Honda Pilot Cold Start Misfire Diagnosis: Coolant in Cylinders

Initial Customer Complaint and Symptoms

• 🎯 The customer reported a rough idle on their 2004 Honda Pilot, specifically when starting it cold after sitting overnight.
• 💡 The Check Engine Light was on, and the vehicle was throwing misfire codes, but the issue only lasted for about 20-30 seconds before clearing up.
• ⚠️ The customer had already attempted repairs, but the problem persisted.

Initial Inspection and Suspicions

• 👃 Upon starting the cold engine, a distinct smell of antifreeze or coolant was detected, accompanied by a faint white plume of smoke.
• 💧 The coolant reservoir and radiator were found to be empty, suggesting a potential coolant loss issue.
• 🔍 Scan tool diagnostics revealed misfire codes for cylinders 3, 4, 5, and 6, along with knock sensor codes, which was not entirely helpful given the initial symptoms.

Diagnostic Process and Findings

• 🛠️ A compression test was performed, and the system held pressure, but the focus shifted to visually inspecting the cylinders.
• 🧐 Using a borescope, the pistons and cylinder walls were examined for signs of coolant contamination.
• 💧 Coolant was clearly visible inside cylinders 4, 5, and 6, indicating a significant internal leak.
• 💥 Cylinder 6 showed a substantial amount of coolant, described as a "lake," confirming it as a major culprit.

Conclusion and Next Steps

• ✅ The diagnosis confirmed that a blown head gasket was allowing coolant to enter cylinders 4, 5, and 6, causing the cold start misfires.
• ⚠️ The engine was cranked over before reinstallation of spark plugs to expel any excess coolant and prevent potential bent rod damage.
• 🚗 The mechanic advised the customer on the necessary repairs, noting that the engine's overall condition (e.g., oil burning) should also be considered.