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Know Your Car
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Under The Hood
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Jon Anderson
(Web Master of this site)

FUNdamentals of Fishing
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FUNdamentals of  Little League Baseball
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The Cooling System

Put a picture of a fan here Jon

The automotive internal combustion engine generates a tremendous amount of heat. This heat is created when the gasoline and air mixture is ignited in the combustion chamber. This ignition (explosion) causes the piston to be forced down inside the engine, levering the connecting rods, and turning the crankshaft, creating power.
 Metal temperatures around the combustion chamber can exceed 1000° F. In order to prevent the overheating of the engine oil, cylinder walls, pistons, valves, and other components by these extreme temperatures, it is necessary to effectively dispose of the heat.

   It has been stated that a typical average-sized vehicle can generate enough heat to keep a 5-room house comfortably warm during zero degree weather (and I'm not talking about using the exhaust pipe).
Approximately 1/3 of the heat in combustion is converted into power to drive the vehicle and it's accessories.

   Another 1/3 of the heat is carried off into the atmosphere through the exhaust system.

The remaining 1/3 must be removed from the engine by the cooling system. The automotive engines have basically dumped the Air Cooled System for the more effective Liquid Cooled System to handle the job.

   In a liquid cooled system, heat is carried away by the use of a heat absorbing coolant that circulates through the engine, especially around the combustion chamber in the cylinder head area of the engine block. The coolant is pumped through the engine, then after absorbing the heat of combustion is circulated to the radiator where the heat is transferred to the atmosphere. The cooled liquid is then transferred back into the engine to repeat the process.

In most vehicles, the coolant is circulated by the water pump, and the thermostat controls the temperature. The thermostat is closed when the engine is cold, allowing coolant to circulate ONLY in the engine block, bypassing the thermostat and radiator. This allows the engine to warm up faster and uniformly so that "hot spots" are eliminated. When the warming coolant reaches the thermostat, the thermostat will begin to open and allow coolant to pass to the radiator. The hotter the coolant gets, the more the thermostat opens, allowing more volume of water to pass to the radiator. The thermostat also controls the length of time that the coolant remains in the radiator so that the heat is dissipated effectively.


Removing the thermostat to increase water flow because your vehicle is overheating is dangerous to your engine and is NOT what you want to do. Not only does the engine take longer to warm up, causing excessive metal-to-metal wear, but once the engine does warm up it can get too hot because the thermostat also controls the length of time that the water is in the radiator so as to dissipate the heat to the atmosphere

The Pressurized System

   Up until the late 1950's, liquid cooled systems did not operate under pressure. This was primarily because these cars had much larger radiators, more open air under the hoods allowing for more natural heat dissipation, and richer fuel mixtures causing lower (and less efficient) combustion temperatures. With the manufacture of smaller vehicles, with smaller radiators, larger engines, and emission controls along with the current use of unleaded fuels, more efficient cooling efficiency became necessary.

   The cure for this was to operate the cooling system under pressure, thus isolating it from atmospheric pressure. A system under pressure can handle higher temperatures, and offers a higher static boiling point.

Most liquids have a specific "boiling point", which is the temperature at which the liquid begins to change to a gas. If pressure is applied to the liquid, it must become hotter before it can boil. Pure water in a cooling system will boil (at sea level) at 212° F. At higher altitudes, atmospheric pressure is less than at sea level.
Example: Water at 5,280 feet will boil at a mere 203° F.
 A cooling system that is less than 15 pounds of pressure however, will now allow the water to reach nearly 250° F before it can boil. Even at this temperature the water is able to circulate through the engine, cooling parts that are at a much higher temperature without the water boiling. As long as the coolant remains in liquid form it can do it's job and transfer heat to the radiator so it can be dissipated. Coolant that is boiling cannot transfer as much heat and engine overheating is likely to occur if the coolant turns to a gaseous state. Steam adjacent to a hot surface, such as a combustion wall, can actually act as an insulator - thus preventing any heat transfer to the coolant.

   Pressurization of the system is achieved by a special radiator filler neck and radiator pressure cap. The filler neck has an upper and lower sealing seat with an overflow tube connection between them. The lower seat is engaged by the pressure controlling valve on the cap and the upper seat (in an open system) is engaged by a spring metal diaphragm in the cap.

The radiator pressure cap features a spring pressure relief valve which closes off the lower sealing seat in the filler neck.


This pressure relief valve allows pressure to build up to a specified level; this permits excess pressure to escape through the overflow tube when it exceeds the range of the pressure valve spring. This valve protects the cooling system from damage from excessive pressure. This pressure relief system also includes a vacuum relief valve that allows air (in an open system) to enter as coolant contracts. This prevents the radiator hoses and tanks from collapsing from the partial vacuum that would be created if air was unable to enter.



The Closed or Reservoir System
the "old style" Open System

   One of the big disadvantages in the old open type pressurized system is that as the system cools, air is allowed back through the overflow tube. These systems are not totally filled with coolant because of the potential for coolant loss through the overflow tube when the coolant heats up and expands. As more coolant is lost through the overflow, less coolant is left to do it's job within the engine. Because of this, and that air can enter the system and reduce cooling system efficiency, overheating can occur. Closed reservoir systems were first used by car manufacturers in the early 1970's.

   A closed or reservoir system has solved the problems listed above. This system is different in that a special radiator cap and overflow reservoir tank. Part of the radiator cap is a second sealing gasket under the shell that contacts the upper sealing seat of the filler neck. What was the overflow hose is now the connection between the radiator and the "bottom" of the reservoir.

   While the open pressurized system is filled to a point 2-3 inches below the top of the radiator, the closed pressurized system is filled completely with coolant and the reservoir is filled approximately half full. When the engine is started and begins to heat up, the coolant expands. As the coolant expands it is forced out through the pressure valve of the radiator cap, through the overflow tube, and into the reservoir. When the engine is turned off and begins to cool, a partial vacuum is created in the radiator by the contracting coolant. The upper sealing gasket in the pressure cap will then allow the vacuum to draw the coolant back into the radiator and engine from the reservoir. As you may have noticed, the actual volume of coolant that displaces during heat-up and cool-down transfer is minimal in most all cases.

   Because of the coolant going back and forth between the radiator and reservoir, practically all air is eliminated from the cooling system. This pretty much guarantees that the engine block, heater core, and radiator are full of coolant instead of air. This allows the most efficient operation of the cooling system. Generally, on closed systems, coolant is added only as required, and then it is added to the reservoir, not the radiator.



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The objective of this Web Page is to familiarize you with basic auto maintenance
-  in some common emergencies -
not to make you an expert in auto mechanics

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