The cooling system carries heat away from the engine and releases it through the radiator, while part of that heat warms the cabin. Here you will find water pumps, thermostats, hoses, fans, caps, and the belts and pulleys that drive the engine accessories.
In this guide: Belt tensioners and idler pulleys, Coolant reservoirs and caps, Cooling system hoses, Fan clutches, Heater cores, Radiator caps, Radiator fans, Serpentine belts, Thermostat housings, Thermostats, Water pumps.
Belt tensioners and idler pulleys
What it is: The tensioner keeps the belt at the right tension, while the idler pulleys guide it along its path. Both run on bearings.
Where it is fitted: At the front of the engine, in the accessory or timing belt run.
Signs of wear: Bearing noise or rumble, a wobbling pulley, a fluttering tensioner arm and a squealing or slipping belt.
Did you know: Noise is easy to pin down with the belt off: spin each pulley by hand; a worn bearing sounds rough, drags or has play.
Shop — Belt tensioners and idler pulleys
Coolant reservoirs and caps
What it is: The overflow tank takes up the excess coolant that expands when hot and returns it as the engine cools. It is where you check and top up the coolant level.
Where it is fitted: In the engine bay, usually beside the radiator or on the inner fender.
Signs of wear: Cracks and leaks in the plastic, a yellowed opaque tank or a leaking cap.
Did you know: On many newer cars the reservoir itself is pressurized and its cap acts as the radiator cap, so the cap must have the correct pressure rating.
Shop — Coolant reservoirs and caps
Cooling system hoses
What it is: Rubber hoses carry coolant between the engine, radiator, heater core and overflow tank. They have to withstand high temperature and pressure.
Where it is fitted: Between the engine and the radiator (upper and lower hose) and in the heater circuit at the firewall.
Signs of wear: Cracks, swollen or soft ends, leaks at the clamps and a dropping coolant level.
Did you know: Check hoses on a cold engine by squeezing them: a hose that feels too soft or crunchy should be replaced even if it is not leaking yet.
Fan clutches
What it is: The viscous fan clutch connects the mechanical fan to its drive and engages it harder when the air behind the radiator gets hot. On a cold engine the fan freewheels more, saving power and fuel.
Where it is fitted: Between the water pump pulley (or a separate bracket) and the fan, behind the radiator.
Signs of wear: Overheating at idle and in traffic, weak air conditioning in hot weather, or a constant loud fan roar.
Did you know: Simple check: with a warm engine switched off, spin the fan by hand; if it turns freely several times, the clutch is most likely worn.
Heater cores
What it is: The heater core is a small radiator that hot engine coolant flows through, warming the air blown into the cabin.
Where it is fitted: Inside the HVAC box, usually behind the dashboard.
Signs of wear: Cold air despite a warm engine, a sweet smell in the cabin, fogged windows, damp carpet and coolant loss.
Did you know: On many cars replacing the heater core means removing the dashboard, so with cold heat first check the coolant level, air in the system and the thermostat.
Radiator caps
What it is: The radiator cap keeps the system under pressure, which raises the coolant's boiling point. Its valve releases excess coolant into the overflow tank when pressure gets too high.
Where it is fitted: On the radiator filler neck at the top of the radiator.
Signs of wear: Coolant loss with no visible leak, boiling coolant, a cracked cap seal or swelling hoses.
Did you know: Never open the cap on a hot engine; the pressurized coolant can spray out boiling hot.
Radiator fans
What it is: An electric fan pulls air through the radiator and A/C condenser when the car is stationary or moving slowly. It is switched on by the engine computer or a temperature sensor.
Where it is fitted: Behind the radiator, in a shroud attached to the radiator.
Signs of wear: The fan does not start at high temperature, runs noisily or vibrates, and the engine heats up in traffic.
Did you know: Before replacing the fan, check the fuse, relay and any control module, as these are a frequent cause of a fan that will not run.
Serpentine belts
What it is: The multi-rib belt transfers drive from the crankshaft to the alternator, power steering pump, A/C compressor and often the water pump.
Where it is fitted: At the front of the engine, running over the accessory pulleys.
Signs of wear: Squealing at start-up or when turning, cracks across the ribs, frayed edges and belt slip.
Did you know: Many American cars have a belt routing diagram on a sticker under the hood; take a photo before removal and check the pulleys and tensioner while you are at it.
Thermostat housings
What it is: The housing holds the thermostat and forms the outlet the radiator hose attaches to. It often also carries the temperature sensor port.
Where it is fitted: Bolted to the cylinder head or engine block where coolant leaves or enters the engine.
Signs of wear: Dampness and coolant residue around the gasket, cracked plastic or a corroded sealing surface.
Did you know: Plastic housings crack easily when the bolts are overtightened, so tighten them evenly and always use a new gasket or O-ring.
Thermostats
What it is: A thermostat is a valve that opens the flow to the radiator only once the engine has warmed up. This lets the engine reach operating temperature faster and hold it steadily.
Where it is fitted: At the coolant outlet or inlet of the engine, usually inside a housing that the radiator hose connects to.
Signs of wear: Stuck open, it causes low engine temperature and weak cabin heat; stuck closed, it makes the engine overheat quickly.
Did you know: Running without a thermostat is a bad idea: the engine runs too cold, fuel use goes up and the computer usually sets a check engine light with code P0128.
Water pumps
What it is: The water pump circulates coolant between the engine, the radiator and the heater core. Without it the coolant stands still and the engine overheats quickly.
Where it is fitted: Usually at the front of the engine, driven by the accessory belt, the timing belt or the timing chain.
Signs of wear: Coolant leaks or dried residue at the weep hole, a whining or squealing bearing, play in the pulley and rising engine temperature.
Did you know: On some V6 engines (e.g. Chrysler 2.7 or Ford 3.5) the pump sits behind the timing cover and is chain-driven, so replacing it means working on the timing, and a failed seal can let coolant into the oil.
Where to next
- Catalogue: cooling system — choose your make, model and generation.
- Parts by VIN — we will check the fit for your car.
- Parts guide — other systems.
We sell aftermarket replacement parts, not genuine parts from vehicle manufacturers. Descriptions are general — check the fit for your vehicle by OEM number or VIN.