Parts of a Cooling System in a Car

All 13 parts on the diagram

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The cooling system 13

Radiator Hot coolant runs down through thin tubes. They go from the top tank to the bottom tank. Fins between the tubes pass the heat to the air.
Water pump It draws coolant from the radiator's bottom. It pushes the coolant through the engine. It sits on the front of the engine block.
Thermostat It sits between the engine and the radiator. While the engine is cold it stays shut, so the engine warms up quickly.
Upper radiator hose It runs from the engine's thermostat housing to the radiator's top tank.
Lower radiator hose It runs from the radiator's bottom tank to the water pump.
Fan That matters most when the car is standing still. This one is on the front of the water pump; many cars have an electric fan instead.
Fan belt It runs from the engine's crankshaft pulley up to the pump's pulley. Here you see it edge on.
Pressure cap It holds the system under pressure, which raises the coolant's boiling point. If the pressure climbs too high, a valve in the cap lets coolant out.
Fan shroud The fan works inside it. The air it moves then has to come through the radiator.
Water jackets The coolant picks up the engine's heat here. They run round every cylinder in the block and over the top in the cylinder head.
Coolant recovery tank A small tube joins it to the radiator's filler neck. As the engine cools, the coolant is drawn back into the radiator.
Heater core Hot coolant from the engine passes through it. Air blown across it warms the inside of the car.
Heater hoses They join the heater core to the engine's cooling system. One takes hot coolant there and one brings it back.

How this list was compiled: Part names checked on 2026-09-30; a name is kept only when two independent sources use it. (1) US Army TM 9-8000, Principles of Automotive Vehicles, 1985, chapter 9, paragraphs 9-3 to 9-12, and paragraph 17-19, https://archive.org/details/tm-9-8000-principles-of-automotive-vehicles-1985 (radiator, coolant or water pump, fan and shroud, thermostat and its housing, water jackets, radiator pressure cap, upper radiator hose, overflow tube, coolant recovery tank, belt drive, heater core and heater hoses). (2) University of Kentucky, Automotive Mechanics Instructional Program, Block 6, Cooling System, 1973, https://archive.org/details/micro_IA41152920_0881 (radiator, radiator pressure cap, thermostat, water pump, fan belt, fan blades, fan shroud, upper and lower radiator hoses, overflow tube and reservoir). (3) Missouri Auto Mechanics Curriculum Guide, Module 9, Heating and Air Conditioning, University of Missouri, 1990, https://archive.org/details/micro_IA41155127_0082 (radiator, thermostat, water pump, upper and lower radiator hoses, heater core, heater hoses, cooling fan shroud, recovery tank). (4) Mid-America Vocational Curriculum Consortium, Diesel Mechanics: Fundamentals, 1982, https://archive.org/details/ERIC_ED220629 (radiator, pressure cap, fan, fan belt, water pump, thermostat, hoses, water jacket). (5) Wikipedia, Radiator (engine cooling) and Internal combustion engine cooling, text only (radiator, water pump, thermostat, fan, heater core). Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; no maker's engine, nothing traced. Licence: own work

How it works

The parts of a cooling system in a car carry the engine's heat away to the air. Coolant goes round a loop: water pump, water jackets, thermostat, radiator and back to the pump.

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Follow the coolant

The water pump pushes coolant into the water jackets, where it picks up the engine's heat. Once the thermostat opens, the hot coolant leaves through the upper radiator hose. It cools as it runs down the radiator and comes back to the pump through the lower radiator hose. Two heater hoses take some of it to the heater core.

Air and pressure

The fan belt turns the pump and the fan, and the fan pulls air through the radiator from inside its shroud. The pressure cap keeps the system under pressure. Coolant it lets out goes to the coolant recovery tank.

An electric fan has no belt. Its own motor turns it, and a switch that feels the coolant's temperature turns it on.
The heater core is built like a small radiator.

Questions and answers

What are the parts of a cooling system in a car?

A radiator with a pressure cap, a water pump and its belt, a thermostat, a fan in a shroud, upper and lower radiator hoses, water jackets inside the engine, a coolant recovery tank, and a heater core on two heater hoses.

What does the thermostat do?

It is a valve between the engine and the radiator. It stays shut while the engine is cold, so the engine warms up quickly, then opens to let coolant through to the radiator.

Why is a cooling system under pressure?

Pressure raises the boiling point of the coolant, which helps keep it from boiling over. The pressure cap on the radiator holds that pressure in.

What is the coolant made of?

Usually water mixed with antifreeze. The most common antifreeze is ethylene glycol, and most antifreeze also has inhibitors in it that slow rust and scale.