Intermittent Stalling/Dying While Otherwise Driving Fine...Restarts, Drives, Randomly Dies Again.

[BlueDove91]

(Truck: '91 MJ 2wd 4.0 HO, SB Base)

SOLVED:

I want to make this post to add into the pool of what is already available for those who are experiencing a similar issue and trying to troubleshoot it. 

 

Several weeks ago, I was driving home going around 40 or 50, when suddenly my truck just shut off. The engine just stopped. I lost all acceleration, but the power was still on. So I pulled over to a stop, turned the truck off, waited a few seconds, and cranked. It started right up. Scratching my head, I shifted into first and pulled back onto the road. I hadn't gone 20 feet before it died again. This time I tried to restart it, and it would crank but not turn over. I shut it off, waited a minute or two this time, and then tried again. It started up just fine, and I pulled back on the road and drove home with no further problems.

It seemed so bizarre to me. I felt that dread surfacing in my gut of "oh no, what is going on now...". I'm sure you know it. But I wrote it off as just a glitch of something. 

After that, the Comanche drove fine for a while. I did feel an occasional "hiccup" here and there, where it seemed to lose power for just the slightest moment, but would correct itself and was fine afterwards.

A couple weeks later, I drove to do a job, finished, went to the store...and when I came back to load up the groceries and go home, the truck started up and then died right away in the parking space. Again, I restarted it, and managed to get out of the parking lot and into traffic -- and it dies again right as I'm entering a roundabout. 

Now it cranks but won't start, so I wait again... and it eventually starts up. I'm several miles down the freeway, going about 65...and it dies again. Power steering goes out, the truck won't restart, I almost run off the road because when turning the key to try to crank it the steering wheel locks up and I suddenly forget what to do. 

Anyway...scary stuff. I managed to pull off safely. Again -- waited a few minutes, until eventually it cranked and turned over, and I was back on my way. 

 

I eventually made it home. But the grace period with this intermittent issue had ended, so it was time to research and fix it. 

As is frequently the case, there were many different opinions/suggestions about what could be the ultimate source of this issue, as well as write-ups from people over the years troubleshooting and chasing all kinds of different things, often to no avail. 

 

My issue wasn't a goose chase, thankfully. But I wanted to post about it in hopes to give one more scenario/solution for this problem. I didn't see anybody post *specifically* about what turned out to be my issue. 

 

With that said: 

The issue DID end up being the Crankshaft Position Sensor. 

BUT, what threw me off was that I had replaced the sensor just last year...as well as the TPS, IAC, Plugs, Wires, Cap, Ignition Coil, Fuel Pump, Fuel Filter, 02 Sensor, Fuel Check, Fuel Pressure Regulator. I did it all. 

One thing that helped me was the thread that showed the list of OBD codes, and how to initiate the code sequence. 

It DID give me a code that had to do with the CPS ( I think code 11?), so at least I figured I'd start there before trying other things. 

I tried using a multimeter to check the CPS...but for the life of me I have still not figured out how to use a multimeter. I'm not even sure mine works right. It didn't tell me jack squat. So I just decided to take the CPS off and replace it. 

 

Once I took off the old CPS, I quickly discovered what the problem was -- in the area where the wires feed into the sensor, the plastic sheathing was worn through and the metal wiring was exposed on all three wires. That would do it, I guess. 

It doesn't seem like there is anything near the wires that could cause friction enough to wear through that sheathing. So I'm assuming it was a heat thing and it just melted through? The way they made the part, they put that corrugated plastic sleeve around the wires, but it doesn't cover them all the way -- it is too short, and leaves the wires exposed at either end (sensor side, and/or plug side). 

 

Anyway, I wrapped silicone self-sealing tape around the exposed area of the wires on the new part in hopes that it will protect it better this time. Because that part wasn't even a year old before failure. 

After putting the new CPS in, there have been no further problems and the truck drives great. 

 

I'll attach some pics of the old CPS here as well to show the issue.

Thanks for reading, hopefully this helps someone. 

 

** I guess one final thought would be: anyone know how those wires could become exposed like that in just a year's time? And so close to where they enter into the sensor? Is my theory about heat/melting the most plausible? Or should I be concerned about something else that needs to be addressed which led to this happening?

Thanks.

 

[pizzaman09]

I am glad you got this sorted out!

 

Random theory since I design rubber parts for a living:  So the cracks are on the outside of the bend on the wires.  I'm guessing the wires were bent over hard in their installed state.  The wires are coated in some sort of rubber, rubber in tension like in the outside of a hard bend is highly susceptible to ozone attack which will make the rubber crack.

 

I doubt it's abrasion because the sensor body looks untouched.

[M.T.Hands]

^^^^ I agree, have seen other installations in different applications that have done similar, some only being exposed to enviromental elements (no engine heat)

 

somebody the other day on the (is a Comanche reliable enough to DD thread) mentioned buying a later model vehicle as their DD, what is sad is none of the stuff being produced today will be around and the electronics are going to make them junk, you can find an old 80 year old tractor that is and will still be running, can you imagine doing any of that with the newer junk, Plastic and rubber just naturally degrade over time, although IMO you sensor should have held up longer, But I read of and have experienced more electrical failures with later model junk, not to mention rodents liking to feast on wiring on equipment

[gogmorgo]

Obviously all we can do is speculate, but I’d agree the wires likely were bent and pulled tight when installed. It’s also a stress concentration point and vibrations flexing it back and forth will do damage. It’s pretty common to see this sort of damage on sensors and connectors, anywhere wires are held firmly in place at the end but allowed to wiggle around.

Its also possible it happened when they were bundled and jammed into the box. Some of these parts sit on shelves for years, and just unbending the insulation can sometimes cause damage.
Just a little nick in the wire plus some road salt and corrosion will end the life of the wires in a hurry. The coiled wire in the sensor is tiny and won’t take much to corrode through.

I’ve also seen similar damage from wire-piercing probes. The only context you’d do that is testing so unless someone installed and tested it and put it back in the box that’s unlikely. 

[Ωhm]

Were wires ever poked with a testlight or DVOM needle probe?

[AnotherOldJeepGuy]

Tagging in here since my new symptom is the same. and this thread seems to be current.  I recently tried cleaning up all the oil/crud in the area below using Gunk engine cleaner.  I went through the process twice.  After this I have this same dying issue.  For the most part it dyes if I completely take my foot off the throttle.  A scenario that is pretty repeatable is to come out of the local grocery, start up fine, start across the lot a bit and take my foot off the pedal and it will die.  Only the area pictured was included in this cleaning.  What is in this area or below it that got messed up?  Is the CPS in this area?

 

[GonzoTheGreat]

7 minutes ago, AnotherOldJeepGuy said:

What is in this area or below it that got messed up?

This is the majority of the vacuum harness. And it sounds like a vacuum leak might be your problem. Give a good, thorough inspection to all of those plastic and rubber hoses. And make sure all the ends are still securely attached. 
 

If you find a leak, the doorman-brand replacement parts are 46003 and 46004, for the two halves of the harness, but some of the generic elbows and little tubes can be found at parts stores.

[AnotherOldJeepGuy]

Thanks @GonzoTheGreat The assembly to the left as pictured is newly replaced, about 6 weeks ago.  It got broken when I replaced the valve cover gasket.  The half to the right is original.  Since this issue didn't happen then, just happened at the cleaning, perhaps I "cleaned" off some crud that was sealing the system.  I'll try looking closer.

[Ωhm]

Check MAP sensor vacuum hose, closely. Is EGR solenoid connected electrically and is plumbed correctly? TPS voltage (flat spot, dropout) while sweeping the throttle, may need checking.

[Salvagedcircuit]

7 hours ago, Ωhm said:

Check MAP sensor vacuum hose, closely. Is EGR solenoid connected electrically and is plumbed correctly? TPS voltage (flat spot, dropout) while sweeping the throttle, may need checking.

I second this. The line going to the MAP sensor is an odd size and very brittle. It can sometimes not seal well. If you find a hole in it, you may be able to get away with an adhesive heatshrink fix, but a long term fix is Cruiser54 tip 31: https://cruiser54.com/?p=280

I have no idea if this problem holds true for HO though.

 

If you end up finding any holes in your other vacuum lines, I do find that adhesive lined heatshrink works well to seal them again. Even non adhesive lined heatshrink works. Use a generous sized piece and a hair dryer. If you use a heatgun, go easy on the heat so you don't melt anything.

 

I had a hole in the vacuum line that runs under the firewall seam. I could not find a replacement hose for this diameter, as it's some weird thin walled hardline. I used an x-acto to clean up the melted hole surface, disassembled the corner elbow with a heatgun and metal spudger and slipped on a piece of heatshrink. It's been working well for 2 years so far.

Good luck!

[AnotherOldJeepGuy]

@Salvagedcircuit I had not thought of heat shrink repairs.  If I can find a leak I will keep that in mind as an option.  The tapped repair from cruiser makes sense but seems a bit more risky, but doable for sure.  I like you house routing much better than mine.  Is this your design or an improvement from the factory?  The way mine is is not modified by me at least, and I am the original owner, so if it is different from stock on mine I'd have to guess its part of some repair shop work in the past I don't remember.  My hose routing interferes with changing the air filter much less that how yours is routed so if I have to rework this area I'll shoot for something that looks like yours.

[AnotherOldJeepGuy]

17 hours ago, Ωhm said:

Check MAP sensor vacuum hose, closely. Is EGR solenoid connected electrically and is plumbed correctly? TPS voltage (flat spot, dropout) while sweeping the throttle, may need checking.

Nothing is unplugged intentionally anyway and no plumbing has been touched any time near when the problem came up, unless accidentally.  I'll look more closely just in case.  I pickup up new Throttle and Crank Position Sensors in the Advanced Auto close out, one for $3 and the other for $1.80 (last one they had of each), so I can experiment with swapping if I need to.  At least the TPS looks easy to get to, I've read the CPS is not.

[Salvagedcircuit]

I used 5/8 heater hose on my vacuum lines harness because both of my u-shaped pieces rotted out after 34 years and the dorman kit only included one side of the vacuum harness. So I notched the front vacuum connector a bit and put the hose over the fitting inside of inside the fitting. It works, but the heater hose is not very flexible.

 

Since you have a renix, also check your throttle body! Early renix throttle bodies had an extra threaded testing input port. The geometry was removed on later renix throttle bodies. I found this out by investigating a blob of silicone the previous owner attached to my throttle body. I had a vacuum leak there. That torx screw you see in the photos is stock and just requires a new 8mm ID, 12mm OD o-ring to seal well. The stock o-ring was very flat and old. I snuged it down tight with the new o-ring and no the screw does not interfere with the butterfly on the throttle body. The o-ring can be found at any TrueValue / Ace / Do-it-Best hardware store.

 

[Eagle_SX4]

25 minutes ago, Salvagedcircuit said:

I thought that screw was for idle speed adjustment and had a cover over it from the factory to prevent adjustment.

[Salvagedcircuit]

15 minutes ago, Eagle_SX4 said:

I thought that screw was for idle speed adjustment and had a cover over it from the factory to prevent adjustment.

Mine was covered in a blob of silicone. There was no cap on mine. The other ones I've seen showed an exposed torx screw. The later ones have this geometry missing completely. I could be wrong, but that's what I found

[Eagle_SX4]

2 minutes ago, Salvagedcircuit said:

Mine was covered in a blob of silicone. There was no cap on mine. The other ones I've seen showed an exposed torx screw. The later ones have this geometry missing completely. I could be wrong, but that's what I found

Interesting. the throttle bodies I have (3) all have this screw and 2 of them had covers over them. The port that the screw regulates goes above the throttle plate giving the engine a bit more or less air to adjust the idle. It might be refered to as the extra air screw.

[Ωhm]

It’s a calibrated air leak. Purpose is for setting Idle Speed Stepper Motor (ISSM) pintle in its ideal spot for ECU to control engine RPM's.

[AnotherOldJeepGuy]

Mine appears to have a recessed cover of some kind.

[AnotherOldJeepGuy]

I found only 1 actual opening so far... a mystery hole just before the large air hose to TB.

 

I added a few snug zip ties where things were fairly loose.  I don't know which if any of these are not really intended to be air tight anyway but they are more snug now.

 

 

Now off for a test drive to NAPA to pick up a part on order that came in.  Will make note if any of the tendency to die has changed.

[AnotherOldJeepGuy]

Test drive completed, no change .  I was not that surprised since most if not all of these connections did not really look like they were intended to be airtight.  But even though they didn't change this particular issue, it probably can't hurt for these to be more snug anyway.

[pizzaman09]

My parent's 97 ZJ 4.0L used to have a similar symptom but would not actually stall.  The issue was always shortly after starting while the engine warmed up.  Usually about the time you had gotten to the edge of a parking lot and ready to pull into a road, it would fall flat on its face and run at about 200rpm.  After years of chasing the issue our mechanic found the exhaust header was cracked and we figured as the header heated up the crack would close.  Replacing the original O2 sensor ended up vastly improving the issues but didn't eliminate it.

 

It's worth checking to see if the manifold bolts are tight or possibly there is a crack in your exhaust header before the O2 sensor.

[AnotherOldJeepGuy]

I put checking that on the to-do list!  Some repairs have to wait until I have both hands functional (got a splint on one at the moment).

 

I took a road trip over the weekend, about 500 miles.  Made it there and back so my issue is not terminal yet!  I have an injector leaking now that may have been leaking less previously.  Don't know if that would cause the problem but it does seem possible.  Anyway, it is a visible leak so I don't need to guess if it needs fixed or not!  Luckily I picked a new one up at the Advanced Auto closeout so I have one on hand.  Always something that can be done to this old truck!  If that gets too interesting I'll probably start a new thread on that repair (or find an existing one to add to).

[AnotherOldJeepGuy]

On 1/13/2025 at 5:25 PM, AnotherOldJeepGuy said:

I'll probably start a new thread on that repair (or find an existing one to add to).