P0308 Fault on 2002 Jeep Grand Cherokee 4.7L V8 with LPG: Diagnosis and Fixes

The P0308 code may indicate continuous misfiring that doesn't trigger a code until it reaches a threshold. It's worth considering valve issues or head gasket leaks—common problems on the 4.7L engine. Valve springs are also known to fail over time and can cause intermittent misfires. A TPS malfunction is unlikely to affect only one cylinder, especially not in isolation.

Thanks for the input, Jeff. The issue is that when a misfire occurs, it's very noticeable—yet it often disappears and the car runs smoothly for long distances. There’s no temperature rise or coolant loss. One forum suggested that a TPS fault could cause slight misfires across all cylinders, which might combine with weak spark in cylinder #8 to trigger the P0308 code.

Can you access an OBD2 scanner and perform a live test during driving? If possible, monitor for misfires across other cylinders when the P0308 appears. Have you replaced the coil pack for cylinder #8? What type of spark plugs were used originally? As a diagnostic step, try swapping the injector on cylinder #8 with cylinder #7—then observe if the code remains on cylinder #8 during the next occurrence.

I realized that P0308 is actually a dual-cycle code: it appears as a pending code first, then triggers a full code and check engine light in the second cycle. I didn’t realize this before—often, even after stopping briefly, the system doesn't require a full key-off restart to re-enter the cycle. After understanding this, I found that the misfire wasn’t intermittent but actually consistent. This led me to suspect a head gasket leak. As a temporary fix, I added a coolant leak sealant in the radiator. After 30 miles, the engine felt significantly smoother and has run without any codes or pending issues for six full stops—suggesting a possible short-term improvement.

The P0308 code returned after six clear driving cycles. I initially thought the leak seal had failed, but I may have been wrong. After further investigation, it appears that cylinder #8 was suffering from a failing spark plug. When I purchased the vehicle, I was told all plugs were replaced—however, upon inspection, several (including cylinder #8) were old and severely worn Champion plugs. I decided to replace all spark plugs anyway. Upon removing cylinder #8’s plug, I found it was badly degraded. This likely explains the intermittent misfire and rough running. The car is now running smoothly again without any codes—though I’m surprised that the leak sealant provided temporary relief.

That's not surprising at all! A spark plug can perform well for one second and fail in the next due to changes in resistance, fuel mixture, or temperature. Never assume that a previous service was completed correctly—always verify with actual inspection. Those old Bosch plugs should be discarded—they were clearly never properly maintained.

I'm having the same issue on my 2002 Jeep Grand Cherokee 4.7L V8. I replaced cylinder #8’s spark plug, but the problem persists—now I’m losing coolant. Any suggestions for diagnosing this? Could it be a head gasket leak or another internal issue?

Yes—it's definitely a headgasket leak. The coolant loss and misfire are strong indicators of internal engine sealing failure.