For Jeep vehicles equipped with the AMC/Jeep 4.2L (258 cubic inch) inline-6 engine, understanding the correct firing order is essential for proper ignition system function and troubleshooting.
The Standard Firing Order
The firing order for the 4.2L inline-6 engine is: 1-5-3-6-2-4.
This means cylinder number 1 fires first, followed by cylinder 5, then cylinder 3, then cylinder 6, then cylinder 2, and finally cylinder 4, before the sequence repeats.
Cylinder Numbering
- Cylinder 1: Frontmost cylinder (closest to radiator).
- Cylinders 2, 3, 4, 5: Sequentially rearward from cylinder 1.
- Cylinder 6: Rearmost cylinder (closest to firewall).
Distributor Rotation and Terminal Placement
The distributor rotor rotates Clockwise (CW). Knowing the rotation direction is critical for correctly positioning the spark plug wires on the distributor cap terminals relative to cylinder number 1.
- Locate the #1 spark plug terminal on the distributor cap. This is your starting point.
- Following the clockwise rotation of the distributor rotor, the terminals correspond to cylinders in this firing order sequence.
Visual Clock Diagram Reference:
- Imagine the distributor cap as a clock face with #1 terminal at approximately the 5 o'clock position.
- Moving clockwise:
- Next terminal (~7 o'clock): Cylinder 5
- Next terminal (~9 o'clock): Cylinder 3
- Next terminal (~11 o'clock): Cylinder 6
- Next terminal (~1 o'clock): Cylinder 2
- Next terminal (~3 o'clock): Cylinder 4
Critical Steps for Service
- Disable the ignition system before disconnecting any wires.
- Always mark spark plug wires and distributor cap terminals individually before removal. Use numbered tags or masking tape. Do not rely solely on position memory.
- When reinstalling wires, connect them to the spark plugs first for correct routing clearance, then connect them to the correctly sequenced distributor cap terminals.
- Ensure wires are securely seated on both the spark plugs and distributor cap terminals.
- Route wires neatly away from sharp edges and excessive heat sources (like exhaust manifolds).
Setting Cylinder 1 to Top Dead Center (TDC) on the compression stroke provides the absolute reference point for verifying distributor rotor position aligns with the #1 cap terminal and confirms correct plug wire sequence during reassembly.