The 4.0L likes copper plugs changed every 30,000 miles or platinum every 50,000 — gapped to 0.035". Most TJs that run rough at idle have fouled or worn plugs as the first thing to check.
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.
1. **Work on a cold engine.** Plug threads are much harder to start in aluminum when the head is hot.
2. **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.
3. **Blow compressed air into the plug well before removal.** Debris in the well goes straight into the cylinder once the plug is out.
4. **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.
5. **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.
6. **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.
7. **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.
8. **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.
9. **Apply dielectric grease inside the plug boot.** Reinstall wire. Press until it clicks.
10. **Start engine and verify no misfires.** Check for codes with an OBD-II scanner if any misfires are present.
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.
| Part | Vendor | Est. price |
|---|---|---|
| NGK V-Power copper plugs — TJ 4.0 (set of 6) | NGK | ~$18 |
| Champion Double Platinum plugs (extended interval) | Champion | ~$28 |
| Bosch Platinum+4 plugs (4.0L spec) | Bosch | ~$35 |
| Denso Iridium plugs — TJ 4.0 | Denso | ~$48 |
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.