If a head gasket is defective, what symptom might appear during a cylinder leakage test?

• A. Air in the cooling system
• B. Bubbles in radiator drain
• C. No test result
• D. Fuel in the cooling system

Answer: (A)

When a head gasket is defective, combustion gases can push past the gasket into the cooling passages during a cylinder leakage test. Those gases enter the cooling system and push air ahead of them, so you end up with air in the cooling system. In practice, this is what you’re looking for: the cooling system contains air or gas, which may also show up as bubbles seen in the radiator or coolant, but the key sign is air in the cooling system. The other possibilities aren’t tied to a head gasket leak in this scenario—fuel in the cooling system points to a fuel-related issue, and no test result would mean no leakage detected.