i know this is not for ZJ type vehicles. but, i'm looking for an old seasoned mechanic to see if what i heard today seems reasonable. i met an old man today who use to have the same exact type of jeep (97 Jeep GC Laredo 4.0L inline 6 2WD) and the same exact problem i have been throwing out on all the forums lately. my vehicle runs PERFECT in every way ( idle,acceleration,fuel mileage,handling, you name it). but, after it has been driven for any length of time ( with absolutely no issues at all) , if it sit's for an hour or more, it just loses all spark. crank the motor up in 10, 20, 30, 45 minutes after turning it off; she fires right up and drives PERFECT again. when the 1 hour mystery syndrome has kicked in, the jeep will not start till the next day. it magically fixes itself over night. until it's driven again & sits for an hour. then it's come back tomorrow. sometimes i get held up on my errands & miss my daily allotment of 45 mins. or less to start the motor. i have to walk home on those occasions. been doing that alot the past couple of months. 3 shops and 15 mechanics later, still no clues as to how to fix the problem. no matter what parts you throw at it, the 1 hour mystery syndrome keeps coming back. so, today this guy is telling me he had a similar problem except for the length of time issue. he changed everything also with no luck. then, by pure chance he changed his oil pressure sending unit & WALLAH!!! he had his old jeep back to normal. so my question is this; is it possible for a bad / going bad oil pressure unit to cause an mystery electrical short that doesn't reveal itself in any way? meaning as in no CEL or trouble codes. can the pcm detect what it thinks is a possible threat to the engine's stability ( oil pressure related ) and put the vehicle in a shutdown ( not limp ) mode til the next day without throwing any code or warning light? and, would this cause total loss of spark til the next day? i appreciate any and all input. again, sorry for putting this on a non-Cherokee site, desperation has forced me to seek all avenues for any solution to this madness. :h ammer:: brickwall:

 

What are the codes?

 

1.get service manual with wiring diagram, test light and multitester.
2.Diagnose when it doesn't start.

Replacement of parts w/o diagnosing first is stupid (been there, done that )
You have 2 choices:
1.diagnose it yourself
2. pay somebody to diagnose it.
Good luck.
P.S. Try not to write 20 lines, I got bored and stopped reading after 5th one.

 

Has any of these shops and mechanics used a DRBIII Diagnostic Tool to try and locate the problem? If not, has anyone checked the CCD Data Bus voltages at the Data Link Connector using a voltmeter to see if the CCD Data bus has been corrupted by a bad module?

I'm not that familiar with a ZJ, but I think the engine (4.0) management system is about the same as the XJ body. If the PCM ignition driver is not active after sitting for an hour it could be the PCM is not receiving an ASD relay signal that the ASD is active.

As far as the oil pressure sensor causing the problem - I don't think so. As long as you're seeing proper oil pressure indication on the instrument cluster the sensor is functing properly, and so is the PCM/CCD Data Bus signals to the instrument cluster. The PCM dosen't monitor oil pressure so you won't get a CEL there. It's a very long shot but if the oil pressure sensor, like any of the 5 volt reference sensors goes bad, there's a slim possibility that it is corrupting the CCD Data Bus. The CPS has a habit of doing this but when it does, it usually takes out the instrument cluster, and the engine won't start.

I would see if a DRBIII tool was used to diagnose the problem. If not it should be done. I'm sure you've spent big bucks having these shops scratch their collective ***** over this, so another $100 may do the trick.

It would be a drill in futility for us to try and diagnose the problem in this thread not knowing what has already been done.

There is a way of checking the CCD Data Bus using a digital Voltmeter and an Ohmeter, but first find out if it has already been accomplished.

 

i get no codes at all. way back when i was having stalling issues, i always got PO351 (primary circuit coil #1 / secondary bank). had a brief bout with PO171 and fuel running too lean. all those issues resolved. if this thing bucked or smoked or misfired, it would make sense to me. to work perfect & just stop is insane. my oil pressure gauge has started fluctuating. it usually sits at about 40 (dead center of gauge). when driving i've seen it rise up to about 50 ( no line on gauge so i'm estimating) and then drop to around 32, then return to 40. dipstick says my oil level is good & not going anywhere. found a little underneath oil filter. that's why i thought the oil pressure sending unit was a possible short causing, no spark culprit.

 

Sortaa sounds like a problem older cars had ignition module heats up shutting down vechicle until it cools down then they would restart and off youd go ....