2005 Jeep Grand Cherokee 4.7L V8 P0308 Misfire: Diagnosis and Fixes
I own a 2005 Jeep Grand Cherokee with a 4.7L V8 engine. The check engine light came on, and I've already replaced all the spark plugs and coil packs. Despite these changes, the vehicle runs smoothly under normal conditions but begins to shake noticeably between 40-50 mph. The OBD2 code P0308 is still present, indicating a misfire in cylinder #8. I'm concerned about what could be causing this issue and whether further diagnostics are needed.
Do you have an OBD2 scanner? The key dance method only shows limited codes, so a proper scanner is essential to retrieve full diagnostic data. You're experiencing a misfire—this can stem from ignition, fuel delivery, or mechanical issues. Start with a fuel system cleaner like TECHRON or SEAFOAM to clear any clogged injectors. Check if you used the correct OEM spark plugs—aftermarket ones may not perform as expected. Also verify that the coil pack for cylinder #8 is properly firing. Use a noid light to confirm ignition signal at cylinder #8. Inspect the condition of the #8 spark plug (wet or dry) and check fuel pressure with the engine off and on, especially looking for drops during idle. Test if injector #8 has a leak—this can be confirmed by swapping injectors (though this is advanced). If the code persists after swap, it may point to mechanical issues. Finally, perform a compression test to rule out internal engine problems.