Fresh spark plugs on the 4.0L will restore clean idle, stronger throttle response, and reliable cold starts — and on a neglected XJ, it's one of the highest-return tune-up jobs you can do in under an hour.
The 4.0L AMC 242 inline-six is a forgiving engine, but spark plugs are a consumable it doesn't forget. The factory service interval is 30,000 miles for copper plugs. On XJs that have been sitting, running rough, or showing hard cold starts, plugs are the first thing to check. The good news: the 4.0L's six cylinders are all on one side of the engine with straightforward access, and the cast iron head means you don't have the aluminum thread-stripping anxiety you get with later engines.
The one hazard on high-mileage or long-neglected XJs is seized plugs. A plug that's been in for 60,000+ miles in a cast iron head can weld itself in with rust and heat. Rush the removal on a cold engine and you can break the plug or pull threads. The fix is straightforward — heat cycles, penetrant, and patience — but it's worth knowing going in so you don't get caught off-guard.
Copper plugs are the correct choice for the 4.0L. Platinum and iridium plugs are designed for modern ignition systems that run higher energy and fire less frequently. The 4.0L's coil rail or distributor-based ignition was engineered around copper. Stick with copper and replace on schedule.
Why it works
One of the best bang-for-your-dollar maintenance jobs on the 4.0L — $15–25 in parts restores noticeable throttle response and idle quality on a neglected engine.
Copper plugs are cheap, widely available, and correct for the ignition system — no reason to spend more on exotic plug types.
Accessing all six plugs from one side of the engine makes this a faster job than most V6 or V8 applications.
Trade-offs
Seized plugs on high-mileage XJs are a real risk. If yours have been in for more than 60,000 miles or an unknown period, budget extra time and have penetrating oil on hand before you start.
The #5 and #6 cylinders at the rear are the tightest reach. A 6" extension and a universal joint or wobbly socket can make it more manageable — a rigid extension at an awkward angle is how plugs get cross-threaded.
Pulling old wires at the same time as plugs is tempting, but inspect them first — if the boots tear during removal, you're replacing the wire set too. Have a set on hand if the wires are original or unknown age.
Tools required
5/8" (16mm) spark plug socket
3" extension
6" extension
Ratchet
Torque wrench
Spark plug gap tool
Compressed air or shop rag (to clear debris from plug wells)
Parts
Part
Vendor
Est. price
Champion RC12ECC Spark Plug (qty 6)
AutoZone / O'Reilly / Rock Auto
~$3
NGK ZFR5N Spark Plug (qty 6, preferred for 2000–2001)
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.