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Any Jeep 4.0 guru's here?

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    Any Jeep 4.0 guru's here?

    A friend recently asked me to help him get his 2003 Jeep Grand Cherokee 4WD 4.0L up and running, it's been in his driveway collecting dust since 2013. He told me the truck would seemingly lose power or buck at random intervals. There were no codes stored in the computer. Not much to go off, but a start.

    I found that these 4.0 engines often have issues with the crank shaft position sensor intermittently failing, and while I hate to just throw parts at a problem it seemed a likely culprit and a good place to start. I replaced the CPS and changed the oil, coolant, fuel filter w/regulator, battery, and transmission fluid then started it up. She ran like a champ for about 10 minutes at idle and then mysteriously shut off. We added seafoam to the gas and filled the tank, hoping to deal with any moisture in there.

    I took it for a test drive and experienced the issue first hand, it will sometime seemingly shut off while driving and restart. Tach, Speedo, and Fuel gauges all drop to zero but digital odometer and stereo continue to function. Occasionally it wont restart and I had to pull over more than once in an hour driving it. On those occasions the truck would not start immediately and would take three or more tries, sometimes dying right after starting, or jumping up rpms in intervals as if I was tapping the throttle harder and harder, despite my foot being firmly planted on it with the pedal depressed. I checked fuel pressure with key on, as well as at idle and it was at 55, actually a bit high for these jeeps from what I've read but my 4.3L runs at 65 so I'm not too concerned with it.

    I did some more research and found that these jeeps occasionally have a PCM with a flawed design that causes it to short out. I removed the PCM and backed out the screws that can cause a short, spacing them with washers. I didnt see any evidence of arcing or a short although I didnt take a magnifying glass to the circuitry by any means but it looks relatively normal.

    I'm not really sure where to go from here, my buddy doesn't really care about cost and is basically encouraging me to throw more parts at it but I wouldn't mind some tips on what else to test. I'll try to address any other questions you might have about the vehicle or things I've done. Thanks in advance for your help!

    #2
    Not one of you had any ideas?

    I'm thinking potentially one of the coil packs is going out, any way to test? I am used to being able to plug a spark plug tester straight into it but these models have three units.

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      #3
      Though I'm very familiar with this engine, I'm not familiar with the WJ chassis. I know more about the ZJ and XJ. What I thought it could be, is the fuel pump. As simple as that sounds, the random shutting off and not starting immediately. As for the gauges, I couldn't really explain besides Chrysler having shitty electronics which also plagued the ZJ. The sputtering problem was fixed right? That could be dirty fuel injectors, or spark plugs. I couldn't tell you how to test the coil pack either.

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