Jeep TJ 4.0L P0302 Misfire Diagnosis: Spark Plugs, Coil Pack & Injector Testing Guide
Hi everyone, I'm working on a 1999-2002 Jeep TJ with a 4.0L engine that has been throwing up DTC P0302 (Cylinder 2 Misfire Detected). The vehicle originally came in with codes P0303 and P0304, indicating misfires in cylinders 3 and 4. After replacing all spark plugs—confirmed as faulty—I cleared the codes and now only P0302 remains. I performed a compression test on all cylinders, and all readings are above 110 psi, ruling out mechanical issues. Injector testing was done using a Noid and resistance checks (ohm), and all injectors appear functional. I replaced the cylinder 2 coil pack with an aftermarket unit, but the P0302 code persists. I'm now wondering whether to install an OEM coil pack instead—though it's more expensive—is that necessary? Given this is a waste spark system, if cylinder 2 is misfiring, the companion cylinder (cylinder 4) should also be affected. I suspect either a dirty or clogged injector in cylinder 2, but I'm not sure how to confirm. I don as a scan tool to check misfire history, current misfire counts, or freeze frame data—would that help diagnose the root cause? Any guidance on next steps would be greatly appreciated.
This is a 4.0L engine? How did you test the injectors—using a Noid, light, or resistance (ohm) check? Testing alone isn't enough; try swapping injector #2 to another cylinder and see if the code moves with it. Are there any clues in the freeze frame data or fuel trim values? For example, is cylinder 2 showing dry or wet misfire? For coil packs, I wouldn’t recommend going OEM just yet—aftermarket units can work well. However, ensure you're using quality parts. Since this is a waste spark system, if cylinder 2 isn't firing properly, the companion cylinder (cylinder 4) should also show signs of misfiring.