Home OF4WD Forum Technical & Trucks Technical Chit Chat CJ randomly cutting out (4.0L)

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  • #1733

    (– by moejeep #5369 –)

    Many people know the 3-4 page story of me trying to fix my CJ. It seems to random stop working and if I leave it for a ½ hour it is perfectly fine.  I have been working on in this week. I have some more details and I am looking for some advice.

    I have hooked up a fuel pressure gauge so I can drive with it connected. When I am having the issue my fuel pressure drops from 30-40 psi to 5-10 psi.

    Previous tests where issue had occurred:
    – Pressure regulator disconnected.
    – Multimeter connected to the fuel pump, (never lost 12V)
    – Full fuel tank
    – Almost empty tank – seams to occur more with a low tank
    – 2 fuel pumps one in tank and one in line.

    When it happens:
    – I can recreate the issue on a 12 km drive. (engine has to be warm)
    – I have recreated it driving around a flat field
    – It happens when I am accelerating.
    – Sometimes causes backfiring.

    Engine is a 1992 4.0L HO from a YJ
    Fuel Tank is a 1985 CJ (small) tank.
    Fuel Pump is YJ pump I bought of JeepKings, not sure the year.
    There is also a Prince Auto sending unit in the tank, but is not working.

    – With this step I don’t have the tray that goes around the fuel pump.
    – The Jeep ran fine 2011 and part of 2012 (not a daily drive, I use it 2-3 times a year)

    Do you think there is an issue with my tank, fuel pump, pressure regulator or injectors?



    (– by Morpheus #5630 –)


    When we chatted the other day, you mentioned that you had previously had an external high pressure fuel pump, is that the one you still are running?

    I was thinking, you could probably replace the complete in-tank sending unit with a carbureted version

    My thought with that is to eliminate that non-working fuel pump assy, but keep the fuel float. I bet you can even buy one of those brand new really cheap.

    The other thing I don't remember if I asked, was if the external pump is mounted at the back of the truck, they are best placed at the back and "pushing" the fuel, not at the front "pulling" the fuel.



    (– by moejeep #5369 –)

    Test 1. with new fuel line away from exhaust – Failed.
    Test 2. with new fuel pump – Failed.

    Fuel System Details:
    – In tank – YJ fuel pump/sending unit, check valve is part of the pump.
    – Fuel filter
    – T connection at the front, attaching my fuel pressure gauge and the fuel rail.
    – From the fuel rail, the injectors and the fuel pressure regulator.
    – From the regulator back into the tank

    The charcoal canister also connects to the tank (straight to the tank, no valves).

    I am thinking of doing these tests tonight:

    Test 3. Disconnecting the pressure regulator, plugging the manifold vacuum, and doing a test drive.
        – as per the manual pressure should be up from 31psi to 39psi.

    Test 4. Clamp the fuel return line and do a test drive. 



    (– by Morpheus #5630 –)

    Crap, time for a fuel cell?



    (– by moejeep #5369 –)

    I think is has to be something in wide open throttle mode causing the problem…

    It is 100% not in the warm-up mode,  but it could be:
    Acceleration mode, Cruse mode, or wide open throttle mode.

    This would explain the happening sometimes…  I can recreating by driving hard and usually happens on a hill.

    As soon a I swap back to original pump I will run full gas tank test… to make sure the pump is not sucking air.
    I currently have around a 1/2 tank.


    (– by moejeep #5369 –)

    Looks like it is resolve… I have 700 + kms with no issues and 4 trails.

    Fuel tanks strainer was the cause….

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