I know there's no shortage of threads on DTC = P1391, but thought I'd document my episode as it may help someone...


Recently (last month) purchased this truck ('99 Dakota 5.2L Automatic 4WD) to serve as work horse around my property (with hilly terrain).


Mistake #1: I violated my own policy to assess vehicle for purchase on cold engine only... 1st check fluids etc, then starting so engine goes through an OL then CL as it would typically on first start of the day. I saw the vehicle for sale online, called the dealer and learning it was still available told him I'd stop by around noon. Well it was clear the engine was warm when I got there. But everything checked out okay, including test drive.


Mistake #2: Being unfamiliar with 1999 vintage dodge dakota, I didn't realize the check engine light was not functioning (when turning on the key a collection of dash lights lit, one of them said "check gauges" which I thought was a little curious for check engine). Later I learned the check engine light wasn't functioning.


The day after purchased, I started the truck and immediately as I exited the driveway it began... tachometer was fluctuating up/down and no power (barely made it up a hill). Returning home, scanned for codes and found P1391.


Two operating symptoms were: 1) Had to let engine warm once each day prior to attempting to drive; during which time the engine would rev up/down searching for what I didn't know 2) When shifting into O/D it would lurch (usually only once)


I bought a tune up kit (spark plugs/wires, CMP sensor, distr. cap/rotor) and oem mopar CKP sensor. Learning that the CKP sensor couldn't be in a worse location, I only installed the tune up kit and it continued to exhibit both symptoms (albeit the lurch into O/D was noticeably less severe).


After searching for some helpful tips on replacing the CKP sensor, I found this video: Although it was pretty clear this job had to be performed by feel only (zero visibility), Lenny (in the video) gave me confidence it could be done


Found the CKP Sensor in the truck, although it looked new, was aftermarket. I installed the OEM Sensor (Mopar Part Number: 56027870), and went for a test drive. ECM was clear of codes (as I disconnected the battery when replacing) and it remained clear of codes after 15 mile drive. I went into O/D many times (both up and down hill). It was very smooth, not even the slightest lurch. I never knew a CKP Sensor could only partially work, but this experience has made it very clear that the aftermarket one in the truck was bad news.


I'll post again after cold start tomorrow morning.


Here's a picture of the CKP sensor I used:

 

Report on cold start of truck this morning:

1. It started and immediately idled normal (no up/down reving as was previously)
2. Again ran very well; smooth shifting through all gears including O/D