Jeep 4.0L P1391, potentially with p0351, p0352, p0353 Crank/Cam Sensor Issue and Fix -
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post #1 of 6 Old 11-06-2015, 01:57 PM Thread Starter
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Jeep 4.0L P1391, potentially with p0351, p0352, p0353 Crank/Cam Sensor Issue and Fix

After struggling with a P1391 code for weeks I thought I'd post what I learned since I've seen no comprehensive method for troubleshooting this issue. Just to setup my situation, I had just completed a 4.0L engine rebuild and swap in my 2000 4.0L XJ. It started and ran, but would continually throw the P1391 code, sometimes accompanied by the p0351, p0352, p0353 codes. It would shut down randomly and run rough sometimes. It even threw a transmission code once just to keep things interesting. A majority of the posts recommended replacing the crank and cam sensors, which I did, and a few recommended phasing the cam sensor housing using the toothpick method, which I did. I had actually replaced the entire cam sensor unit during the rebuild and it used a cap it came with instead of the toothpick, but same deal. Still nothing after trying new sensors and checking the phasing. I read several great posts, one of which referenced this article. Besides of being an awesome article (the author is THE MAN), he mentions (along with one or two other posts) using the dealer scan tool to set the phasing. Running out of options, I drove my jeep down to the dealer and talked with the mechanic who said when they get this signature they just replace both sensors. I said I'd really like him to do the phasing procedure (takes 5 minutes) and I watched as he did it. If anything, it was worse after that. They didn't charge me for the assistance though, which I really appreciated. Now totally fed up, I went back to the article I mentioned earlier and read it again. I realized I really needed to dig into the problem and ordered an inexpensive scope here. I picked up the ~$60 SainSmart DDS120, but you can use any scope that has two inputs. That is key, 2 inputs. The test I describe next I truly believe anyone can do. All you need to do is find the inputs to the PCM for the crank and cam sensor, then back probe them using a pin, nail, or other method. Once you have both signals, you get to watch them and how they interact which will actually tell you what the problem is instead of just throwing parts at it. Here is the setup:
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Note I just used a couple of alligator clips to ensure everything was grounded properly. I found attaching everything at the PCM easiest, but you could grab the signals from closer to the crank and cam sensors.
After getting it setup and adjusting the scope software, I could see the crank signal (the blue signal with fast groups of 4) and the cam signal (the long yellow signal that captures 3 groups of crank signals. They all looked great. Every set of 3 crank signals is one revolution. If you were to look at the flex plate you'd see 3 sets of 4 holes in the ring. If you had a failing crank or cam sensor, you'd see abnormal groupings or drop outs of the signal. HOWEVER, you can see the rise in the cam signal AMONGST the first group of crank signals.
That is bad. It needs to be between two sets of 4. However, check out the orientation of my cam sensor, picture perfect. AND the dealer had set it, right??
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So, the next step is to loosen the hold down bolt and rotate the unit like a distributor. You can watch the signals shift on the screen as you do so. What I saw was surprising, as I loosened the bolt, the signals shifted to the left and looked great, but when I tightened it down again the shifted right into the group of crank signals. So, I could now conclude that the root cause was incorrect phasing, and that just tightening down the clamp caused it to change. This also meant the mechanic at the dealer was running his procedure correctly, but he needed to check the scan tool both before and after tightening the clamp. Here is where the cam sensor housing wound up, ironically where I remembered it was before the engine swap.
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Here is what you are looking for on the scope with a properly set cam sensor.
You may run into this issue after replacing the cam sensor or the cam sensor drive, after a rebuild, or just over time as the timing chain stretches. This post applies directly to any 4.0L Jeep without a distributor.

This is actually my first post, and I hope this adds to the body of knowledge out there. I appreciate everyone who works hard on their posts, and especially the folks and follow through and post what ultimately resolved their problem. Thanks!!

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post #2 of 6 Old 07-14-2016, 06:05 PM
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2000 XJ Cherokee 
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Thank you for this post. I was having the same problem and now it is gone. The gasket was too thin, the sensor needed to be turned on by the scanner and it rotated while being tightened down. This saved me a lot of money in parts

2000 xj
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post #3 of 6 Old 05-24-2018, 07:26 AM
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Dude you are a LEGEND! I was about ready to give up hope on ever getting my 2001 4.0 xj running without bogging down or ever getting rid of the ever-present p1391 code of death, but simply turning the cam sensor slightly seems to have fixed everything. It now sounds and feels so much better, and the MIL is off and staying off for the first time since I bought it! Thank you so much for this thorough write up man.
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post #4 of 6 Old 05-27-2018, 08:12 PM Thread Starter
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Thanks for the feedback! Glad my post helped.
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post #5 of 6 Old 10-10-2018, 07:00 PM
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First off, Thanks for the great post.

I have always had a rough idle and occasional stalling in my newly built stroker. I had suspected that it might be my cam and crank sensor timing so I did some searching and found this post. I recently bought an oscilloscope and followed what xjgoat had done. Hers what I found.

What is up with my Crank signal? I had a spare crank sensor so I changed it and had the same results on the scope.

Any ideas xjgoat? Is it me operating the system? bad ground, PCM?
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post #6 of 6 Old 10-11-2018, 09:33 AM Thread Starter
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Try making your scope settings for your channel 2 match my channel 1 settings (crank signal). And update your sampling rate to match mine. I bet that cleans up your signal. Based on your picture it looks like everything is phased right most likely.
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4.0l , cam position sensor , crank position sensor , p1391

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