2012 Buick Enclave Misfire Diagnosis: Beyond Coils & Plugs

Initial Assessment and Scan Tool Data

• 💡 The 2012 Buick Enclave arrived with a flashing check engine light and a severe misfire, despite new coils and plugs already being installed.
• 🔋 The vehicle had a completely dead battery, requiring a jump start, which likely caused spurious codes like window calibrations.
• 🔍 Scan tool diagnostics revealed a P0300 code (random/multiple cylinder misfire) and indicated that cylinder #6 was the primary culprit.

Fuel System and O2 Sensor Analysis

• ⛽ O2 sensor data suggested the issue was likely not an injector fault, as the system struggled to go lean even when commanded.
• 📉 Bank two fuel trim was significantly negative (-15% short-term), indicating the engine was compensating for a perceived rich condition, further ruling out a simple injector failure.
• 📈 Rail pressure was observed to increase after cycling the key, suggesting the high-pressure fuel system was functioning correctly.

Compression Test and Initial Findings

• 👂 The engine's cranking cadence suggested a significant loss of compression in one or more cylinders.
• 📍 Cylinder #6 was identified as the likely source of the misfire, and a compression test was performed.
• 💥 The compression test yielded 0 PSI for cylinder #6, confirming a major internal engine issue.

Internal Inspection with "Mr. Wiggles"

• 🔬 A borescope inspection using "Mr. Wiggles" was conducted to visually inspect the cylinder's internal components.
• 🧐 Initially, no obvious holes in the piston were visible, but the camera revealed valves that appeared to be open when they should have been closed.
• ⚠️ The inspection suggested a potential issue with valves not closing properly, possibly due to carbon buildup or mechanical damage.

Leak Down Test and Exhaust Valve Leakage

• 💨 A leak down test indicated 90% leakage in cylinder #6.
• 💨 Surprisingly, the leak was not audible from the crankcase or intake, which is typical for ring or intake valve issues.
• 🔥 By feeling for heat from the exhaust tips, it was confirmed that the significant blow-by was escaping through the exhaust valves.

Diagnosis and Next Steps

• ⚙️ The diagnosis points to a problem with the exhaust valves not sealing properly, potentially due to damage or a valve seat issue, despite visual inspection not revealing obvious damage.
• 🔧 The recommended next step is to remove the valve cover to inspect the valve train for any mechanical issues before proceeding to more extensive engine disassembly.
• 📉 Given the age of the vehicle (2012), the cost of repair may outweigh the vehicle's value, and the decision will ultimately rest with the customer.