DFFT Cooling Systems 1 — Questions and Answers
Question 1: A vehicle's temperature gauge rises to the red zone shortly after startup. The coolant level is correct. What should be checked first?
- The thermostat for being stuck closed (Correct answer)
- The radiator cap pressure rating
- The water pump belt tension
- The coolant concentration ratio
Correct answer: The thermostat for being stuck closed
A thermostat stuck closed prevents coolant from circulating through the radiator, causing rapid engine overheating even when the coolant level is normal.
Question 2: A vehicle's heater produces cold air even after the engine reaches operating temperature. What is the most likely cause?
- A thermostat stuck open preventing the engine from reaching proper temperature (Correct answer)
- A clogged heater core due to coolant contamination
- Low coolant level preventing flow to the heater circuit
- A faulty blend door actuator in the HVAC system
Correct answer: A thermostat stuck open preventing the engine from reaching proper temperature
A thermostat stuck in the open position allows constant coolant flow through the radiator, preventing the coolant temperature from rising enough to produce warm air through the heater core.
Question 3: White sweet-smelling smoke comes from the exhaust, and the coolant level is decreasing without visible leaks. What is the diagnosis?
- A failed head gasket allowing coolant to enter the combustion chamber (Correct answer)
- A cracked radiator causing internal leakage
- A faulty EGR cooler mixing coolant with exhaust gases
- Condensation in the exhaust system evaporating during warm-up
Correct answer: A failed head gasket allowing coolant to enter the combustion chamber
When coolant enters the combustion chamber through a failed head gasket, it vaporizes into steam with a characteristic sweet smell and exits through the exhaust as white smoke.
Question 4: A cooling system pressure test shows the system loses pressure steadily with no external leaks visible. What does this indicate?
- An internal head gasket leak allowing coolant to escape into the combustion chamber or oil passages (Correct answer)
- A faulty radiator cap that is not holding pressure
- Air trapped in the cooling system causing false pressure readings
- A crack in the coolant reservoir allowing evaporation
Correct answer: An internal head gasket leak allowing coolant to escape into the combustion chamber or oil passages
Pressure loss without external evidence of coolant means the coolant is disappearing internally, most likely through a head gasket leak into the combustion chamber or engine oil.
Question 5: A vehicle overheats in traffic but cools down at highway speed. What component is most likely at fault?
- A faulty or non-operating electric cooling fan (Correct answer)
- A clogged radiator restricting coolant flow
- A failing water pump with worn impeller vanes
- A thermostat stuck partially closed
Correct answer: A faulty or non-operating electric cooling fan
At highway speed, ram air cools the radiator sufficiently without fan assistance; overheating only in traffic, where airflow is minimal, points directly to a cooling fan failure.
Question 6: What is the function of the radiator pressure cap?
- To raise the boiling point of the coolant by pressurizing the cooling system (Correct answer)
- To prevent coolant from circulating until operating temperature is reached
- To regulate the flow of coolant between the radiator and expansion tank
- To protect the radiator from being damaged by impact during front-end collisions
Correct answer: To raise the boiling point of the coolant by pressurizing the cooling system
By pressurizing the system (typically 13–16 psi), the cap raises the coolant's boiling point above 212°F, preventing boiling and allowing safe operation at higher temperatures.
DFFT Cooling Systems 1
A vehicle's temperature gauge rises to the red zone shortly after startup.
The coolant level is correct.
What should be checked first?