So I've recently taken the jeep in for diagnostic of p1391 throwing a CEL every second start after clearing it.

Cam crank sensors are synced, signal gets to PCM, jeep starts, runs, and drives fine.

The diagnostic shows everything running cleanly.

They suggest maybe having a dealer trying to update mine before replacing it.


My question is what do I need to be concerned with as far as this goes, they kept saying a new computer needs to be programmed?

 

Yeah they say programmed, all it is is they put in the mileage and the vin number. They are not updateable in terms of the programming they run.

 

In this instance, what do they suggest trying to flash it before replacing it?

 

Not sure I understand your question because you only said "They suggest maybe having a dealer trying to update mine before replacing it."and then I said "Programmed" which I should have said "update". But as far as I know, if your profile is correct and we are talking a 99 xj,

By the way, who is "they"?

The only info I know of that the pcm will take is the vin and mileage. I'm not 100% on that. But I do know that the 96 and 97 zj does not even need that, when you buy a pcm it's just plug it in and go, because the vin and mileage are stored in the bcm. Regardless of that, many sellers of pcm's will still insist in their advertising that you must provide that info so they can "flash" or "program" the new pcm.
I don't think in 99 they were as sophisticated as a new Iphone that needs operating system updates. Not to doubt the word of a DEALER who this site knows so well, are always so honest, correct, skilled, trained and informed.

 

https://autoservicecosts.com/obd2-codes/p1391/

there is an attached youtube on fixing jeep with this code!,,, didnt view it

 

Sorry, "they" the shop I got to run the diagnostic, suggested having the dealer try updating my 99 PCM. If that didn't work they said the dealer would still have to do something to a replacement PCM.

I guess a pcm can't be flashed by a shop? Idk, he said they couldn't.

 

The only time i have heard of a PCM in an XJ needing "programmed" is when you add or remove SENTRY, which is pretty much all you can do with them. Aside from that we are stuck with whatever was programmed into them from the factory. I would like to think that if one could do more, the forums would be loaded with threads from those that have had it done. If they have, they are being awfully quiet about it.

 

The diagnostic results a lab scope showing ckp,cmp are in sync. This also showed signal being received at the pmc and the pmc continuing to set the code. A skipped tooth is very very very unlikely with this engine and also would result in a sync issue.

The harness has great continuity to pcm. Grounds are good, voltage and signal are strong. With this and sensors reading properly, replacing them is not a logical step. This is again where the PCM having an issue comes into play and is as the shop said 99% the culprit.

Is there any pcm testing a dealer can do to see if this is absolutely the case? Other wise can I just grab a 99 PCM from the JY and be on my way?

 

To the best of my knowledge the Jeep ECU cannot be "re-programmed", other than VIN being added (the PCM/ECU rebuilders mentions this)

I dont think it matters that much if the VIN is not correct, my 3 x '96 models, the reader will not detect the VIN anyway

For the price of a PCM from a wrecker?Ebay, its probably worth a try...I would make sure the part num on the PCM matches...plenty on Ebay

 

How are you clearing the CELs?

 

Cause IMO you need to clear the Adaptive Memory.

Also IMO I sincerely doubt that the only symptom of a bad ECU is it forgot what the camshaft voltage should be.

 

And how old and what brand are the crank and cam sensors?

 

CEL cleared with scanner and disconnecting the battery, multiple multiple times. All sensors are Mopar under 2yr old.

 

would not be surprised if you have chain stretch (depending on miles)

I own a '03 GM Commodore with Alloytec motor...they are known to have this issue

If P1391 means "detecting CMP and CPS out of sync" ..they probably are!

It would be cheaper to put a used PCM in it, if that doesnt work, onto the next thing

 

Try this one:

https://www.jeepforum.com/forum/f67/...emory-1265118/