battery neg immediately to the inner fender well. Theres also 2 more grounds on the driver side just a few inches closer to the brake booster from the PCM also on the inner fender well

 

I'll give pulling on the harness a shot. Hopefully that shows me where it's shorting out. I'll report back the next time I'm around it.

I cleaned all the grounds when I replaced the engine with a wire brush pretty well, especially the one by the dipstick. I did not, however, scrape the paint off where the grounds connect to the chassis like I've seen recommend.