Hummer H2. Manual - part 1414

The instrument panel cluster (IPC) illuminates the ENGINE COOLANT HOT indicator in the message center 
when the IPC determines that the coolant temperature is greater than 121° C (251° F). The IPC receives a class 
2 message from the powertrain control module (PCM) indicating the coolant temperature. 

LOW COOLANT 

The IPC illuminates the LOW COOLANT indicator in the message center when the IPC detects a low coolant 
level condition from the PCM. The IPC receives a class 2 message from the PCM indicating the coolant level. 

ENGINE OVERHEATED 

The IPC illuminates the ENGINE OVERHEATED indicator in the message center when the IPC determines 
that the coolant temperature is greater than 128° C (262° F). The IPC receives a class 2 message from the PCM 
indicating the coolant temperature. 

REDUCED ENGINE POWER 

The IPC illuminates the REDUCED ENGINE POWER indicator in the message center when the IPC detects a 
reduced engine power condition from the PCM. The IPC receives a class 2 message from the PCM requesting 
illumination when the engine temperature reaches 128° C (262° F). 

Coolant Heater 

The optional engine coolant heater (RPO K05) operates using 110-volt AC external power and is designed to 
warm the coolant in the engine block area for improved starting in very cold weather 29° C (20° F). The coolant 
heater helps reduce fuel consumption when a cold engine is warming up. The unit is equipped with a detachable 
AC power cord. A weather shield on the cord is provided to protect the plug when not in use. 

Cooling System 

The cooling system's function is to maintain an efficient engine operating temperature during all engine speeds 
and operating conditions. The cooling system is designed to remove approximately one-third of the heat 
produced by the burning of the air-fuel mixture. When the engine is cold, the coolant does not flow to the 
radiator until the thermostat opens. This allows the engine to warm quickly. 

Cooling Cycle 

Coolant is drawn from the radiator outlet and into the water pump inlet by the water pump. Coolant will then be 
pumped through the water pump outlet and into the engine block. In the engine block, the coolant circulates 
through the water pump outlet and into the engine block. In the engine block, the coolant circulates through the 
water jackets surrounding the cylinders, where it absorbs heat. 

Some coolant is also pumped from the water pump to the heater core, then back to the water pump. This 
provides the passenger compartment with heat and defrost. 

The coolant is then forced through the cylinder head gasket openings and into the cylinder heads. In the cylinder 
heads, the coolant flows through the water jackets surrounding the combustion chambers and valve seats, where 
it absorbs additional heat. 

 

2004 Hummer H2 

2004 ENGINE Engine Cooling - Hummer H2

  

Coolant is also directed to the throttle body. There it circulates through passages in the casting. During initial 
start up, the coolant assists in warming the throttle body. During normal operating temperatures, the coolant 
assists in regulating the throttle body temperature. 

Coolant 

The engine coolant is a solution made up of a 50-50 mixture of DEX-COOL and suitable drinking water. The 
coolant solution carries excess heat away from the engine to the radiator, where the heat is dissipated to the 
atmosphere. 

Radiator 

The radiator is a heat exchanger. It consists of a core and two tanks. The aluminum core is a tube and fin 
crossflow design that extends from the inlet tank to the outlet tank. Fins are placed around the outside of the 
tubes to improve heat transfer to the atmosphere. 

The inlet and outlet tanks are a molded, high temperature, nylon reinforced plastic material. A high temperature 
rubber gasket seals the tank flange edge to the aluminum core. The tanks are clamped to the core with clinch 
tabs. The tabs are part of the aluminum header at each end of the core. 

The radiator also has a drain cock located in the bottom of the left hand tank. The drain cock unit includes the 
drain cock and drain cock seal. 

The radiator removes heat from the coolant passing through it. The fins on the core transfer heat from the 
coolant passing through the tubes. As air passes between the fins, it absorbs heat and cools the coolant. 

Surge Tank 

The surge tank is a plastic tank with a threaded pressure cap. The tank is mounted at a point higher than all 
other coolant passages. The surge tank provides an air space in the cooling system that allows the coolant to 
expand and contract. The surge tank provides a coolant fill point and a central air bleed location. 

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.

 

2004 Hummer H2 

2004 ENGINE Engine Cooling - Hummer H2

  

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 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. 

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. 

 

2004 Hummer H2 

2004 ENGINE Engine Cooling - Hummer H2

  

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 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. 

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 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. 

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. 

SPECIAL TOOLS AND EQUIPMENT 

SPECIAL TOOLS 

Special Tools 

Illustration

Tool Number/Description

J 24731 

Tempil Stick

 

2004 Hummer H2 

2004 ENGINE Engine Cooling - Hummer H2