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Thread: Difference Between 4.0 and 4.2

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    Chiks=Trouble XJ=:) CrewDawg15/22's Avatar
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    USAF Difference Between 4.0 and 4.2

    Here I go again. What is the biggest difference between the 4.2 and the 4.0 eng? Have any Jeeps come with the 4.2 in the last 10 to 15 years and are those fuel injected? I cracked my block this weekend and the cheapest I can find locally is a 4.2 but it has the mechanical fuel pump where my coil goes? Can I make this work? Or should I just keep looking? Found a 4.0 on ebay but it would have to ship from Main and not sure what the shipping cost would be.
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    F it! It ain't my Jeep. Ricky Booby's Avatar
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    4.2 = carb
    4.0 = fi

    Not too sure on other major differences.
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    4.0 should be a dime a dozen, and if that's what you have - that's what I would stick with. You shouldn't be having that difficult of a time locating an engine.
    The 4.2's didn't come from the factory with high hp/tq. The 4.0's have the High Output option, and as mp101v stated they have FI.
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    Recovery Whore RescueJP's Avatar
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    If you take that 4.2 block and combine that with your existing 4.0 head and manifolds and fuel rail u have a fuel injected engine that is actually making more low end tourqe.
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    Opressor of "Men" AberdeenLiberty's Avatar
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    Quote Originally Posted by mp101v View Post
    4.2 = carb
    4.0 = fi

    Not too sure on other major differences.
    Plus it's .2" bigger.

    I also hear that the stock 4.2 carb is junk and should be swapped out.

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    Chiks=Trouble XJ=:) CrewDawg15/22's Avatar
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    Quote Originally Posted by rescueyj View Post
    If you take that 4.2 block and combine that with your existing 4.0 head and manifolds and fuel rail u have a fuel injected engine that is actually making more low end tourqe.

    I got an article out of an OFFRoading site that says if I do this I need to plug some of the water passages on the 4.0 head because the 4.2 does not use them if I am using a 4.0 head gasket does this really need to be done? I know that Chevy has some extra holes in the head that are blocked off by the head gasket (aparent when you remove the head and see the outline on the gasket) but have never heard of having to plug them off. Also are there any 4.2 blocks that do not have the fuel pump on the block. Mine is still going doesnt seem to be leaking as much after I welded it but would still like to get a new (to me) block so I figure I have time to play with. Cheapest I can find is 250 at the pic-a-part but there is no guarantee and dont know if it even runs. I think I am gonna go check it out see if I can atleast find a complete eng maybe one that was hit in the side or rear of the vehicle.
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    Senior Member HillBillE's Avatar
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    All 258's (4.2's) have the mechanical fuel pump, but you can just put a plate over the opening. (Thiers a cam lobe just inside that runs the fuel pump)

    A cover plate will keep the oil in and dirt out, and won't harm a thing.

    Yes, you'll need to plug the water passages, but from what I've seen/read, it's fairly easy to do. There's lots of write ups on the web, as doing the 4.0 head swap is popular with the CJ/early YJ guys.

    They quit using the 4.2 around 88-89 I believe, at least in Wranglers.

    Some of the American Eagles had the 4.2 also. The 258/4.2 is an AMC engine, and was used in CJ's, early YJ's and some AMC cars.

    And yes, the later 258 carbs stink (Carter BBD) but the stepper motor can be neutralized using the 'Nutter Bypass' which eliminates the computer.


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    Senior Member HillBillE's Avatar
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    With over 2.8 million XJ's produced, and all the ZJ's (early Grand Cherokees) and MJ's ( Jeep Comanche pick up) you should be able to find a 4.0 block/engine for you rig, close to home and reasonably priced.

    Some early XJ models are Renix engines, but the block will be the same, from 88 ( I believe ) on up, any 4.0 from an XJ, ZJ should be a direct swap. Good luck!


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    Difference Between 4.0 and 4.2


    simple version answer is = 0.2

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    Think I found one went to the pic-a-part and saw a few candidates. Looking at two one is a 92 with 102000 on it that looks like it was abandoned and picked up by the city. The other is a 94 with 120000 that has a big ol stencil on it that says "hold stolen" guess no one claimed it and it was taken to the junk yard. I can always take all the new pistons out of this one and transfer them over just wish I could hear the replacement eng run. Ive had bad luck with junk yard motors.
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