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P0308 Fault in 2005 Jeep Grand Cherokee 4.7L After Overheating and Engine Misfire

Model: 2005 Jeep Grand Cherokee 4.7L Fault Code: P0308 Posted: 2011-06-03 09:35

Hello, I own a 2005 Jeep Grand Cherokee Limited with a 4.7L engine and over 229,000 miles. Recently, I checked my coolant level and failed to tighten the reservoir cap properly, causing the engine to overheat. After cooling down, the vehicle became very difficult to start—eventually starting only with a misfire. A diagnostic scan revealed fault code P0308, indicating a misfire on cylinder 8. I replaced the spark plug, coil pack, fuel injector, and camshaft position sensor for cylinder 8, but the issue persists. I'm now wondering if I missed something critical in the diagnosis. Freeze frame data: - Fuel System 1: OL (Out of Limit) - Fuel System 2: OL (Out of Limit) - Calculated Load %: 53.73% - ECT (*F): 125°F - STFT B1 (%): 0.00 - LTFT B1 (%): -15.63 - STFT B2 (%): 0.00 - LTFT B2 (%): +15.63 - MAP (in Hg): 18.01 - Engine RPM: 294 - Vehicle Speed (mph): 0 - Spark Advance (*): -2.00° - IAT (*F): 89°F - TPS (%): 12.16% I'm not familiar with interpreting these values—can anyone explain what the fuel trim readings mean and whether they point to a deeper issue? Note: The engine overheating history raises concerns about internal component damage, especially in aluminum heads common on 4.7L engines.

Related fault codes
P0308
Comments (4)
Anonymous 2011-06-03 17:14

The LTFT B1 is at -15% and LTFT B2 is at +15%, which indicates a significant imbalance between the two sides of the engine's fuel trim. This suggests a major issue affecting fuel delivery—one side running lean, the other rich—likely due to a mechanical failure such as a blown head gasket, especially after severe overheating. If the misfire persists despite replacing all standard components, this points strongly toward internal engine damage.

Anonymous 2011-06-03 19:57

At this stage, performing both leak-down and compression tests is recommended to identify any cylinder-specific mechanical failures, such as valve seat loss or piston ring issues. These tests can help determine if the problem lies in sealing or internal combustion components.

Anonymous 2011-06-09 11:28

I removed the valve cover and found that the exhaust rocker arm for cylinder 8 was lying on top of the cylinder head. After removing the upper engine assembly, I discovered that the valve seat had completely dropped out of its pressed-in position. This is a known issue affecting both 3.7L and 4.7L engines in Jeep Grand Cherokee models—particularly those with aluminum cylinder heads.

Anonymous 2011-06-09 18:16

Yes, this type of failure is quite common, especially after repeated overheating events. The combination of heat stress and material fatigue in aluminum heads can lead to valve seat collapse or rocker arm dislodgement over time.