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Long story short, my son in Florida is in dire need of a vehicle.  As such, as much as I hate to let go of the MJ, she needs to be driven, and my son loves the truck, and will do his best to take care of it.  I'm in Pennsylvania, so he's going to have to drive it 950 miles to Florida in about a week, and then intends to daily it for the next year or two.  I'm pushing him to get a membership here, as it will prove invaluable to him through his ownership of the truck.

 

Anyway, the truck has been running perfectly since some work I did to it last year.  Last night I took her out to ensure everything was running tip-top.  I planned a four mile circuit over hills and curves, and really pushed the truck over the first two miles.  I was on the skinny pedal, shifting at high RPMs, taking corners hard, braking hard, etc.  Really having some fun - and the truck ran great, until it didn't.

 

Somewhere around mile three when I started to take it a little easier, the MJ shut off while driving, and refused to start again.  When it shut off, it was traveling at 30 MPH at 2500 RPM in second gear.  There was a slight surge in the RPMs to around 2800 for about half a second, then the engine shut off.  I was able to pull it to the side of the road, and then it had to be towed home.  I was very disappointed.  I feel bad - I called it a Ford (fix or repair daily, found on road dead).

 

While waiting for the tow, I checked a few things, and could not get it to start.  It was cranking, but not starting (like a perfectly working truck with the CPS disconnected).  I have gone over the fuel system 100% recently and know it's functioning as there is plenty of fuel in the rail under pressure.  I swapped the fuel pump and auto shutdown relays without any change.  I unplugged the O2, MAP, CPS and CMP sensors and plugged them back in just for the hell of it.

 

This morning the truck started on the first try.  It's been running in the driveway for 45 minutes and idling perfectly at around 800 RPM.  No hiccups.  I ran it for two minutes at 4000 RPM about 20 minutes ago and had no issues.  It's still running out there.

 

When I last maintained the vehicle, I checked compression (excellent), vacuum (excellent), fuel pressure (excellent), changed the O2 sensor and downpipe, put a new distributor (with CMP) cap and wires in, new fuel filter and fuel hose, new ballast resistor and checked/tested all sensors.  OBDI scans showed no codes, no issues, all sensors reporting properly.  Approximately 140K on the engine.  Everything I've done to the truck is in my build thread, linked to in my signature.

 

My thought is a bad CPS or bad ignition coil, possibly heat related.  Beyond that, I have no ideas and would appreciate thoughts from the group on the situation.  The truck has to do a big trip in a week and has to be reliable after that.  I really don't want to fool with the hassle of replacing the CPS, so I'm looking for other ideas first.

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Check the Distributor. There is an induction ring in there. Also, try and wiggle the shaft. I think specs are about .001". Also, I had issues with the electronic ignition block on the steering column on a 2000 XJ. Would not start. 

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16 minutes ago, 75sv1 said:

Check the Distributor. There is an induction ring in there.

 

Thank you.  I replaced the entire distributor six months ago.  The 'ring' is the camshaft position sensor (CMP).  All good there.  Properly indexed.

 

Just ran for another 30 minutes.  Temp steady at 220  degrees or so.   No issues.  Also ran OBDI diagnostics and found no issues.  No codes.  It's like last night didn't happen. 

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Take it out again, you won't be able to figure out what's failing unless you get it to fail, once it has failed check the coil for heat and see if there is spark. My money is on the coil.

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I will say, when I owned my salvage yard, we sold many "white box" distributors that were made in China.  They are notorious for crapping out in less than a year.  Though more expensive, you are better off to rebuild a oem unit.  It got so bad that Platinum/Dorman quit selling distributors for a bit because of it.  That being said, it's def. electrical.  My 89 Renix did the same thing a while back.  First I thought it was the coil - replaced it, seemed fine until it wasn't.  Finally replaced ignition module and it was all good.

 

I'd lean on the distributor if it wasn't a reputable brand, if not CPS.  Heat, bad quality control, & poor soldering means intermittent open circuit... the absolute worst problem to solve!

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Thanks for the input.  The only other thing I thought to add was that I tried last night in the driveway after the tow to get it to start, and it wouldn't.  It had to sit overnight before it would start this morning (see OP about that).  So there was nothing that the tow job did (like rattle something loose) to fix the situation.

 

The truck started up this evening, and I ran it for 30 minutes without issues - 20 minutes of which I had the throttle open, and the engine running at 2800 RPM.  I just tried again this evening and ran it at idle for 30 more minutes, and no problems - it's running smoothly at any RPM - no variations.

 

So in all, I've run it today in excess of 2 hours 4 different times, with the throttle at 4000 RPM for 2 minutes, with the throttle at 2800 RPM for 20 minutes.  The truck started perfectly each time.  The truck ran perfectly for the duration each time.  It was 90 degrees in my driveway this afternoon and after running the truck the hood was hot after the 4000 RPM cycles.  Hotter than it likely was last night.  Engine temps have never gone above normal operating temp (~220 degrees).

 

The distributor is not a rebuilt OEM unit, it's a generic part PPDST4696 listed under Amazon ASIN B00VVBOJT0.  A bad CMP in the distributor would produce rough idle, and a bad distributor would cause misfires.  A bad MAP or O2 would produce rough idle as well.  I believe if the CPS was going out, I'd be having rough idle issues before a total failure.  And then it wouldn't just start working perfectly again after a failure. 

 

After thinking more on it, I'm leaning towards the coil like @jdog.  I have a spare on my YJ that I can borrow.  I'm just not keen on paying for another $80 tow if I get it to fail, and it's not the coil.

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I had this exact problem with another vehicle and it was the coil.  Run the truck as much as you can and check if the coil is noticably overheating.  In my case, the coil got killed by a burnt and shorting plug wire, but that's not much of an issue with the 4.0.

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I did go through a no start on my '98 XJ. I did have issues with it stalling out a few times before. I went wheeling early in the year, Winterfest. Plenty of water. Lost my muffler. I got the home and had the muffler repaired. Then I got it back home. Next day it would not start. I bought a new coil. When taking it off, I got a spark from one of the grounds. Yeah, I know I should have had the - terminal off. Anyways, I replaced both the coil and the alternator. I had a new one laying around. Rans good now. 

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I've run the truck again in the driveway at idle (800 RPM) for another hour.  No variations in idle, no problems.  Engine temp is steady at 220 degrees.  All sensors look good via OBDI diagnostics.  Under the hood is hot.  The coil is hot - but no hotter than the distributor cap.  I wouldn't call it excessively hot, probably normal for the conditions in the engine bay considering the load for the past hour.

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The truck has now been running at idle for two hours in the driveway.  The sun came out and it's hot out again - around 90 degrees.  Engine temp is up around 225-230 degrees and (for the first time ever in my ownership) the aux fan has kicked on.  The engine bay is hot.  No signs of failure, idle is perfect...

 

How can I go for a three mile drive one night and have a total failure to start until the next day, then have the vehicle run like a champ standing still while under vigorous conditions for hours on end?  This makes no sense to me.  I understand this is symptomatic of a bad coil under load, but if that were the case, I'd expect some indication of a problem by now.

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I am thinking a loose part. You may have hit a bump that knocked a wire loose. The tow back home knocked the part back. If you haven’t driven it then the part is still connected. While it’s running (but still cold) start tugging on the wires. 

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I got nothin', sorry. My intermittent shutdown issues have tended to be ignition coil related too, but this is a rough one. At least mine have had the courtesy to be a little more regular and consistent.

 

Do you know the rough age and brand of the coil?

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On 6/4/2021 at 10:19 PM, Minuit said:

Do you know the rough age and brand of the coil?

 

Thanks for chiming in, brother.  Coil is OE.  Not sure if it is original to the truck (1991, 231k) or engine (1991, 140k XJ swapped unit).  

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I had exactly the same thing happen. It turned out that the wiring harness that feeds the distributor  had a break in it. I would wiggle all the wiring that feeds the coil and distributor and see if the engine stops.

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Another vote for distributor. Truck would shut down while driving and not seem to restart until it cooled down.  Replaced he dizzy with a reman from O'reilly's. That one lasted 6mos before taking a dump.  Swapped it out under their lifetime distributor warranty.  That 2nd unit has lasted 4 years now, trouble free.

 

The lesson here is don't overlook the distributor just because you replaced it recently. They are pretty much all remans with chinese sensors these days.  Just look for the best warranty available.

 

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My ‘91 would randomly die when I had it years ago. I’d wiggle things and check things… nothing. One day the main fuse in the power distribution block finally popped. Found it, changed it, never had a problem again. 

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Not particularly pertinent information to a Comanche, but I recently helped diagnose a very similar issue on a 87 Toyota MR2.  After rebuilding the distributor and replacing the coil, the dying while driving after a short time persisted.  The issue turned out to be the throttle position sensor was worn out at the closed position.  Turning in the throttle plate stop screw 1/16th of a turn allowed the sensor to sit in a fresh position and fixed the problem!

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Thanks for all the comments.  I haven't had any time to drive it yet to try to produce a failure, so I'll post an update at a later date when that happens.  Coil and/or distributor seem to be the most likely culprits.  A couple of other things I thought about:

 

- When the distributor was installed, I could not use the TDC mark on the harmonic balancer to find TDC.  It was as if the harmonic balancer had rotated on it's own (it should be replaced - the inner seal is blowing out).  I had to find TDC using cylinder one.  All indications are that timing is correct.

 

- There may be old gas in the tank.  I know I put a half tank of gas in in last year, but that was on top of gas that was a couple of years old.  I've only driven the truck about 70 miles over the last three years.  Maybe bad unmixed gas?  Probably not.

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