Introduction

Volkswagen GTI, Golf, and Jetta engines are liquid-cooled, and rely on a closed system of circulating coolant to maintain an even engine temperature and help transfer heat away from the engine. To provide adequate cooling system performance over a range of temperature conditions, to supply lubrication to the system's moving parts, and to prevent the buildup of mineral deposits and other contaminants, the coolant recommended for use year-round is a mixture of phosphate-free anti-freeze and clean water. The closed system becomes pressurized as its temperature increases. The pressure in turn raises the boiling point of the coolant and allows engine temperature to exceed the coolant's normal boiling point.

The coolant pump is mechanically driven by the engine, and operates whenever the engine is running. Coolant circulates through the engine to the radiator, the heat exchanger or heater core in the passenger compartment heating system, and back to the pump. On some models, coolant is also circulated through a heat exchanger in the engine's lubrication system to help moderate engine oil temperature. Before the engine is up to normal operating temperature, the flow of coolant is controlled by a thermostat. The thermostat forces coolant flow to bypass the radiator, returning directly to the coolant pump, until the engine is warm.

A radiator cooling fan provides auxiliary air flow through the radiator. For compatibility with the transverse engine design, and to minimize power consumption, the fan is electrically operated and thermostatically controlled so that it runs only when the extra air flow is required to maintain proper coolant temperature.