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    ENGINE:3.9 V6
    DESCRIBE ISSUE.... The vehicle is not mine but trying to figure this out. Apparently, it started running rough and got to the point where it needed to be towed home. CEL is on and only code is a P1391. The cam and crank sensor have been changed but the problem remains the same. I testest the plugs at both sensors and have the appropriate 5.5 v and everything seems good. If anybody has dealt with this before please let me know. I am not sure where to go from here. Thanks
  2. this is a loss of signal either CMP or CKP .

    Did you scan for codes OR try the key trick? Need to know ALL codes.
    Ideally a lab scope to view the CMP and CKP wave forms as we tug and shake harness.
    A scanner could observe signal as we move the harness - check the connectors tight, corrosion free and first several inches of wire as that is often where they fail
    there is 3 wires at sensors..... did you check the other 2?

    Anything that may lend a clue-
    recent work? replaced ... adjusted.... cleaned....
  3. I use to see this alot when the distributor bushing got loose. I would look there also. As Kev mentioned, was the distributor removed? If the distributor drive/rotor bushing is loose, it may very well be the source of this intermittent. Here is what it looks like. Screenshot (83).png

    Another thing I have noted, when installing the new crank sensor, maybe some corrosion is/was present and not removed, leaving an excessive air gap for the crank sensor. Look for that also. So even if a new one is in there, it will code. I had this happen to me last week on a 5.7l vortec. And this is my day job. Even if I am not prone to overlooking such simple things, I did in this case and a come back occurred which I absolutely hate.

    Client came back with same misfire codes on same cylinders. I had to remove the new sensor and look. And, yes, there was corrosion that had to be removed with some picks and sand paper. When I re installed it, the misses went away.
  4. However, you state it had to be towed, not normally a "intermittent" problem. Maybe it got to the point of be a hard condition. You can use a voltmeter if a scope is not available. Wiggle testing may help find a open or short. Here is how to do that for the cam shaft sensor. Screenshot (84).png
  5. having to be towed - has me thinking CKP.
  6. Yeah, that was my afterthought also, but the code is a intermittent fault and cam rotor bushing should not be overlooked.

    But when crank goes out for good, towing is usually the only option. But op says it's new. So maybe some further in depth inspection is due as I write before.
  7. "have the appropriate 5.5 v"

    I gotta ask... where does that 5.5V spec come from? 5.5V is a pretty odd-ball number for electronics.
  8. Yeah, usually 5v is the norm. Actually, re-read my previous snapshot and it should fluctuate 0-5. So that is again confirming the norm. However, some ppl don't have the best of DVM's, so that may account.

    Who knows, op will post back soon enough if he is serious about a proper fix.

    Stay cool, Kev says temps are way hot down in the states. Up here, a cool 14c this morning and now at 22c.
  9. Sorry to take so long getting back, been busy as hell all day.
    Kev, yes that was scanned and the ONLY code. I am not sure what the Key trick is.

    When I said, "had to be towed" it was just because it was running so poorly. It was not a "no start". We drove it on the trailer to get it home.

    One thing I forgot to mention is the tach jumps all over when acting up. Like its signal is shorting out too.
    Thing is I can go start it and take it for a drive and it will be fine for a mile or two. Then the tach starts jumping around immediately followed but the engine cutting in and out.
  10. tach would also have me looking at CKP....

    Senosrs come from DODGE I am no fan of aftermarket.
  11. Again, I must second Kev, this is another case I had not so long ago on another 5.7 vortec. I had installed an aftermarket CKP, from the "standard" (red box) brand if memory serves well, it was defective right out of the box. Good thing car-quest guaranteed the part, towing cost and also labor cost for the client. I did not get another one from them however. I went AC delco.

    How about you start back probing now Dan3412. It would be nice to see that when the tach is fluctuating what a voltmeter is reading directly at the crank sensor.

    Here is a trouble shoot tree that I should have posted before. Pay special attention to possible cause and step 614. Wiggle test the harness while observing tach. See pdf ACE !.
  12. Everyone here knows I am a fan of this Sherlock Holmes quote. when you have eliminated the impossible, whatever remains, however improbable, must be the truth? In this trade, and the area of specialty in which I work, we are really alot like detectives. No stone should be left un-turned.

    So don't overlook anything such a possible defective flywheel also. However improbable it may seem, if it is overlooked, you are always back to square one in the trouble shoot.
  13. to get nick going.....
    Spock used that line on star trek saying he was quoting an ancestor.
  14. Thanks guys i will get on all this tomorrow and let you know what i find or dont find.