1998 Jeep Cherokee

Engine Mechanical problem
1998 Jeep Cherokee 6 cyl Four Wheel Drive Automatic 149000 miles

I have a 1998 Jeep Cherokee Sport with a 6 cylinder engine. Recently, I have had problems when I have driven it in hot weather. Several times, after I have stopped the car and turned it off, I have been unable to restart it. Once, the engine cut out on me while driving. The one time it cut out on me while driving, it was a very hot day and I had been driving for about an hour up a mountain. I had a mechanic check the battery and the alternator, and both were just fine, no problems at all there. I thought it might be vapor lock and have tried lifting the hood and placing wet rags on the gas lines, which has worked all but once which was the same time that the engine cut out on me while driving. I had a mechanic perform a computer diagnostic, which indicated that the crankshaft sensor is sometimes not registering. The mechanic said that if the crankshaft sensor is not registering, the computer will not allow the engine to start or to continue running. I asked the mechanic to replace the sensor at that time, but he could not get the part and I was far away from home. We waited a couple of hours until things cooled off and then I was able to start the car again and drive home. This has happened at least half a dozen times over the past 2 months and each time it has happened in extremely hot weather this summer. (I haven't had this problem in previous summers.) I have a few questions:

1. If this is vapor lock, why does it usually occur after I have turned the car off?
2. Is this something caused by the heat, or is it merely a coincidence that this has occurred on the hottest days this summer?
3. Could this be caused by a faulty crankshaft sensor or did the crankshaft sensor become faulty because of this?
4. Should I be looking somewhere else in the engine or electrical areas?
5. What can I do to fix this? (If you have more than one idea or suggestion, please let me know.)
September 16, 2009.

If the computer scanner indicated a crank sensor is not being seen it could be a few things. First make sure that the wiring is good and it is not shorted or broken. If it's ok, hopefully you need a new crank sensor. If the sensor is replaced and you still have this issue - then the reluctor ring is probably to blame. The reluctor ring is connected to the crank shaft (inside of the engine). It is the thing that the crank sensor reads to tell the computer where the crank - at a certain time - during the rotation of the crank shaft. This ring can become loose and it can be affected by temperature. Usually the problem of a loose reluctor ring is experienced during the first few months of the engine life. Being that it is a 1998, it is rare that this would be the cause, but it could be. If it is the reluctor ring, the only way to get to it is to remove the crank shaft. Cross your fingers.
Vapor lock can happen, but being that your fuel pump is located inside of the fuel tank and with the computer diagnostic indication, I would pretty much rule that out.

Sep 16, 2009.