yamaholic

Howdy all-
This is my first post on cherokeeforum, from the research I've done so far it definitely seems to have the most useful tech questions/answers on the interwebs so hoping that someone can help me with a few questions...

Here's my plight-two days ago my Jeep (01 XJ automatic, all stock motor ~125k) died on the way to work, about 5 miles after a cold (40 degree morning maybe?) start. It's an automatic so I was surprised that before I had a chance to try to restart it the motor spun up from 0 (on the tach) and started running again. It did this a few times, the last I had to put it into neutral and crank for a while to restart. I had about 20 miles left of my commute, so I stopped and let it run for awhile but it didn't seem to have any more issues. It ran the rest of the way to/from work with no problem. No trouble codes yet. Yesterday was a different story...I tried taking off for work and only made it a few miles before it died completely, luckily I was able to coast into a parking spot but it wouldn't restart. I checked fuel pressure at the rail (thinking the fuel pump was shot) with a screwdriver and it seemed to have plenty. It would crank and sounded like it would occasionally fire on a cylinder or two at a time but never enough to stay running. I ended up hopping on the bus and by the time I got back that night it fired up like it was brand new. It died multiple times on the way home after warming up, and would only restart if I let it cool down. It also threw the P1391 crank/cam shaft intermittent loss code. I cleared the code, and today it runs the same (despite everything I've checked) except will only start if dead cold and runs for about a minute or two before completely done. It ran (and died 20+ times) today without throwing another code. Letting it sit until cold it still fires up and runs perfect, no missing or loss of power until it shuts off completely.

Here's what I've checked so far (thanks much to the posts on this forum!)-
-Fuel pressure (always check that when it dies and it's always there, not with a gauge but I'm 99% sure it's fine)
-Replaced crank and cam position sensors with best locally available parts even though both originals seemed to test fine...just to be sure since so many seem to fail
-Swapped out fuel pump and auto shutdown relays, you can hear the fuel pump cycle when turning the ignition on and we managed to flood the motor when trying to test coil pack input voltage so I'm reasonably sure the fuel system is ok
-Checked resistance across any sensor I had test data for (coolant, intake temp etc) all tested in spec
-Looked for loose connections at the PCM, all sensors, coil pack etc.
-Pulled the plugs (Bosch Platinum +2, they've worked for 30k+ miles since I've had it) and they don't look perfect but ok
-Tested the primary on the coil pack, seems to be ok

So here's where I'm at...I've tried testing the secondary on the coil pack but possibly I'm doing it wrong? I haven't seem any good instructions on the internet...I always get infinite resistance from any of the pins in the connector to any of the plug contacts. That being said my meter runs off a watch battery so I'm sure it has very little test current (but it goes to Mohms). Also, I don't get more than a few hundred millivolts to pin 2 on the pack connector, but here's my real question - what should I see on these pins? I'm assuming 2 should have 12v dc for at least a few seconds when turning the ignition to on (we tried cranking the motor just to be sure) but I never see anywhere near that, it makes my autorange meter fluctuate in the millivolt range then goes to 0. Also the other pins should go to ground as the motor turns right? I've only checked for voltage and basically get the same on all pins, fluctuating millivolts. And since I've swapped the CKP/CMP sensors that basically leaves the PCM or coil pack I'm guessing? Like I said the issue is completely heat sensitive so I'm thinking either the coil packs fail when they warm up or the PCM does. Is there any way to test the PCM? How does the pack get its 12v (assuming that's what it should have)? And since I've already wasted $ replacing parts I didn't need to is there a good way to test the coil pack? Thanks much for any insight you can give, and if you ever need any security/remote start/car audio help let me know!

yamaholic

Ugh...this was frustrating-surprised I didn't get any responses to this post, seems like there isn't any good info on testing coil packs on the internet (that I've been able to find anyway). Regardless, turns out my crankshaft position sensor was bad...and the replacement I bought from O'Reilly auto was defective as well! Took the Jeep into the shop after 2 days of testing and swapping parts, they swapped the sensor with a known good one and it fired right up and runs like new...

tjwalker

Thanks for updating us with the resolve. Glad you got it fixed! That code did point to the crank sensor.

As it is NOT unusual to get a bad aftermarket crank sensor, I always recommend buying a MOPAR crank sensor directly from Jeep.