Cooling Systems

Cooling is the most diagnosable system in the subtest: each symptom points at one component, and the reasoning is short.

The path

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Follow the loop:

1. Coolant absorbs heat passing through passages in the block and head.
2. It leaves through the upper hose, past the thermostat.
3. It flows down through the radiator, giving its heat to air drawn through by the fan.
4. It returns through the lower hose to the water pump.
5. The pump drives it back into the block, and the loop repeats.

Hot coolant leaves at the top and cool coolant returns at the bottom, which is why the upper hose is the hot one. A question about which hose is which is answered by the direction of flow.

A branch feeds the heater core, which is a small radiator inside the cabin - which is why a car's heater works by taking engine heat, and why a heater blowing cold can mean low coolant.

The components

The thermostat

A closed thermostat is the normal state of a cold engine.

It blocks flow to the radiator so the engine warms quickly, then opens at its rated temperature and lets coolant circulate.

Warming up quickly is the point: a cold engine wears faster, burns fuel less efficiently and produces more emissions. The thermostat is not only a safety device, it is an efficiency device.

Stuck open: the engine runs cold and never reaches operating temperature.

Stuck closed: the engine overheats, because coolant cannot reach the radiator.

Those two symptoms are the question, and they follow directly from what the valve does.

The radiator cap

The cap seals the system and holds it above atmospheric pressure.

Raising the pressure raises the boiling point, which lets the coolant run hotter without boiling - and hotter coolant sheds heat to the air faster.

Never open a hot radiator cap. The system is pressurized and above the normal boiling point, so releasing the pressure makes the coolant flash to steam violently. That is a real safety point and it is asked.

Coolant

Coolant is water plus antifreeze, usually ethylene glycol, typically about half and half.

It does three things: lowers the freezing point, raises the boiling point, and inhibits corrosion.

Plain water alone is a poor choice, not because it cools badly - water's high specific heat makes it excellent - but because it freezes, boils lower and corrodes the system.

Diagnosing

Each symptom points at one component.

The idle-against-speed distinction is the neatest piece of reasoning here. At speed, air is forced through the radiator by motion; at idle, only the fan moves it. So overheating that appears only at idle is a fan fault, and overheating that appears only at speed is not.

Know it by sight

What you can skip

Across the 59 questions on this topic in our bank:

• Coolant chemistry. Ethylene glycol is named three times; the different coolant formulations by chemistry are not asked.
• Pressure cap ratings. Pounds per square inch come up three times. Know why the cap is pressurized - it raises the boiling point - not a number for it.
• Fan clutches come up twice.

Where people lose points

Thinking the thermostat's job is only to prevent overheating. It gets the engine warm quickly too.

Saying a stuck-open thermostat causes overheating. It causes running cold.

Thinking the radiator cap is just a lid. It sets the system pressure.

Saying plain water cools better and forgetting freezing, boiling and corrosion.

Confusing the water pump with the radiator. The pump moves, the radiator sheds.

Opening a hot cooling system.

Work one in under a minute

A vehicle overheats in stop-and-go traffic but runs at normal temperature on the highway. What is the most likely cause?

At speed, air is forced through the radiator by the vehicle's motion, and cooling is adequate.

At idle, there is no ram air, so the fan is the only thing moving air through the radiator.

Cooling fails only when the fan is the sole source of airflow, so the fan is not working.

Note that a thermostat or pump fault would cause overheating at both speeds, which is what rules them out.

Where this leads

The same heat-transfer ideas run through the lubrication system, and the drive belt that turns the pump reappears in the engine's accessory drive.