Lubrication and Cooling Systems Flashcards

A diesel engine shows normal oil pressure when cold, but at hot idle the gauge reads very low and rises sharply with rpm. Which condition is the most likely cause?

Answer: Excessive main and rod bearing clearance

Worn bearings let hot, thin oil escape too easily, so pressure drops at idle. It recovers as pump output rises with rpm.

Why is supplemental coolant additive (SCA) maintenance especially important in a diesel engine with wet cylinder liners?

Answer: It protects the liner outer surfaces from cavitation erosion

Liner vibration creates vapor bubbles on the coolant side that collapse and pit the metal. SCA forms a protective film that resists this cavitation damage.

A technician suspects a cracked cylinder head or failed head gasket is allowing combustion gases into the coolant. Which test is the most direct way to confirm this?

Answer: Using a chemical block tester on the radiator fill opening

A block tester's fluid changes color when combustion gases (CO2) are present in the air above the coolant. This confirms a combustion leak.

Engine oil is found in the coolant, but there is no coolant in the oil. The leak is at the engine oil cooler. What best explains the direction of the leak?

Answer: Oil pressure is higher than coolant pressure at the cooler

Fluid leaks from the higher-pressure side to the lower-pressure side. Oil pressure normally exceeds coolant pressure, so oil enters the coolant.

What is the primary purpose of a pressure-rated radiator cap on a diesel engine cooling system?

Answer: To raise the coolant boiling point by pressurizing the system

Pressurizing the system raises the boiling point of the coolant, allowing higher operating temperatures without boiling.

A thermostat removed from a diesel engine is being bench tested in a container of water. What indicates that the thermostat is good?

Answer: It begins to open near its rated temperature as the water is heated

A good thermostat is closed when cold and begins opening near its stamped temperature as the water heats. It reaches full lift at a higher temperature.

A diesel engine overheats at highway load. With the engine hot and shut off, the technician can spin the viscous fan clutch freely by hand with almost no drag. What is the most likely problem?

Answer: The fan clutch has lost its silicone fluid and is not engaging

A healthy viscous clutch has noticeable drag when hot. Free spinning means the fluid has leaked out, so the fan will not drive airflow.

An oil analysis report shows elevated sodium, potassium, and boron, along with a drop in viscosity. What does this pattern most likely indicate?

Answer: Coolant leaking into the engine oil

Sodium, potassium, and boron are common coolant additive elements. Finding them in used oil points to a coolant leak.

What is the function of the bypass valve in a full-flow engine oil filter?

Answer: To let oil flow around a clogged filter element and avoid oil starvation

If the element is restricted or the oil is very thick, the bypass valve opens so oil still reaches the engine, though unfiltered.

Which statement about an ethylene glycol and water mixture in a diesel cooling system is correct?

Answer: A 50/50 mix gives freeze protection down to about -34°F (-37°C)

A 50/50 mix of ethylene glycol and water protects to roughly -34°F. Undiluted glycol freezes at a higher temperature than a 50/50 mix.

A diesel engine has high oil pressure at cold start that drops to normal as the engine warms up. Which of the following explains this?

Answer: Oil is thicker when cold, so flow resistance and pressure are higher until it warms

Cold oil is more viscous, so it resists flow and produces higher pressure until warming thins it. The relief valve limits the peak.