Spark Plugs - 4.0L (Champion vs NGK Debate)
Six plugs, gap 0.035 in. Champion RC12LYC was OE; NGK ZFR5N is the modern enthusiast pick that resists carbon fouling.
Champion RC12LYC (copper) and later RC12ECC were the original equipment plug. Many XJ owners and Chrysler service literature switched to NGK ZFR5N after a wave of misfire complaints traced to Champion plug quality issues in the late 1990s. Either plug, gapped to 0.035 inches, works in any 4.0L. NGK V-power copper plugs are usually preferred on stock and lightly modified engines for sharp throttle response; iridium upgrades (NGK IZFR5G-11 or similar) extend service life to 60-100k but the 4.0L does not need them. The plugs sit under the intake/exhaust manifold runners and are tight to access on cylinders 5 and 6. Use a 5/8 spark plug socket with a swivel and short extension. Anti-seize on the threads (a tiny dab only) is wise on aluminum heads. Torque to 27 ft-lb. Change every 30,000 miles with copper, longer with iridium. A persistent P0300 random misfire on a healthy XJ is often tired plugs and/or wires.
Why it works
- Restores smooth idle and MPG
- Cheap
Trade-offs
- Rear cylinders 5/6 are awkward access
Tools required
- 5/8 spark plug socket
- 3/8 ratchet with swivel
- torque wrench
- gap gauge
Parts
| Part | Vendor | Est. price |
|---|---|---|
| NGK ZFR5N spark plug (qty 6) | RockAuto/Summit | ~$18 |
| Champion RC12LYC alt | any | ~$18 |
Sources
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Written and maintained by an AZ wheeler and driveway wrencher. How we source guides. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.