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

kcorcoran

kcorcoran

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Joined
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AZ
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.
 
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!
 
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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...

95 2.5l Jeep Wrangler with AC - Years back my Jeep would randomly cut out. Lights, radio, everything still worked. Normally happen on the freeway. Before it loss all speed it started to run again. Ended up being a bad coil. Replaced the coil and it stopped cutting out. good luck
 
BillB said:
95 2.5l Jeep Wrangler with AC - Years back my Jeep would randomly cut out. Lights, radio, everything still worked. Normally happen on the freeway. Before it loss all speed it started to run again. Ended up being a bad coil. Replaced the coil and it stopped cutting out. good luck
Click to expand...

I replaced the coil when I did all the ignition parts, plugs, wires, cap, rotor, pickup coil. The original one was cracked and arcing to the motor. Maybe I got a defective one. shrug.
 
I would wait and see how the replacement PCM goes. I had a spare PCM I used for years that went bad recently. It now just cranks and cranks and will never start. Swapped back to my old pcm and starts up fine. Not exactly the same symptoms as you, but it goes to show that the PCM can cause other problems and fail in other ways than just the typical alternator regulator gone bad or capacitors needing replacement.
 
Swapped in known working PCM, started up (like it already had), ran for 5 mins and just stopped.

  • map sensor - 4.9 v
  • tps - 4.7 v
  • cps - 7.4v

since there's no signs of WHY it stalls or warning, or conditions that i can replicate i feel like i literally need to hook up some sort of diagnostic computer to the f'ing thing and record all the things doing on until the point it stalls to see what is causing the stall. i was reading about some adapted to hook to the port on a YJ and run some open source diagnostic softeware that will record all the engine shit. going to try to look that up tonight.

at this point every shingle thing on the jeep has been replaced. if anyone can think of something i should replace that coudl cause intermittent random stalling at speed, at idle, in the heat, in the 40s-50s, during the day, at night, on a train, in a plane, in a house, with a mouse... i'll f'ing replace it.

like i siad.. truck starts... runs... then doesnt.. then starts... or it doesnt. wait 30 mins, starts again. rinse and repeat.. it's the most frustrating shit ever.

if anyone knows of a more active YJ forum where there's more folks with ideas, please let me know.

i absolutely have to get this fixed by this week and it's stressing me out. :)

found the powertrain diagnostic procedures manual last night but dont know how successful I'll be without a DRB II to hook up since a lot of steps call for that.
 
Last edited:
I didn’t see in your list of repair attempts if you checked and/or cleaned out the pcv on your 4.0. This could also give you the same issue if it clogs and starves the engine for air.

It’s kind of tough to offer other advice since you have done so much but I would also go back to the coil you installed and check it and everything it connects to. Especially since your old one was cracked and arcing. It’s possible there is a greater electrical issue at play there with your wires or grounds. The part could also just be an unfortunate bad replacement.

Lastly, check and or replace all of your cables and grounds if you can (like with a big 7 type replacement kit). Oftentimes, the cables can be bad way up under the insulation and we can’t see the issue but it can cause sporadic engine trouble.
 
Ghostwriter623 said:
I didn’t see in your list of repair attempts if you checked and/or cleaned out the pcv on your 4.0. This could also give you the same issue if it clogs and starves the engine for air.

It’s kind of tough to offer other advice since you have done so much but I would also go back to the coil you installed and check it and everything it connects to. Especially since your old one was cracked and arcing. It’s possible there is a greater electrical issue at play there with your wires or grounds. The part could also just be an unfortunate bad replacement.

Lastly, check and or replace all of your cables and grounds if you can (like with a big 7 type replacement kit). Oftentimes, the cables can be bad way up under the insulation and we can’t see the issue but it can cause sporadic engine trouble.
Click to expand...

Oh yeah sorry. if i remember there's like 2 of them. i literally went shopping and was replacing anything that was dried up, dusty, cracked, etc when I pulled everything apart to do the exhausht manifold. I'm the type that feels like if I'm taking it all apart, nothing is going back on unless it's clean or new. lol.

I didn't check the IAC but after all that I pulled up the PDC and got rid of the wierd wiring hack with the extra relay the PO put where the ABS syst power relay goes. they spliced that into the starter relay which was getting constant power 24/7.

i replaced the CPS with a NTK one and even though I didn't have the oblong hole on the tab I put the paper spacer in there anyhow.

replaced the CPS harness connector, pulled all the grounds to check them even though i knew they were all done in the past year.checked continuity on all the connectors, coil, pickup coil, tps, IAC, the other one next to it.

buttoned it all up and it started first show and ran smoother that ever. Honestly wondering if that wierd hack wiring job was pushing all kinds of electrical gremlins through the system.

so i figured I'd change the ign switch and turn signal switch and cylinder because key was hard to turn and binding that it wouldnt snap back after starting.
upper shaft bearing was disintegrated so ordered a rack kit for the top bearing and a lower bearing. then realized trying to do the ign switch on top side of the column as well as dicking with the turn signal harness was stupid so I pulled to column.

too bad I dindt read the service manual that warns you 10 times not to bang, hammer or jolt the column during service because getting the lower bearing off had me tapping and banging the lower shaft. finally got it all back together and in the jeep only to realize the column shifter detent/lock that keeps it from jumping out of park no longer worked. Clearly I moved somethign inside that misaligned and rendered the shifter gate useless.

FML.

luckily the guy I got the spare PCM from had a complete tilt column for $250 so I grabbed that. not I will VERY CAREFULLY replace the switches with my new stuff since the ones on the new column are pretty weathered. bearings and rest of the wheel are tight so not doing shit else to it but cleaning and painting and placing it back in place like i'm putting the final card on a 10 tier house of cards. lol

i'm sure this won't be the last of the issues but there's not much left to cause issues. knock on wood.
 
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