1995 YJ 4.0: Runs great but randomly dies, then won't start for a while

kcorcoran said:

Random engine cut off then it won't start for a while.

I'm really at a loss here and can't think of anything else to check/replace.
My kid drives 35 miles one way to college 2x a week, That's as much as the jeep gets used.
Gotta get it fixed or we're playing taxi for the rest of the semester.


Here's what my son's '94 4.0 does currently.

• flip on kill switch
  • i wired a switch to the 1sr relay (i think, I have to look) which completes circuit to energize it so you can start the vehicle. had a 2-3 day parasitic drain I couldn't find other than when you would connect the battery, the first relay would energize and stay that way. the switch was easier that disconnecting the battery constantly.
• turn key forward
  • CEL comes on
  • fuel pump primes
• crank
  • started pretty much immediately, seems to run very well
• drive and die
  • head out, could be 2 miles away, could be 40, but without rhyme or reason the jeep will randomly just die. it's as if someone just turned the key off. there's no loss of engine power, stumbling, or warning. it just turns off. All lights, etc stay on.
• try to restart
  • CEL comes on, fuel pump primes, motor cranks and will not start.
• wait and wonder
  • try swapping around/out the ASD relay
  • disconnect the battery
  • wait a bit
• try to restart
  • CEL on, pump primes, motor cranks.. whoa it started! ... or damnit, it didn't start! (basically it's hit or miss but eventually it starts as I found after towing my son home from 6 miles away and got home and it fired up in the driveway.
• let it idle
  • another crap shoot. sometimes it will idle for 20 mins, then die. sometimes it will idle for 4 mins and then die. rando.

Here's what I've done... literally everything.

• last night I recapped the PCM since i do a lot of cap kits on classic arcade monitors and had the caps on hand
  • reinstalled, started right up (cold engine). idled for about 3 mins. died. So I did what I do above, sat down, searched the forums, grabbed a drink Came back maybe 20 mins later... started right up.
  • Idled for about 25 mins and then died. This time though when i went to restart it, it fired back up first time which it generally doesn't do.. but then it died again after idling for about 5 mins, wouldn't start after that so i closed it up and went to bed.
• in the last year since getting it on the road after buying it (running and driving) I've replaced....
  • exhaust manifold gasket
  • all new injectors, fuel pump, fuel filter, and soft line. Flushed the tank
  • new radiator, temp sensor (both), thermostat (currently out because it tends to run hot and haven't figured out why), temp sensor, fan clutch
  • oil pressure sensor
  • new air filter, coil, plugs, wires, cap, rotor, both pcv valves, camshaft position sensor/pickup
  • all new relays
  • all new vacuum line
  • new battery cables
  • crank position sensor
  • map sensor
  • meticulously cleaned intake, when i had everything apart
  • pulled the main harness connector form the firewall, clean everything up because I'm weird like that
  • pulled the PDC to check all the wiring and remove some of the old janky DIY aux lighting wiring

The vehicle has AC which I'm guessing is factory aftermarket as it looks like wiring was putt spliced in under the PDC similar to what I saw in a dealer install manual or something.


I'm picking up a spare PCM tonight just in case there's something else wrong with that pcb.

I wish there was a way to leave it running with some sort way to record signals to various sensors/systems so see exactly what's happening when it stalls.

I took some video to show how it's running and left it running to try to catch the gremlin. You can see that there's no warning, it's like someone just turned the thing off with the key.

Would appreciate any help to get this thing to not die randomly so my kid can finish his degree.

Click to expand...

Here are a few suggestions that might help pinpoint the cause:

1. Ignition Switch: Since the engine shuts off abruptly and all electrical components (lights, etc.) remain powered, this points towards a potential ignition switch issue. The contacts within the switch might be failing intermittently. It might be worth trying to replace the ignition switch if you haven't already.

2. Ground Connections: Check for any loose or corroded ground connections, especially around the engine block and the battery. A weak ground can cause erratic behavior in the electrical system, resulting in stalling.

3. Fuel Integrity: Since you've replaced the entire fuel system, check for any kinks or clogs in the fuel lines. Also, ensure the fuel tank doesn't have any debris that might intermittently block fuel flow. It might be worth conducting a pressure test on the fuel system to rule out any issues.

4. Crank Position Sensor (CPS): You mentioned replacing it, but if you have a suspicious one on hand, it can be worth swapping it out again, especially if it was a used or aftermarket part. Some sensors can fail intermittently, which aligns with your stalling issue.

5. Check for Codes: Ensure you're scanning for any trouble codes stored in the PCM, even if the check engine light isn't on. Sometimes, the codes might give you a clue about what's failing.

6. Heat Soak Issues: Since the vehicle sometimes runs fine before stalling, it might be worthwhile to consider components that behave differently when heated. The CPS and PCM are two worries; if you have the spare PCM, it could be a good diagnostic step to switch it out temporarily and see if it changes behavior.

7. Relay Connections: Double-check the wiring and connections to the relays you mentioned you swapped. Sometimes, a loose connection can mimic a failing relay.

In regard to recording signals while it's running, you might consider using an OBD-II scanner that monitors real-time data. Some advanced scanners let you record parameter values while driving, which could certainly provide insight during a stall.

Keep us updated on your progress and best of luck getting it sorted out for your son!