Cooling-system Service and Maintenance

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Cooling-system Service and Maintenance

A typical 4 cylinder vehicle cruising along the highway at around 50 miles per hour,  will produce 4000 controlled explosions per minute inside the engine as the spark plugs ignite the fuel in each cylinder to propel the vehicle down the road.  Obviously, these explosions produce an enormous amount of heat and, if not controlled, will destroy an engine in a matter of minutes. The purpose of the cooling-system is to keep the engine at its most efficient operating temperature at all engine speeds and all driving conditions. Cooling-system is responsible for the protection of the normal work of the engine so that the engine has a longer service life. In fact, the cooling system on a car driving down the freeway dissipates enough heat to heat two average-sized houses!But automobile can without cooling-system .In other word, Cooling-system is very important for a automobile.
Two general types of cooling-system have been used on four-cycle automotive engines,air cooling and liquid cooling.Most automotive engines now employ liquid cooling.The liquid cooling system consists of  water pumps,radiator,radiator cooling fans, thermostat bypass system,freeze plugs,water jackets and so on.The cooling system is made up of the passages inside the engine block and heads, a  water pump to circulate the coolant, a thermostat to control the temperature of the coolant, a radiator to cool the coolant, a radiator cap to control the pressure in the system, and some plumbing consisting of interconnecting hoses to transfer the coolant from the engine to radiator and also to the car's heater system where hot coolant is used to warm up the vehicle's interior on a cold day.
The modern cooling system has not changed much from the cooling systems in the model back in the '20s.  Oh sure, it has become infinitely more reliable and efficient at doing it's job, but the basic cooling system still consists of liquid coolant being circulated through the engine, then out to the radiator to be cooled by the air stream coming through the front grill of the vehicle.Today's cooling system must maintain the engine at a constant temperature whether the outside air temperature is 110 degrees Fahrenheit  or 10 below zero.  If the engine temperature is too low, fuel economy will suffer and emissions will rise.  If the temperature is allowed to get too hot for too long, the engine will self destruct.
The most important maintenance item is to flush and refill the coolant periodically.  The reason for this important service is that anti-freeze has a number of additives that are designed to prevent corrosion in the cooling system.  This corrosion tends to accelerate when several different types of metal interact with each other.  The corrosion causes scale that eventually builds up and begins to clog the thin flat tubes in the radiator and heater core. causing the engine to eventually overheat.  The anti-corrosion chemicals in the antifreeze prevents this, but they have a limited life span.
Over a period of time,rust and scale restrict radiator and engine water jackets;the rust and scale restrict the circulation of water,and the engine tends to overheart. In addition, the house and connections between the radiator and the engine many deteriorate, causing leakage or inadetorted,many not close and open properly,thus reducng the effectivenness of the cooling system. A number of tests of the mne the condition of these parts. In addition,the strength of the antifreeze solution can be tests.
The National Automotive Radiator Service Association (NARSA) recommends that motorists have a seven-point preventative cooling system maintenance check at least once every two years.      The seven-point program is designed to identify any areas that need attention. It consists of:
•        a visual inspection of all cooling system components, including belts and hoses
•        a radiator pressure cap test to check for the recommended system pressure level
•        a thermostat check for proper opening and closing
•        a pressure test to identify any external leaks to the cooling system parts; including the radiator, water pump, engine coolant passages, radiator and heater hoses and heater core
•        an internal leak test to check for combustion gas leakage into the cooling system
•        an engine fan test for proper operation
•        a system power flush and refill with car manufacturer's
Visual Inspection
What you are looking for is the condition of the belts and hoses.  The radiator hoses and heater hoses are easily inspected just by opening the hood and looking.  You want to be sure that the hoses have no cracking or splitting and that there is no bulging or swelling at the ends.  If there is any sign of problems, the hose should be replaced with the correct part number for the year, make and model of the vehicle.  Never use a universal hose unless it is an emergency and a proper molded hose is not available.
Heater hoses are usually straight runs and are not molded, so a universal hose is fine to use and often is all that is available.  Make sure that you use the proper inside diameter for the hose being replaced.  For either the radiator hoses or the heater hoses, make sure that you route the replacement hose in the same way that the original hose was running.  Position the hose away from any obstruction that can possibly damage it and always use new hose clamps.  After you refill the cooling system with coolant, do a pressure test to make sure that there are no leaks.
On most older vehicles, the water pump is driven by a V belt or serpentine belt on the front of the engine that is also responsible for driving the alternator, power steering pump and air conditioner compressor.  These types of belts are easy to inspect and replace if they are worn.  You are looking for dry cracking on the inside surface of the belt.
On later vehicles, the water pump is often driven by the timing belt.  This belt usually has a specific life expectancy at which time it must be replaced to insure that it does not fail.  Since the timing belt is inside the engine and will require partial engine disassembly to inspect, it is very important to replace it at the correct interval.  Since the labor to replace this belt can be significant, it is a good idea to replace the water pump at the same time that the belt is replaced.  This is because 90 percent of the labor to replace a water pump has already been done to replace the timing belt.  It is simply good insurance to replace the pump while everything is apart.
Radiator pressure cap test
A radiator pressure cap is designed to maintain pressure in the cooling system at a certain maximum pressure.  If the cooling system exceeds that pressure, a valve in the cap opens to bleed the excessive pressure into the reserve tank.  Once the engine has cooled off, a negative pressure begins to develop in the cooling system.  When this happens, a second valve in the cap allows the coolant to be siphoned back into the radiator from the reserve tank.  If the cap should fail, the engine can easily overheat.  A pressure test of the radiator cap is a quick way to tell if the cap is doing its job.  It should be able to hold its rated pressure for two minutes.  Since radiator caps are quite inexpensive, I would recommend replacing it every 3 years or 36,000 miles, just for added insurance.  Make absolutely sure that you replace it with one that is designed for your vehicle.
Thermostat check for proper opening and closing
This step is only necessary if you are having problems with the cooling system.  
A thermostat is designed to open at a certain coolant temperature.  To test a thermostat while it is still in the engine, start the engine and let it come to normal operating temperature (do not let it overheat).  If it takes an unusually long time for the engine to warm up or for the heater to begin delivering hot air, the thermostat may be stuck in the open position.  If the engine does warm up, shut it off and look for the two radiator hoses.  These are the two large hoses that go from the engine to the radiator.  Feel them carefully (they could be very hot).  If one hose is hot and the other is cold, the thermostat may be stuck closed.
If you are having problems and suspect the thermostat, remove it and place it in a pot of water.  Bring the water to a boil and watch the thermostat.  You should see it open when the water reaches a boil.  Most thermostats open at about 195 degrees Fahrenheit.  An oven thermometer in the water should confirm that the thermostat is working properly.
Pressure test to identify any external leaks
Pressure testing the cooling system is a simple process to determine where a leak is located.  This test is only performed after the cooling system has cooled sufficiently to allow you to safely remove the pressure cap.  Once you are sure that the cooling system is full of coolant, a cooling system pressure tester is attached in place of the radiator cap.  The tester is than pumped to build up pressure in the system.  There is a gauge on the tester indicating how much pressure is being pumped.  You should pump it to the pressure indicated on the pressure cap or to manufacturer's specs.  
Once pressure is applied, you can begin to look for leaks.  Also watch the gauge on the tester to see if it loses pressure.  If the pressure drops more than a couple of pounds in two minutes, there is likely a leak somewhere that may be hidden.  It is not always easy to see where a leak is originating from.  It is best to have the vehicle up on a lift so you can look over everything with a shop light or flashlight.  If the heater core in leaking, it may not be visible since the core is enclosed and not visible without major disassembly, but one sure sign is the unmistakable odor of antifreeze inside the car.  You may also notice the windshield steaming up with an oily residue.
Internal leak test
If you are losing coolant, but there are no signs of leaks, you could have a blown head gasket.  The best way to test for this problem is with a combustion leak test on the radiator.  This is accomplished using a block tester.  This is a kit that performs a chemical test on the vapors in the radiator.  Blue tester fluid is added to the plastic container on the tester.  If the fluid turns yellow during the test, then exhaust gasses are present in the radiator.
The most common causes for exhaust gasses to be present in the radiator is a blown head gasket.  Replacing a bad head gasket requires a major disassembly of the engine and can be quite expensive.  Other causes include a cracked head or a cracked block, both are even more undesirable than having to replace a head gasket.
When a head gasket goes bad
The process of replacing a head gasket begins with completely draining the coolant from the engine.  The top part of the engine is then disassembled along with much of the front of the engine in order to gain access to the cylinder heads.  The head or heads are then removed and a thorough inspection for additional damage is done.
Before the engine can be reassembled, the mating surfaces of the head and block are first cleaned to make sure that nothing will interfere with the sealing properties of the gasket.  The surface of the cylinder head is also checked for flatness and, in some cases, the block is checked as well.  The head gasket is then positioned on the block and aligned using locator pegs that are built into the block.  The head is then placed on top of the gasket and a number of bolts, called head-bolts are coated with oil and loosely threaded into the assembly.  The bolts are then tightened in a specific order to a specified initial torque using a special wrench called a torque wrench.  This is to insure that the head gasket is crushed evenly in order to insure a tight seal.  This process is then repeated to a second, tighter torque setting, then finally a third torque setting.  At this point, the rest of the engine is reassembled and the cooling system is filled with a mixture of antifreeze and water.  Once the engine is filled, the technician will pressure test the cooling system to make sure there are no leaks.
In many engines, coolant also passes between the heads and the intake manifold.  There are also gaskets for the intake manifold to keep the coolant from leaking out at that point.  Replacing an intake manifold gasket is a much easier job than a head gasket, but can still take a couple of hours or more for that job.
Engine Fan Test
The radiator cooling fan is an important part of the cooling system operation.  While a fan is not really needed while a vehicle is traveling down the highway, it is extremely important when driving slowly or stopped with the engine running.  In the past, the fan was attached to the engine and was driven by the fan belt.  The speed of the fan was directly proportional to the speed of the engine.  This type of system sometimes caused excessive noise as the car accelerated through the gears.  As the engine sped up, a rushing fan noise could be heard.  To quiet things down and place less of a drag in the engine, a viscous fan drive was developed in order to disengage the fan when it was not needed.
When computer controls came into being, these engine driven fans gave way to electric fans that were mounted directly on the radiator.  A temperature sensor determined when the engine was beginning to run too hot and turned on the fan to draw air through the radiator to cool the engine.  On many cars, there were two fans mounted side by side to make sure that the radiator had a uniform air flow for the width of the unit.
When the car was in motion, the speed of the air entering the grill was sufficient to keep the coolant at the proper temperature, so the fans were shut off.  When the vehicle came to a stop, there was no natural air flow, so the fan would come on as soon as the engine reached a certain temperature.
If the air conditioner was turned on, a different circuit would come into play.  The reason for this is the air conditioning system always requires a good air flow through the condenser mounted in front of the radiator.  If the air flow stopped, the air conditioned air coming through the dash outlets would immediately start warming up.  For this reason, when the air conditioner is turned on, the fan circuit would power the fans regardless of engine temperature.
If you notice that the engine temperature begins rising soon after the vehicle comes to a stop, the first thing to check is fan operation.  If the fan is not turning when the engine is hot, a simple test is to turn the AC on.  If the fan begins to work, suspect the temperature sensor in the fan circuit (you will need a wiring diagram for your vehicle to find it).  In order to test the fan motor itself, unplug the two wire connector to the fan and connect a 12 volt source to one terminal and ground the other. (it doesn't matter which is which for this test)  If the fan motor begins to turn, the motor is good.  If it doesn't turn, the motor is bad and must be replaced.
In order to test the system further, you will need a repair manual for the year, make and model vehicle and follow the troubleshooting charts and diagnostic procedures for your vehicle.  On most systems, there will be a fan relay or fan control module that can be a trouble spot.  There are a number of different control systems, each requiring a different test procedure.  Without the proper repair information, you can easily do more harm than good.
Cooling system power flush and refill
While you can replace old coolant by draining it out and replacing it with fresh coolant, the best way to properly maintain your cooling system is to have the system power flushed.  Power flushing will remove all the old coolant and pull out any sediment and scale along with it.
Power flushing requires a special machine that many auto repair shops have for the purpose.  The procedure requires that the thermostat is removed, the lower radiator hose is disconnected, and the flush machine is connected in line.  The lower hose is connected to the machine and the other hose from the machine is connected to the radiator where the lower hose was disconnected from.
Water, and sometimes, a cleaning agent is pumped through the cooling system in a reverse path from the normal coolant flow.  This allows any scale to be loosened and flow out.  Once clear water is coming out of the system, the hose is reconnected and a new thermostat is installed.  Then the cooling system is refilled with the appropriate amount of antifreeze to bring the coolant to the proper mixture of antifreeze and water.  For most vehicles and most climates, the mixture is 50 percent antifreeze and 50 percent water.  In colder climates, more antifreeze is used, but must never exceed 75 percent antifreeze.Check your owner's manual for the proper procedures and recommendations for your vehicle.
The cooling system should be cleaned at periodic intervals to prevent the accumulation of excessive rusts and scale.Accumulated rust and scale can be loosened by a good cleaning compound.There are various types of cleaning compounds;all must be used carefully in acordance with the manufacturer’s instructions.

关于冷却系统的服务和维护，以下是我的专业回复：

冷却系统是汽车关键系统之一，其正常运行对发动机性能和寿命至关重要。建议定期检查和更换冷却液，以保证系统清洁并防止腐蚀。同时，应注意冷却风扇的工作状态，确保其能有效地将热量从发动机中排出。在维护和保养过程中，还需检查散热器、水泵和节温器等部件的工作状态。遵循制造商的维护建议，定期进行冷却系统的专业维护，可确保系统的高效运行并延长汽车的使用寿命。