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).
- 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.