Water pump

How it works

The water pump circulates coolant between the engine and the radiator. It is a small impeller driven mechanically by the engine (directly by a shaft or via a gear), housed in a cover on the side of the engine. Its shaft passes through the case: seals isolate it, one on the coolant side and one on the engine-oil side. A small weep hole under the pump lets coolant or oil out if a seal leaks: a trace there means the pump needs a rebuild. A seal leaking on the oil side can let coolant mix with the oil, which then looks milky. A worn pump (tired bearing, damaged impeller) circulates coolant less well and the engine runs hot. Well-kept coolant, free of scale and corrosion, extends its life. It is rebuilt with a kit (seals, bearing) or the pump is replaced.

Maintenance tips

• ✓Inspect the weep hole under the pump: a trickle of coolant or oil means a seal needs replacing.
• ✓If the engine oil looks milky, suspect a water-pump or head-gasket seal and don't ride.
• ✓Use good-quality coolant, never plain tap water.
• ✓An engine that runs hot with a clean radiator and a working fan should have its pump checked.
• ✓Rebuilding the pump (seals, bearing) means draining the coolant and often the oil too.