Chevrolet Silverado / GMC Sierra. Service manual — part 1686

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

radiator to increase engine cooling. Air baffles are also used to direct airflow through the radiator
and increase cooling capability. Air seals prevent air from bypassing the radiator and A/C
condenser, and prevent recirculation of hot air for better hot weather cooling and A/C condenser
performance.

Water Pump

The water pump is a centrifugal vane impeller type pump. The pump consists of a housing with
coolant inlet and outlet passages and an impeller. The impeller is mounted on the pump shaft and
consists of a series of flat or curved blades or vanes on a flat plate. When the impeller rotates, the
coolant between the vanes is thrown outward by centrifugal force.

The impeller shaft is supported by one or more sealed bearings. The sealed bearings never need to
be lubricated. Grease cannot leak out, dirt and water cannot get in as long as the seal is not
damaged or worn.

The purpose of the water pump is to circulate coolant throughout the cooling system. The water
pump is driven by the crankshaft via the drive belt.

Water Pump (6.6L Diesel Engine)

The water pump is a centrifugal vane impeller type pump. The water pump is gear driven by the
crankshaft gear. The pump consists of a housing with coolant inlet and outlet passages and an
impeller. The impeller is a flat plate mounted on the pump shaft with a series of flat or curved
blades or vanes. When the impeller rotates, the coolant between the vanes is thrown outward by
centrifugal force. The impeller shaft is supported by bearings. Splash of the engine oil lubricates
the bearings. The bearings and shaft are sealed to prevent engine oil to mix with the coolant. If
the seal fails, coolant will leak out the vent hole in the water pump housing.

The purpose of the water pump is to circulate coolant throughout the cooling system.

Thermostat

The thermostat is a coolant flow control component. It's purpose is to help regulate the operating
temperature of the engine. It utilizes a temperature sensitive wax-pellet element. The element
connects to a valve through a small piston. When the element is heated, it expands and exerts
pressure against the small piston. This pressure forces the valve to open. As the element is cooled,
it contracts. This contraction allows a spring to push the valve closed.

When the coolant temperature is below the rated thermostat opening temperature, the thermostat
valve remains closed. This prevents circulation of the coolant to the radiator and allows the
engine to warm up. After the coolant temperature reaches the rated thermostat opening

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

temperature, the thermostat valve will open. The coolant is then allowed to circulate through the
thermostat to the radiator where the engine heat is dissipated to the atmosphere. The thermostat
also provides a restriction in the cooling system, after it has opened. This restriction creates a
pressure difference which prevents cavitation at the water pump and forces coolant to circulate
through the engine block.

Thermostats (6.6L Diesel Engine)

The thermostats are coolant flow control components. The purpose of the thermostats are to
regulate the correct operating temperature of the engine. The thermostats utilizes a temperature
sensitive wax-pellet element. The element connects to a valve through a piston. When the element
is heated, it expands and exerts pressure against a rubber piston. This pressure forces the valve to
open. As the element is cooled, it contracts. This contraction allows a spring to push the valve
closed.

The 6.6L diesel engine requires two thermostats for correct coolant flow. The front thermostat is
a dual purpose thermostat. The front thermostat controls the coolant flow to the bypass port and
to the water outlet. The rear thermostat only controls the coolant flow to the water outlet.

When the coolant temperature is below the rated thermostat opening temperature, the front
thermostat valve remains closed to the water outlet and is opened to the bypass port. The bottom
portion of the thermostat is raised off of the bypass port while at the same time the top portion
closes the coolant flow to the water outlet. The rear thermostat also is closed to the water outlet
during engine warm-up. This prevents circulation of the coolant to the radiator and allows the
engine to warm up quickly. After the coolant temperature reaches 82°C (180°F) the front
thermostat primary valve opening temperature, the front thermostat primary valve will start to
open. The coolant is then allowed to circulate through the thermostat to the radiator where the
engine heat is dissipated to the atmosphere. As the engine coolant reaches 85°C (185°F) and more
coolant demand is required the front thermostat secondary valve begins to close the bypass port
and the rear thermostat begins to open coolant flow to the water outlet. The thermostats will
continue to control the coolant flow by opening and closing. The front thermostat will be fully
open when the coolant temperature reaches 95°C (203°F) the rear thermostat will be fully open
when the coolant temperature reaches 100°C (212°F). The thermostat also provides a restriction
in the cooling system, even after the it has opened. This restriction creates a pressure difference
which prevents cavitation at the water pump and forces coolant to circulate through the engine
block.

Engine Oil Cooler

The engine oil cooler is a heat exchanger. It is located inside the left side end tank of the radiator.
The engine oil temperature is controlled by the temperature of the engine coolant that surrounds

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado

the oil cooler in the radiator.

The engine oil pump, pumps the oil through the engine oil cooler line to the oil cooler. The oil
then flows through the cooler where the engine coolant absorbs heat from the oil. The oil is then
pumped through the oil cooler return line, to the oil filter, to the engine block oil system.

Engine Oil Cooler (6.6L Diesel Engine)

The engine oil cooler is a heat exchanger. The engine oil cooler is mounted to the left lower
corner of the engine. The oil filter is attached to the oil cooler housing. The engine coolant flows
around the oil cooler element. The oil cooler element is a series of plates. The engine oil
temperature is regulated by the temperature of the engine coolant that surrounds the oil cooler as
the engine oil passes through the cooler.

The engine oil pump, pumps the oil through the engine oil feed line to the oil cooler. The oil then
flows down through the cooler while the engine coolant absorbs heat from the oil. The oil is then
pumped through the oil return line, to the oil filter, then to the main engine oil passage.

Transmission Oil Cooler

The transmission oil cooler is a heat exchanger. It is located inside the right side end tank of the
radiator. The transmission fluid temperature is regulated by the temperature of the engine coolant
in the radiator.

The transmission oil pump, pumps the fluid through the transmission oil cooler line to the
transmission oil cooler. The fluid then flows through the cooler where the engine coolant absorbs
heat from the fluid. The fluid is then pumped through the transmission oil cooler return line, to the
transmission.

Turbocharger Bypass Valve (6.6L Diesel Engine)

The turbocharger bypass valve is a temperature control valve. The valve is located in the
turbocharger coolant inlet hose at the water outlet tube.

The purpose of the valve is to close the coolant flow through the turbocharger. Closing off the
coolant flow through the turbocharger avoids turbocharger overcooling.

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Cooling - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra & Silverado