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1999 Jeep Grand Cherokee Cylinder 8 Missfire: Diagnosis & Fixes After Coil Pack and Plug Replacement

Model: 99 Cherokee Fault Code: P0308 Posted: 2015-01-23 13:35

I own a 1999 Jeep Grand Cherokee with a persistent cylinder 8 missfire. I've already replaced the coil pack and spark plug for cylinder 8, but the issue persists. I'm wondering if there's a relay involved or another component that could be causing this. Additionally, I recently removed the catalytic converter during an exhaust repair—could this be contributing to the problem? Any suggestions on what steps to take next? I'd appreciate guidance on diagnostics and potential fixes.

Related fault codes
P0308
Comments (4)
Anonymous 2015-01-24 07:13

The Powertrain Control Module (PCM) has individual diodes that manage each cylinder. If the missfire continues, a PCM replacement might be necessary—though this should be considered only after other diagnostics.

Anonymous 2015-01-24 13:02

Before considering a PCM replacement, reinstall the catalytic converter first. Check for spark at cylinder #8 using a noid light. Perform a compression test on cylinder #8 and adjacent cylinders. Verify that the fuel injector is pulsing with a noid light. Also assess the condition of the old spark plug—was it wet, dry, black, or tan? Do you have an OBD2 scanner? Are there any other diagnostic trouble codes (DTCs) present? A missing catalytic converter would typically trigger a specific code in the system.

Anonymous 2015-01-24 16:30

kev2 wrote: hold off on the PCM change- first reinstall the cat converter. Check for spark at #8 plug, compression test #8 and adjacent cylinders, with a noid light is injector pulsing... how did old plug look wet,dry, black,tan? do you have a scanner- what other codes are you getting? missing cat would set a code.. The plug was black the cat was bad in process of new exhaust. Haven't checked Injectors yet

Anonymous 2015-01-25 10:14

A blackened #8 spark plug suggests fuel delivery or combustion issues. First, confirm whether the new plug is actually receiving a spark. Then perform a compression test on cylinder #8 and adjacent cylinders. Additionally, conduct a fuel pressure check and an injector leak-down test to rule out fuel system problems.