Surge Tank
Le Mans Master
Joined: Jan 2005
Posts: 5,611
Likes: 5
From: Thousand Oaks California
The surge tank works any time the coolant expands or contracts in volume.
As per Service Information:
Surge Tank
The surge tank is a plastic tank with a pressure cap mounted to it. The tank is mounted at a point higher than all other coolant passages. The surge tank provides an air space in the cooling system. The air space allows the coolant to expand and contract. The surge tank also provides a coolant fill point and a central air bleed location.
During vehicle use, the coolant heats and expands. The coolant that is displaced by this expansion flows into the surge tank. As the coolant circulates, air is allowed to exit. This is an advantage to the cooling system. Coolant without bubbles absorbs heat much better than coolant with bubbles.
Pressure Cap
The pressure cap is a cap that seals and pressurizes the cooling system. It contains a blow off or pressure valve and a vacuum or atmospheric valve. The pressure valve is held against its seat by a spring and protects the radiator by relieving pressure if it exceeds 18 psi. 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, could cause the radiator hoses to collapse.
The pressure cap allows pressure in the cooling system to build up. As the pressure builds, the boiling point of the coolant goes up as well. Therefore, the coolant can be safely run at a temperature higher than the boiling point of the coolant at atmospheric pressure. The hotter the coolant is, the faster the heat moves from the radiator to the cooler passing air. However, if the pressure exceeds the strength of the spring, the pressure valve rises so that the excess pressure can escape. When 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 cooling system. This equalizes the pressure in the cooling system with atmospheric pressure, thus preventing the radiator hoses from collapsing.
Also,
Low Coolant Message
The engine cooling system utilizes an algorithm based low coolant level system which eliminates the need for the coolant level sensor previously positioned inside the surge tank. The algorithm measures the difference in engine coolant temperature readings over time as a function of engine speed, to determine if the cooling system has a low coolant condition. After vehicle start up, the engine control module (ECM) sends an engine RPM and engine coolant temperature message through the body control module (BCM) to the HVAC control module. The HVAC control module uses engine coolant temperature and engine RPM to determine if an increase in engine coolant temperature is due to a low coolant level condition. This system is capable of losing 2 full liters of coolant before triggering the Low Coolant indicator. Because the surge tank holds approximately 0.8 liters when filled to the proper level, its possible that the Low Coolant indicator will not be triggered, even though the surge tank is completely dry.
As per Service Information:
Surge Tank
The surge tank is a plastic tank with a pressure cap mounted to it. The tank is mounted at a point higher than all other coolant passages. The surge tank provides an air space in the cooling system. The air space allows the coolant to expand and contract. The surge tank also provides a coolant fill point and a central air bleed location.
During vehicle use, the coolant heats and expands. The coolant that is displaced by this expansion flows into the surge tank. As the coolant circulates, air is allowed to exit. This is an advantage to the cooling system. Coolant without bubbles absorbs heat much better than coolant with bubbles.
Pressure Cap
The pressure cap is a cap that seals and pressurizes the cooling system. It contains a blow off or pressure valve and a vacuum or atmospheric valve. The pressure valve is held against its seat by a spring and protects the radiator by relieving pressure if it exceeds 18 psi. 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, could cause the radiator hoses to collapse.
The pressure cap allows pressure in the cooling system to build up. As the pressure builds, the boiling point of the coolant goes up as well. Therefore, the coolant can be safely run at a temperature higher than the boiling point of the coolant at atmospheric pressure. The hotter the coolant is, the faster the heat moves from the radiator to the cooler passing air. However, if the pressure exceeds the strength of the spring, the pressure valve rises so that the excess pressure can escape. When 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 cooling system. This equalizes the pressure in the cooling system with atmospheric pressure, thus preventing the radiator hoses from collapsing.
Also,
Low Coolant Message
The engine cooling system utilizes an algorithm based low coolant level system which eliminates the need for the coolant level sensor previously positioned inside the surge tank. The algorithm measures the difference in engine coolant temperature readings over time as a function of engine speed, to determine if the cooling system has a low coolant condition. After vehicle start up, the engine control module (ECM) sends an engine RPM and engine coolant temperature message through the body control module (BCM) to the HVAC control module. The HVAC control module uses engine coolant temperature and engine RPM to determine if an increase in engine coolant temperature is due to a low coolant level condition. This system is capable of losing 2 full liters of coolant before triggering the Low Coolant indicator. Because the surge tank holds approximately 0.8 liters when filled to the proper level, its possible that the Low Coolant indicator will not be triggered, even though the surge tank is completely dry.
Le Mans Master
Joined: Feb 2007
Posts: 8,514
Likes: 235
From: Currently somewhere in IL,IN,KY,TN,MO,AR,MS,AL, or FL
What he said. If coolant isn't flowing to the surge tank within a few minutes after starting the engine then you have air in the system and need to find out where it is coming from.
Racer
Joined: Apr 2006
Posts: 486
Likes: 2
From: The Villages FL
The surge tank is always part of the flow circuit of the cooling system. At engine start up and to when the T-stat starts to open (187F) there is some controled flow in the system due to a bleed hole in the T-stat butterfly valve. A maximum allowable flow rate occurs, for any given engine RPM, when the stat is full open at 212F. The surge tank's purpose is to provide expansion volume for the heated coolant. Provide a location for air and coolant vapor (steam) to purge from the coolant. Finally, provide a convenient system fill location with a system pressure control valve/cap. In summary, the surge tank is in play during all engine operating temperatures.
