How to Change the Spark Plugs for a 1998 Jeep Wrangler TJ

Check the gap between the center and ground electrodes on all six new Champion RC-12LYC, or equivalent spark plugs, using a spark plug gap tool. The correct gap for the 4.0-liter's spark plugs is 0.033 to 0.037 inches. Use the spark plug gap tool to make the gap wider or narrower, as needed.

Find the coil rail assembly on the top center of the engine's valve cover. Remove the four coil rail-retaining bolts, using a ratchet and socket. Pry upward on the coil rail, using a 12-inch pry bar, to free it from the spark plugs. Slide the locking tab on the wiring harness -- on the rear of the coil rail -- toward the passenger's side of the Wrangler to unlock it. Press the locking button on the coil rail wiring harness and pull the harness away from the coil rail. Pull the coil rail off the top of the engine, exposing the spark plugs below it, and place it in a secure location.

Remove one spark plug, using a ratchet, 6-inch extension and a spark plug socket. Pull the spark plug from the spark plug socket and push a new plug into the socket until the socket's rubber insert holds the plug in place.

Hand-tighten the spark plug into the engine, using a 6-inch extension and spark plug socket. Connect a torque wrench to the 6-inch extension and tighten the spark plug from 26 to 30 foot-pounds.

Repeat Steps 3 and 4 to replace the remaining five spark plugs.

Plug the wiring harness back into the receptacle on the rear of the coil rail. Slide the locking tab toward the engine to lock the harness into place. Line the coil rail boots up with the tops of all six spark plugs and press the coil rail downward until it seats on the spark plugs. Tighten the coil rail-retaining bolts from 20 to 21 foot-pounds, using a torque wrench and socket.

Check the gap on the base of all four new Champion RC-12LYC, or equivalent spark plugs, using a spark plug gap tool. The prescribed gap on the 2.5-liter engine is 0.033 to 0.037 inches. Adjust the gap on any incorrectly gapped spark plugs by widening or narrowing the gap with a spark plug gap tool.

Trace a spark plug wire until you reach where the thick rubber boot connects the wire to the spark plugs. Grab the rubber boot and pull upward with a slight twisting motion to remove the spark plug wire.

Inspect the spark plug wire for any visual defects, including cracking, splitting, burning or brittleness. If any defects exist, replace all four spark plug wires one-by-one, as you replace each spark plug. This helps you retain the correct firing order.

Remove the spark plug under where the spark plug wire once was, using a ratchet, 6-inch extension and a spark plug socket. Pull the old spark from the spark plug socket and push a new RC-12LYC , or equivalent spark plug into the socket until the socket's insert holds it in place.

Use the spark plug socket and 6-inch extension to hand-tighten the spark plug into the engine. Attach a torque wrench to the 6-inch extension and tighten the spark plug from 26 to 30 foot-pounds of torque.

Repeat Steps 2 through 5 to replace the remaining three spark plugs in the Wrangler's engine. Replacing the spark plug one-by-one keeps you from mixing up the Wrangler's firing order.