ASE A9 Lubrication and Cooling Systems — Questions and Answers

Question 1: 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?

A stuck-closed oil pressure relief valve
Excessive main and rod bearing clearance (Correct answer)
A restricted oil pump pickup screen
An oil level slightly above the full mark

Correct 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.

Engine oil pressure is created by resistance to flow through the bearing clearances, not by the pump alone. When bearings are worn, oil escapes quickly as it heats and thins. This shows up as low pressure at hot idle, where pump speed is lowest. A stuck-closed relief valve would cause high pressure. A restricted pickup would normally cause low pressure at all temperatures, with pump cavitation noise. Slight overfilling does not typically lower pressure.

Question 2: Why is supplemental coolant additive (SCA) maintenance especially important in a diesel engine with wet cylinder liners?

It raises the engine's operating temperature for efficiency
It increases the coolant's freezing point
It protects the liner outer surfaces from cavitation erosion (Correct answer)
It lubricates the injectors through the coolant

Correct 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.

In a wet-sleeve engine, the outer surface of each liner is in direct contact with coolant. Combustion forces make the liner vibrate, forming low-pressure bubbles that implode against the surface and erode it. Nitrite-based SCA builds a protective layer that reduces this pitting. Coolant should be tested regularly with test strips and SCA added as needed. Too little SCA risks liner pitting, while too much can cause deposits.

Question 3: 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?

Checking coolant specific gravity with a hydrometer
Measuring coolant pH with a test strip
Using a chemical block tester on the radiator fill opening (Correct answer)
Checking the thermostat opening temperature

Correct 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.

A chemical block tester draws air from the coolant fill neck through a test fluid that changes color when it reacts with exhaust gases. This confirms that combustion gas is entering the cooling system through a head gasket, crack, or liner seal. Hydrometer and pH readings indicate freeze protection and coolant condition, not combustion leaks. A thermostat check addresses temperature regulation. Other signs of a leak can include pressure surges in the upper hose and coolant loss.

Question 4: 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?

Oil pressure is higher than coolant pressure at the cooler (Correct answer)
Coolant pressure is higher than oil pressure at the cooler
The thermostat is stuck open
The radiator cap is set too high

Correct 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.

In a liquid-to-liquid oil cooler, a breach in the core allows fluid to move from the higher-pressure circuit to the lower-pressure one. With the engine running, oil pressure is generally much higher than coolant pressure, so oil contaminates the coolant. With the engine off and the system hot and still pressurized, coolant can sometimes seep into the oil. The thermostat position and cap rating do not determine the direction of the leak.

Question 5: What is the primary purpose of a pressure-rated radiator cap on a diesel engine cooling system?

To raise the coolant boiling point by pressurizing the system (Correct answer)
To reduce the coolant's freezing point
To increase coolant flow through the radiator
To bypass coolant around the thermostat

Correct 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 liquid boils at a higher temperature when under greater pressure. The cap holds system pressure up to its rating and then vents through the pressure valve. A weak cap lets pressure escape, lowering the boiling point and causing boil-over and coolant loss. A vacuum valve in the cap also lets coolant return from the overflow as the system cools. A cap has no effect on freeze protection or flow rate.

Question 6: A thermostat removed from a diesel engine is being bench tested in a container of water. What indicates that the thermostat is good?

It stays fully closed at 20°F above its rated temperature
It is open at room temperature
It begins to open near its rated temperature as the water is heated (Correct answer)
It opens fully at the first sign of warm water

Correct 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.

To bench test, suspend the thermostat in water with a thermometer without letting it touch the container, then heat the water slowly. It should be closed when cold and begin to open within a few degrees of its rated temperature. It should reach full lift at a higher specified temperature. A thermostat that is open when cold, or one that fails to open when hot, should be replaced. A stuck-closed unit causes overheating, while a stuck-open unit causes slow warm-up.

Question 7: 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?

The fan clutch has lost its silicone fluid and is not engaging (Correct answer)
The water pump bearing has seized
The thermostat is stuck open
The radiator cap is leaking

Correct 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.

A viscous fan clutch transmits torque through silicone fluid sheared between plates. As air temperature behind the radiator rises, a bimetallic spring opens a valve and the clutch locks up. When fluid has leaked out, the clutch free-spins and cannot move enough air under load, so the engine overheats. Fan clutch failure typically shows up at low speed and high load, when ram air is least available. A seized water pump would not spin freely, and a stuck-open thermostat would cause overcooling.

Question 8: An oil analysis report shows elevated sodium, potassium, and boron, along with a drop in viscosity. What does this pattern most likely indicate?

Excessive piston ring blow-by
Fuel dilution from a leaking injector
Normal wear from the oil filter media
Coolant leaking into the engine oil (Correct answer)

Correct 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.

Coolant corrosion inhibitors commonly contain sodium, potassium, and boron. When these appear in used oil in unusual amounts, coolant has entered the lubrication system, often through a failed oil cooler, head gasket, or cracked liner. Glycol contamination also degrades the oil film and can cause bearing damage. Fuel dilution would show a viscosity drop and fuel presence without these additive metals. Ring blow-by usually raises soot and insolubles. The engine should be pressure tested to find the leak.

Question 9: What is the function of the bypass valve in a full-flow engine oil filter?

To maintain a minimum oil level in the filter housing
To let oil flow around a clogged filter element and avoid oil starvation (Correct answer)
To increase oil pressure after the filter
To send filtered oil directly to the turbocharger only

Correct 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.

A full-flow filter has a spring-loaded bypass valve that opens when the pressure drop across the element exceeds a set value, such as with a clogged filter or cold, thick oil. This ensures that engine bearings are supplied with oil rather than being starved. The trade-off is that unfiltered oil, with any debris, will circulate. This is why filters should be changed on schedule and why a bypass valve stuck open can allow dirty oil into the engine.

Question 10: Which statement about an ethylene glycol and water mixture in a diesel cooling system is correct?

Using undiluted glycol gives the best freeze protection
A 50/50 mix gives freeze protection down to about -34°F (-37°C) (Correct answer)
Water alone gives better heat transfer, so no glycol is needed for corrosion control
Glycol raises the specific heat of the coolant above that of water

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.

Glycol and water protect better together than either does alone. A 50/50 mix freezes near -34°F, and the maximum protection is reached at roughly 60 to 70 percent glycol. Pure glycol freezes at a much higher temperature and transfers heat poorly. Water has better heat transfer than glycol, but it offers no freeze protection, no boil-over margin, and no corrosion or cavitation protection. Glycol actually has a lower specific heat than water, so mixtures carry slightly less heat.

Question 11: 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?

Oil is thicker when cold, so flow resistance and pressure are higher until it warms (Correct answer)
The oil pump is failing
The bearings are worn
The oil cooler is leaking

Correct 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.

Oil viscosity falls as temperature rises. Cold oil resists flow through clearances and passages, which produces a higher pressure reading until the engine warms up. The pressure relief valve limits the maximum pressure by returning excess oil to the pump inlet or sump. A reading that returns to the specified range when hot is normal. A worn pump or bearings would cause low pressure rather than high, and a leaking oil cooler would cause contamination of the coolant or oil.

ASE A9 Lubrication and Cooling Systems

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?

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