ABYC Diesel Practice Exam

What may cause exhaust gases to leak into the coolant system?

1. A defective thermostat

2. A cracked radiator

3. A defective cylinder-head gasket

4. Improper coolant mixture

The correct answer is: A defective cylinder-head gasket

The presence of exhaust gases leaking into the coolant system can be attributed to a defective cylinder-head gasket. The cylinder-head gasket serves the critical function of sealing the joint between the engine block and the cylinder head, preventing any leakage of combustion gases, oil, and coolant. If this gasket becomes damaged or fails, it can allow exhaust gases to escape from the combustion chamber and mix with the coolant in the system. This can lead to overheating, contamination of the coolant, and potentially severe engine damage if not addressed. Other factors, such as a defective thermostat, a cracked radiator, or an improper coolant mixture, do not typically lead to exhaust gases infiltrating the coolant system. A defective thermostat might cause overheating or improper engine temperatures but would not create a pathway for exhaust gases. A cracked radiator primarily results in coolant loss through leaks, and an improper coolant mixture could affect cooling efficiency but would not introduce gases from the combustion process into the coolant. Hence, the root cause identified here is directly related to the integrity of the cylinder-head gasket.