During the cooling system pressure test, gauge pressure rises sharply when the engine is started. Which of the following would most likely cause this condition?

During the cooling system pressure test, gauge pressure rises sharply when the engine is started. Which of the following would most likely cause this condition?

• A. A worn water pump
• B. A faulty thermostat
• C. A cracked cylinder head
• D. A faulty radiator pressure cap

Answer: (C)

When the cooling system test shows the gauge pressure rising sharply as soon as the engine is started, it points to a breach that allows combustion gases to enter the cooling circuit. If the cylinder head is cracked, hot combustion pressures can push into the coolant passages, causing a rapid increase in coolant system pressure as the engine begins firing and cylinder pressures rise. This kind of sudden spike happens because the path for high-pressure gas from the combustion chamber into the cooling system is direct and immediate, which is exactly what a cracked head would introduce. Other problems like a worn water pump affect circulation, a faulty thermostat changes flow and overheating behavior, and a faulty radiator cap alters how pressure is regulated—none of these typically cause a sharp, startup-induced pressure surge from inside the engine.

When the cooling system test shows the gauge pressure rising sharply as soon as the engine is started, it points to a breach that allows combustion gases to enter the cooling circuit. If the cylinder head is cracked, hot combustion pressures can push into the coolant passages, causing a rapid increase in coolant system pressure as the engine begins firing and cylinder pressures rise. This kind of sudden spike happens because the path for high-pressure gas from the combustion chamber into the cooling system is direct and immediate, which is exactly what a cracked head would introduce. Other problems like a worn water pump affect circulation, a faulty thermostat changes flow and overheating behavior, and a faulty radiator cap alters how pressure is regulated—none of these typically cause a sharp, startup-induced pressure surge from inside the engine.