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.
Steps
Let the engine cool completely before starting. Hot plugs in a cast iron head are harder to remove and risk thread damage. Give it at least an hour after driving.
Gather your parts. You need 6 plugs. Verify the gap on each new plug with a gap tool — factory spec is 0.035 inches. Most plugs come pre-gapped, but check anyway. Adjust by bending the ground electrode carefully; never force it.
Locate the plugs. On the 4.0L, all six plugs are on the driver's side of the engine, fired by a coil rail (1997–2001) or distributor cap and wire set (1991–1996). The #1 cylinder is at the front of the engine; #6 is at the rear near the firewall.
Blow out the plug wells. Before pulling any wires, use compressed air or a rag to clear dirt and debris from around each plug. Any grit that falls into a plug hole goes directly into the cylinder.
Remove plug wires or coil connectors one at a time. Pull from the boot, not the wire — grip the boot at the base and twist slightly while pulling. Yanking the wire itself will eventually tear the internal conductor. On 1997–2001 coil rail setups, unplug the electrical connector and remove the coil rail bolts (usually 3–4 bolts, 8mm) if accessing plugs individually or if clearance is tight.
Break loose each plug. Use a 5/8" (16mm) spark plug socket with a short extension. Apply steady counterclockwise pressure. If a plug feels stuck — not tight — stop. Apply penetrating oil around the base of the plug and let it sit for 30–60 minutes before trying again. Do not force a seized plug in a hot engine.
Remove all six plugs. Once broken loose, spin them out by hand or with the ratchet. Keep them in order if you want to read them for diagnostic info (oil fouling, carbon fouling, or lean/white deposits tell you about engine condition).
Inspect the threads and wells. Look into each plug hole with a flashlight. Threads should look clean and intact. If you see cross-threaded damage or stripped threads, stop — that requires a Heli-Coil or Time-Sert repair before proceeding.
Thread each new plug in by hand first. This is the most important step to avoid cross-threading. The plug must spin freely for the first several turns using your fingers. The 4.0L head has enough depth that you can start plugs by threading them onto a piece of vacuum hose or the socket with no ratchet attached to keep them straight.
Torque to spec. Once hand-tight, use a torque wrench and snug to 27 ft-lbs. The cast iron head handles this torque fine. Do not over-torque chasing a "snug" feeling — the threads in cast iron are more forgiving than aluminum but they're not bulletproof.
Reinstall plug wires or coil rail. Press wire boots firmly onto each plug until you feel or hear them seat. Reconnect coil rail connectors and reinstall rail bolts if removed. Torque rail bolts to approximately 75 in-lbs (6 ft-lbs).
Start the engine. It may take an extra crank or two on the first start after plug work. Let it idle for a few minutes and confirm it's running smoothly. If you get a rough idle or misfire, check that all boots are fully seated and all connectors are plugged in.
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.