During vehicle use, the coolant heats and expands. The increased coolant volume flows into the
surge tank. As the coolant circulates, any air is allowed to bubble out. Coolant without air
bubbles absorbs heat much better than coolant with bubbles.
Pressure Cap
The pressure cap seals the cooling system. It contains a blow off or pressure relief valve and a
vacuum or atmospheric valve. The pressure valve is held against its seat by a spring, which
protects the radiator from excessive cooling system pressure. The vacuum valve is held against its
seat by a spring, which permits opening of the valve to relieve vacuum created in the cooling
system as it cools off. The vacuum, if not relieved, might cause the radiator and/or coolant hoses
to collapse.
The pressure cap allows cooling system pressure to build up as the temperature increases. As the
pressure builds, the boiling point of the coolant increases. Engine coolant can be safely run at a
temperature much higher than the boiling point of the coolant at atmospheric pressure. The hotter
the coolant is, the faster the heat transfers from the radiator to the cooler, passing air.
The pressure in the cooling system can get too high. When the cooling system pressure exceeds
the rating of the pressure cap, it raises the pressure valve, venting the excess pressure.
As the engine cools down, the temperature of the coolant drops and a vacuum is created in the
cooling system. This vacuum causes the vacuum valve to open, allowing outside air into the surge
tank. This equalizes the pressure in the cooling system with atmospheric pressure, preventing the
radiator and coolant hoses from collapsing.
Cooling Fan and Clutch
The engine cooling fan and clutch are driven by the crankshaft via the drive belt. The cooling fan
draws air through the radiator to improve the transfer of heat from the coolant to the atmosphere.
As the fan blades spin, they pull cool, outside air past the radiator core. The fan clutch drives the
cooling fan. The fan clutch controls the amount of torque that is transmitted from the crankshaft
to the fan blades. The clutch allows more torque to engage on the fan when the engine operating
temperature increases and/or the vehicle speed is low. As the torque increases, the fan turns more
quickly. The fan clutch decreases the torque applied to the cooling fan when the engine
temperature decreases and/or the vehicle speed is high. As the torque decreases, the fan speed
decreases.
Air Baffles and Seals
The cooling system uses deflectors, air baffles and air seals to increase cooling system capability.
Deflectors are installed under the vehicle to redirect airflow beneath the vehicle and through the
2008 Chevrolet Silverado 1500
2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado