System Operation: Notes

Engine coolant flows primarily from the engine to the radiator circuit and back to the coolant pump. Coolant is sent from the coolant pump through the engine block and cylinder heads. A separate circuit from the engine also feeds the heater core and turbochargers with coolant. The coolant pump, operated by engine rotation through the accessory drive belt, circulates the coolant. The coolant thermostat is a control valve actuated by coolant temperature. When the thermostat is closed, coolant flow bypasses the radiator circuit and returns to the coolant pump. When the thermostat is opened, coolant flows through the radiator circuit to transfer engine-generated heat to the outside air.

The degas bottle holds surplus coolant and removes air from the cooling system. It also allows for coolant expansion and system pressurization, replenishes coolant to the cooling system and serves as the location for service fill.

The thermostat monitor is a function of the PCM  and is designed to verify correct thermostat operation. The monitor executes once per drive cycle and has a monitor run duration of 300-800 seconds. If a malfunction occurs, DTC  P0125 or P0128 sets, and the MIL  illuminates.

The engine's cooling circuit consists of an additional coolant bypass solenoid valve and coolant shutoff solenoid valve. Using these two valves, the coolant flow through the engine is specifically restricted or stagnated in the warm-up phase. This restriction of the coolant flow makes it possible for the engine components to warm up faster. The result is a significant reduction in the emissions of harmful pollutants and an improvement in fuel economy (i.e. reduced friction) during the warm-up phase. Both solenoid valves are controlled by the PCM  .

The following input parameters are used to do this:

• Coolant temperature
• Ambient air temperature
• Engine speed
• Engine load
• Status of the air conditioning system

Warm-up regulation is performed in four phases. Phase 1 or 2 occurs after initial engine start-up depending on the ambient air temperature.