TJ 4.0L Spark Plug Replacement — Interval, Gap, and Which Plugs to Buy

The AMC/Chrysler 4.0L inline-six is a robust engine with a straightforward ignition system: six plug wires, six conventional plugs, one distributor. No coil-on-plug complexity. Plug replacement is a genuine 45-minute job even on the first attempt.

The 4.0L specifies a gap of 0.035 inches. Stock NGK copper plugs are the benchmark — they run well, cost under $20 for a set of six, and gapping them fresh out of the box is recommended even if the package says "pre-gapped." Heat cycles cause plugs to drift. The Chrysler service interval for copper plugs is 30,000 miles; platinum or iridium plugs extend that to 50,000 to 60,000 miles. For a daily driver, copper at 30k is a perfectly reasonable maintenance plan. For a rig that gets less use, platinum or iridium makes sense — less chance of them sitting for years and seizing into the head.

One consistent TJ problem: the fourth and fifth plug wires deteriorate from heat exposure and crack internally. Rough idle or a misfire code on cylinder 4 or 5 is often the plug wire, not the plug. Replace wires at the same time if they're more than 8 years old.

• 5/8" spark plug socket with rubber insert (the insert holds the plug for extraction — a regular socket lets it fall)
• 3/8" ratchet with 6" and 3" extensions
• Torque wrench
• Spark plug gap gauge
• Anti-seize compound (thin coat on threads — prevents seizing into the aluminum head)
• Dielectric grease for the plug boot ends

1. Work on a cold engine. Plug threads are much harder to start in aluminum when the head is hot.

1. Remove plug wires one at a time. Twist the boot 90 degrees before pulling — don't yank straight off. Lay the wire aside in order so you don't mix up cylinder positions.

1. Blow compressed air into the plug well before removal. Debris in the well goes straight into the cylinder once the plug is out.

1. Remove plug with socket and ratchet. If a plug feels tight, add penetrant and wait 15 minutes rather than forcing it. Stripped threads in an aluminum head are an expensive repair.

1. Check the old plugs. Tan or gray electrode = healthy. Black and sooty = rich mixture or oil burning. White or blistered = running lean or overheating. Worn electrode (rounded, large gap) = past interval.

1. Gap new plugs to 0.035". Use the gap tool to check — adjust by bending the ground electrode (outer tip) only. Never adjust the center electrode.

1. Apply thin coat of anti-seize to plug threads. Not needed with copper plugs in steel heads, but the 4.0 has an aluminum head — anti-seize prevents galling.

1. Thread plugs in by hand first. Any binding means cross-threaded — back out and re-start. Torque to 18 ft-lb. No torque wrench? Snug + 1/4 turn after seating on a new gasket.

1. Apply dielectric grease inside the plug boot. Reinstall wire. Press until it clicks.

1. Start engine and verify no misfires. Check for codes with an OBD-II scanner if any misfires are present.

• Plug 6 (rear-most) is the tightest access point on the 4.0. Use a universal joint on the extension if needed. A wobble extension makes it manageable.
• If any plug comes out with oil on the threads or the boot end, you likely have a valve cover gasket leak getting into the plug wells. That's a separate repair, but worth knowing.
• Don't over-torque into the aluminum head. 18 ft-lb is the spec — plugs feel "not that tight" at 18 ft-lb, which is correct.

NGK copper plug set (6): $15–$20. Denso iridium set: $45–$55. Plug wire set if replacing: $30–$50 (NGK or Accel). Full tune-up parts (plugs + wires): $50–$80. One of the better dollar-per-result maintenance jobs on the 4.0.

Tools required

• 5/8" spark plug socket (with rubber insert)
• 3/8" ratchet
• 6" and 3" extensions
• torque wrench
• plug gap tool
• anti-seize compound

Parts

Sources

• Jeep TJ 4.0L tune-up specs
• NGK spark plug gap guide

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• TJ 4.0L Oil Change — Specs, Interval, and What to Use
• TJ Battery Replacement and Upgrade Guide
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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.