4.0L Engine Date Codes

Engine code location

The date code is on the passenger side of the engine, on the block, just forward and up from the distributor on a machined flat surface.

The digits of the code identify:

  • 1st Digit-The year (8 = 1998).
  • 2nd & 3rd Digits-The month (01 – 12).
  • 4th & 5th Digits-The engine type/fuel system/compression ratio (MX = A 4.0 Liter (242 CID) 8.7:1 compression ratio engine with a multi-point fuel injection system).
  • 6th & 7th Digits-The day of engine build (01 – 31).

FOR EXAMPLE: Code * 801MX12 * identifies a 4.0 Liter (242 CID) engine with a multi-point fuel injection system and 8.7:1 compression ratio that was built on January 12, 1998.

Revised 1-31-2016

7 thoughts on “4.0L Engine Date Codes”

  1. Cruiser,

    Just wanted to thank you for all your work on this site. I have a 87 Cherokee and I’m currently going down your list as we speak. Your a jewel of knowledge sir!

  2. Found the code on my supposed Wrangler motor. 303MX11
    Assuming the ‘3’ at the front is for 1993? You responded to my post on CC that a TJ 4.0 L from 2001 would not work in my MJ. Curious though, how do you tell which decade to put the first digit in? Learning a lot here and appreciate all of the work you and some other folks have put in!

  3. Cruiser-I thought I posted this question the other day but I don’t see it here. I finally got a picture of the number on the block of my 4.0 L. The leading number is an 8. Ho do you know if that is 1988 or 1998? Since the motor is not original to my 1987 MJ I’ve just been a little curious about how to determine the decade of manufacture.
    Thanks

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