DFFT Cooling Systems 2 — Questions and Answers
Question 1: Coolant is found mixed with engine oil, appearing as a milky substance on the dipstick. What is the likely cause?
- A failed head gasket or cracked cylinder head allowing coolant into the oil passages (Correct answer)
- Water condensation in the crankcase from short trips
- A cracked intake manifold gasket on a manifold-cooled design
- A faulty oil cooler allowing coolant to mix with oil
Correct answer: A failed head gasket or cracked cylinder head allowing coolant into the oil passages
Coolant in the oil produces a milky, creamy appearance; the most common cause is a head gasket failure that opens a pathway between the water jacket and oil passages.
Question 2: Electrolytic corrosion is found on the radiator cores and cooling system fittings. What is the cause?
- Depleted corrosion inhibitors in old coolant allowing electrochemical reactions between dissimilar metals (Correct answer)
- Using the wrong type of antifreeze in the system
- Overfilling the coolant expansion tank causing overflow
- Air pockets in the cooling system causing localized boiling
Correct answer: Depleted corrosion inhibitors in old coolant allowing electrochemical reactions between dissimilar metals
Modern coolant contains corrosion inhibitors that become depleted over time; without inhibitors, the coolant becomes electrolytic and causes galvanic corrosion between aluminum and steel components.
Question 3: A water pump is removed and found to have eroded impeller vanes. What caused this condition?
- Cavitation from low coolant levels or depleted coolant causing erosion of the impeller (Correct answer)
- Normal wear from long service intervals
- Using antifreeze concentrate without proper dilution
- Excessive belt tension over-loading the pump bearings
Correct answer: Cavitation from low coolant levels or depleted coolant causing erosion of the impeller
Cavitation occurs when vapor bubbles form and implode against the impeller surface; this is caused by low coolant, incorrect mixture ratio, or overly high pump speed, and erodes the impeller metal over time.
Question 4: After replacing a thermostat, the cooling system must be bled. Why is this step critical?
- Air pockets prevent proper coolant flow, causing hot spots and inaccurate temperature readings (Correct answer)
- Bleeding removes any contaminants introduced during the repair
- The thermostat requires a full flow of coolant to calibrate its opening temperature
- Air pressure in the system can prevent the thermostat from opening
Correct answer: Air pockets prevent proper coolant flow, causing hot spots and inaccurate temperature readings
Air is a poor conductor of heat compared to coolant; trapped air pockets prevent effective heat transfer, cause overheating in specific areas, and can lead to erratic temperature gauge readings.
Question 5: A vehicle's cooling system is pressurized to 16 psi and holds for 10 minutes, but the engine still overheats. What should be suspected?
- A combustion gas leak into the coolant caused by a head gasket failure (Correct answer)
- A clogged radiator reducing heat rejection capacity
- A water pump impeller spinning freely on its shaft
- A blocked heater core reducing total coolant flow
Correct answer: A combustion gas leak into the coolant caused by a head gasket failure
A head gasket can fail in a way that only leaks combustion gases into the coolant without leaking coolant externally; this introduces non-condensable gas that reduces cooling efficiency.
Question 6: What is the correct antifreeze-to-water ratio for most US passenger vehicles for year-round protection?
- 50% antifreeze and 50% distilled water (Correct answer)
- 70% antifreeze and 30% water for maximum protection
- 30% antifreeze and 70% water for warmer climates
- 100% antifreeze for best freeze and boil protection
Correct answer: 50% antifreeze and 50% distilled water
A 50/50 mixture of ethylene glycol antifreeze and distilled water provides freeze protection to approximately -34°F and boil protection to approximately 265°F while maintaining proper heat transfer.
DFFT Cooling Systems 2
Coolant is found mixed with engine oil, appearing as a milky substance on the dipstick.
What is the likely cause?