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Chris, Shop Foreman
Category: Jeep
Satisfied Customers: 12513
Experience:  Jeep Certified Master Tech
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The engine in my 89 YJ was replaced and Im trying to determine

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The engine in my 89 YJ was replaced and I'm trying to determine pedigree of this 4.0 straight 6,unfortunately the info I've found is a little confusing.Some info indicates a 1993 and some a 1999 engine(i was told it came out of a cherokee 02,it came complete with the computer and of course is fuel injected). My engine ID#409MX09 and the casting#53008405.Can anyone help me determine exactly what year I have and of course any other info would be great?
I was told the MX is for the 4.0 and the 9 at the end is for 1999,I have no idea if this is true or what the rest of the numbers mean? Thanks USMM

Hello and welcome to just answer Jeep. Here is the information from the factory service manual for determining the build date of your 4.0L engine. From the # XXXXX provided, it looks as though this came out of a 1994 Jeep that is a 4.0L, built on September the 9th.

Hope this information is helpful, and let me know if you need more assistance with this.






The 4.0 Liter (242 CID) six-cylinder engine is an In-line, lightweight, overhead valve engine. This engine is designed for unleaded fuel.

The engine cylinder head has dual quench-type combustion chambers that create turbulence and fast burning of the air/fuel mixture. This results in better fuel economy.

The cylinders are numbered 1 through 6 from front to rear. The firing order is 1-5-3-6-2-4 (Engine Firing Order).

The crankshaft rotation is clockwise, when viewed from the front of the engine. The crankshaft rotates within seven main bearings. The camshaft rotates within four bearings.

The engine Build Date Code is located on a machined surface on the right side of the cylinder block between the No.2 and No.3 cylinders (Build Date Code Location).

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).
  1. FOR EXAMPLE: Code * 801MX12 * identifies a 4.0 Liter (242 CID) engine with a multi-point fuel injection system, 8.7:1 compression ratio and built on January 12, 1998.
Customer: replied 6 years ago.
Thanks,this is great.I'm restoring this YJ and am sure I will have plenty of questions.Just one more for now if you don't mind.What is the cylinder test compression for this engine? Thanks again

No problem I will be here if you need more help. Compression test readings should be around 140 to 150 PSI and each cylinder should be within 25 lbs of each other. If the readings are quite a bit lower than these specs, then suspect ring or valve problems. If they are a lot higher, than suspect carbon build up issues. The 4.0L is a very tough engine, and if it was maintained well should be good for well over 200,000 miles! My wife and I had a 1991 Cherokee a few years ago and it had 247,000 miles on it and was still running great when I sold it.



Chris, Shop Foreman
Category: Jeep
Satisfied Customers: 12513
Experience: Jeep Certified Master Tech
Chris and other Jeep Specialists are ready to help you
Customer: replied 6 years ago.
Thanks again CJ,You are definitely my go to source.Great site.

You are very welcome, and thank you for the accept! If you need more help with this please let me know,



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