2000 Stock XJ, just passed 160,000 miles, my daily driver.
Last December, ten minutes down the road from my house, the engine just shuts down. No sputtering, choking, or warning of any kind; engine just dies. So I coast to a stop, try cranking it again; it cranks and cranks, but will not come back to life. Almost like the fuel tank's empty (that did happen once prior) but I know that wasn't the case. Ended up getting it towed to my mechanic; a couple hours later when they got to it, it started right up. They spent a couple days poking at it, reported fuel pressure was all up to spec, and couldn't recreate what happened. The alternator was throwing some odd voltage and was a bit old, so I had that replaced. Since then it's been fine.
Today, coming to a stop at an intersection, it happened again. No stutters, chocking, or bad firing; just quietly shut down and wouldn't restart. Cranks and cranks, voltage looks fine, but won't turn over. This time I waited it out; I installed a switch&relay a month ago that runs the electric fan an extra 3 minutes (which seems to have solved my heat soak trouble, at least I haven't had a rough start since using it), so I kept that going to cool things off, tried cranking it every 3 minutes or so. I got something closer to turning over after half an hour. Randomly after that I tried hooking up the backup jump battery I carry, which for whatever reason did the trick; it started right back up and ran perfectly the rest of the way home. Check back a couple hours later and it's still starting just fine.
So, it's the worst kind of problem; seems random, can't recreate, and no warning symptoms of any kind. CPS sensor seems to be a common suggestion, Curious if there's any other thoughts.
Thanks

 

Has your daily driver had anything done to it, other that oil changes or gas put in it?
Have checked anything or cleaned/replaced anything?

 

The crankshaft position sensor indeed is the primary suspect here. What you have described is not uncommon with the Jeep 4.0 liter engine. When it fails in this way, it usually does not leave any codes but I always recommend you use a code reader or scan tool to check.

Original Mopar crank sensors are by far the most reliable but I think I remember seeing that they are very difficult to source. Good luck and keep us updated!

 

Next time it happens, hook up the multimeter to the CPS GY/BK wire to see if a signal is being sent while cranking. It's a very brief signal, so what you get for a value will vary depending on your set-up (it'll probably be like 2.0 - 3.0 DCV) but if it's -0- CPS is dead.

 

No major changes on the jeep. I've had it since 80,000 miles; general expected wear/tear, no major unexpected replacements aside from a transmission rebuild around 70,000 miles ago. Scanner didn't pull up any codes once I got it home. Read some suggestions that the CPS is vulnerable to overheating; that would sort of square with yesterday's failure (backroads, lot of stop signs, and I think the temperature gauge was reading a touch higher than usual. And I was able to get it restarted once things cooled down, I'm honestly not sure if the jump battery had anything to do with it). But I drove it to half an hour to a job and back this morning, smooth running the whole way though.

Given how infrequently it seems to happen (at the moment at least) I may not have much else to post, but I'll try replacing the CPS; I did find an OEM after a little hunting, seems pretty well agreed that aftermarket brands are garbage. It looks like a fairly infuriating job too...
Thanks for the tips.

 

Not wanting to fire the parts cannon, I too suspect the CPS.

I had a similar issue with my '88. Drive around for a bit and have the engine quit like I turned off the key. Open the hood, wait 20 mins or so, and it would fire right back up. Sometimes it would run the rest of the day no problem, sometimes die again in like 10-15 minutes. In hindsight, it was worse in the afternoon than in the morning. I attributed this to the ambient temperature. Cool in the morning may not happen for local drives. Hot in the afternoon more likely to happen. Since most of my drives were pretty short (just around town running errands), it was never predictable. This was in 2004.

At the time, I got a Mopar one from my local Jeep dealer. They're no longer available for mine. The Mopar one for your 2000 is still available but pricey!

https://tinyurl.com/2ev7muuc

As an alternative, I've had good luck with NTK or Standard sensors. Both cheaper and available on RockAuto.

 

Thanks for the link. Symptoms sound exactly like mine. I'm checking availability now, hopefully get it installed before the end of the month...

 

Check all of the ignition related pigtail connectors. Even unplug, spray with contact cleaner, let dry, plug back in. If any of the locks on the connectors are broken, get them replaced or figure out how to lock the connection in.

Just had your symptoms, not the first time, and replaced ignition coil, pickup coil, and crank sensor. The problem was the connector to the ignition coil was broken and nothing fixed it until I replaced the female connector.