What could cause bubbles in the radiator drain during a cylinder leakage test?

• A. Poor sealing of the coolant filter
• B. Poor sealing liner O-ring
• C. Defective head gasket
• D. Scored cylinder wall

Answer: (C)

During a cylinder leakage test, bubbles in the radiator drain indicate combustion gases are entering the cooling system. The most likely cause is a failing head gasket, which seals the cylinder head to the engine block and keeps the combustion chamber separate from the cooling passages. When that gasket leaks, exhaust gases can push into the cooling system and cause the coolant to bubble as it escapes through the radiator drain. The other options don’t provide a direct path for exhaust gas into the cooling system during this test: a coolant filter seal issue isn’t part of the cylinder gas path, a liner O-ring leak would more commonly cause coolant or oil leaks rather than combustion gases in the cooling system, and a scored cylinder wall affects compression and performance but not the presence of bubbles in the radiator during a leak test.