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Thread: 4.2 to 4.0 yj swap??

  1. #1
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    4.2 to 4.0 yj swap??

    a buddie of mine has an 88 yj with a very tired 4.2L straight 6. we are tossing the idea of throwing in a 4.0 outta 1992 jeep cherokee. just wanting to know if anyones done this before and can give light on any hangups in the swap. we mostly have everything covered but the main consern is if the flex plate out of the 4.2 will bolt up to the 4.0. we suspect it will but just looking for first hand knowledge.

    thanks.

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    Well, I used a 4.0L Flywheel from a cherokee when I did my swap from a 4.2 to a 4.0L in my YJ and it worked good. Not sure if it goes the same for Flexplates, but I assume so.

    Any other questions about the swap feel free to ask me.

    Blake
    Last edited by Bling Bling Blake; 01-09-2007 at 10:57 PM.
    Impossible is just a big word thrown around by small men who find it easier to live in the world they've been given than to explore the power they have to change it. Impossible is not a fact. Its an opinion. Impossible is not a declaration. Its a dare. Impossible is potential. Impossible is temporary. Impossible is nothing.

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    first dibs on yer 4.2
    What can I say, I'm Skitarded...

  4. #4
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    I did the swap but I used the flex plate from the 4.0

    If you are going to try and locate the CPS off the bellhousing of the tranny, you are going to need the 4.0 flex plate. Otherwise, you will be needing a CPS relocation kit (from He$co). I think the 4.2 flexplate should work though. The relocation kit runs about $300US (i.e. around the price of a used 32RH tranny)

    Other things to consider:
    Exhaust - exhaust manifold is different from the 4.2 to the 4.0
    Wiring (interior, gauges, lights, etc... - I did alot of splicing)
    Fuel system (4.0 needs higher pressure - inline pump or replace one in tank)

    I would say grab everything you can think of from the donor vehicle. Grab all the wiring and everything from the engine bay. Better to junk it later than try to search for it. Don't forget to grab the MAP sensor from the firewall. The 4.0 mounts exactly the same as the 4.2

    IMO a swap well worth it!
    Last edited by Appleton; 01-10-2007 at 03:19 AM.

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    The 4.2 flex plate wont work with the 4.0 because it does not have the teeth for the CPS pick up and i believe there is also balancing issues .
    Half the fire wall/part of the hood & front grill its all CJ

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    we want to keep the stock yj auto tranny so thats why we wanted to know if the flex plate off the 4.2 would fit the 4.0

    as for fuel pump, we never even thought about that, it shouldnt be too big a deal to modify the cherokee pump to fit in place of the yj pump.

    we have a complete cherokee donor vehicle. wouldnt do it any other way.

    thanks for the quick replys.

    will keep you updated.

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    You'll need the whole 4.0 including "accessories" with the right pulleys, outputs, and mountings. You'll also need:

    1. 4.0 Flywheel or flexplate with the appropriate CPS.

    2. 4.0 Bell Housing or fab your own CPS opening.

    3. Clutch components that will fit the selected bell housing & ensure the front bearing retainer on the tranny will work with the selected clutch type - internal or external slave.

    4. At least the engine wiring harness including the computer that works with the 4.0 sensor set - ODB1 or 2. This harness should have the wiring going back to the the in-tank fuel pump.

    5. Fuel tank with internal pump or at least the pump. You can buy 3rd party external fuel pumps as well.

    6. Fuel lines - intake and return or just intake if the 4.0 has newer fuel rail with only the input. The regulator is with the pump so it doesn't need the return.

    That's all I can think of right now. It would be cheaper if you were able to find a doner vehicle that's got a rotten body & frame, with reasonable "guts".

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