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► Most cooling systems rely on coolant, a special mix of chemicals (anti-freeze) and water.
► Coolant absorbs heat from the engine, is cooled in the radiator, and flows back to the engine to absorb
more heat.
► Heat travels in one of three ways: conduction, convection, or radiation.
► Coolant works to keep an engine from overheating and from freezing.
► Coolant must contain anti-freeze to prevent the water content from freezing and to reduce corrosion.
► Anti-freeze contains either ethylene glycol (toxic) or propylene glycol (nontoxic).
► The combination of water and anti-freeze lowers the freezing point and raises the boiling point of both
components.
► Manufacturers can create more efficient combustion by raising the engine’s operating temperature,
creating more pressure, and causing coolant to boil at a higher temperature.
► Radiator caps maintain a specified pressure throughout the cooling system, generally 13–17 psi (89.6–
117.2 kPa).
► Changing coolant regularly prevents acid buildup and electrolysis.
► The stationary parts of the cooling system (heater core and radiator) are connected to the engine via
radiator and cooling hoses.
► Modern vehicles have replaced the thermo-siphon process for moving coolant through the engine with a
water pump that forces coolant through the system.
► An engine thermostat regulates coolant circulation, keeping it in the engine until the engine reaches
operating temperature.
► Engineers have developed a reverse-flow cooling system in which coolant is first pushed through the
cylinder head, thereby better equalizing temperature between the block and head, which extends the life
of the head gasket.
► Engines with the reverse-flow cooling system must have a surge tank to capture steam and reconvert it to
coolant.
► Rotary engines use similar cooling systems to piston engines, with a radiator, thermostat, radiator hoses,
and water jackets.
► The radiator’s function is to allow coolant to pass through it and to conduct heat away from the engine.
► Cooling tubes in the radiator core run in a vertical (down-flow) or horizontal (cross-flow) design.
► Radiator pressure caps contain a spring-loaded valve to allow excess coolant to pass into the overflow
container, and a vacuum valve to allow coolant to be pulled from the overflow container back into the
radiator when it is needed.
► A surge tank is situated as the highest component so that it collects any air present in the system and
allows for easy air removal.
► In a recovery system, coolant flows into an overflow container and then back into the radiator so that no
coolant is lost.
► The thermostat’s valve is controlled by a wax pellet that melts and expands and forces the valve open
against spring pressure.
► Thermostats installed on the inlet side of the engine better control the amount of cold water flowing into
the engine.
► The water pump uses centrifugal force to drive coolant into the water jackets.
► A fan clutch is driven by an accessory belt and uses a viscous fluid to control speed changes of the fan,
determined by air temperature from the radiator.
► A solenoid-controlled fan clutch replaces the bimetallic spring with an electric solenoid controlled by the
power train control module.
► Radiator hoses must be correctly clamped to the radiator assembly or a leak will develop.
► Some heater hoses contain a coolant control valve to regulate coolant flow when the air conditioner is
turned on.
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The three types of drive belts for the water pump are V-belts, serpentine belts, and toothed belts.
To ensure minimal slippage, drive belts are tightened around the pulley by tensioners.
Either a temperature gauge or a temperature light can function as a coolant temperature indicator.
Coolant passages are critical to the transfer of heat energy so coolant must be serviced regularly to
prevent the passages from becoming clogged.
The heater box contains air doors and actuators that work to control passenger compartment airflow.
IAT coolant is the standard green coolant used in many vehicles and must be changed every 2 years or
24,000 miles (39,000 km).
OAT coolant is an extended-life coolant that should be changed every 5 years or 150,000 miles (241,000
km).
HOAT coolant (yellow) combines inorganic and organic additives and is also an extended-life coolant.
POAT coolant, the newest type, is a very long-life coolant that should be changed every 7 years or
250,000 miles (402,000 km).
Cooling systems need regular maintenance to ensure engine longevity.
Common cooling system diagnostic concerns include coolant leaks, thermostat that is stuck closed or
open, faulty water pump, inoperative cooling fan, or failed head gasket.
Test pH concentration of the coolant to ensure the anti-freeze acid inhibitors are working correctly.
Measure the voltage of the coolant to determine if electrolysis is occurring due to acidic coolant or faulty
grounds.
Always follow EPA regulations for handling and disposing of coolant.
Coolant condition and level should be checked regularly; coolant should be at the correct level mark
(upper or lower) according to engine temperature.
When removing and replacing a radiator, be sure to properly catch, handle, and dispose of or replace the
coolant.
Do not attempt to remove or replace the thermostat until the engine has cooled for at least 30 minutes.
Potential engine drive belt problems can be belts that are cracked, oil soaked, glazed, torn, or bottomed
out.
Coolant freeze point can be verified by a hydrometer or refractometer, both of which indicate the specific
gravity of the fluid, revealing the ratio of anti-freeze to water.
Removing and reinstalling the heater core can be challenging; taking pictures as you go is highly
recommended for ease of reassembly.