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A Short Course on
Fuel Systems

by Charles Ofria

This article is broken down into four sections:

What is a Fuel System?

The function of the fuel system is to store and supply fuel to the enginewhere it can be mixed with air, vaporized, and burned to produce energy. The most common fuels in use today are gasoline and diesel, however a number of new  fuels are hitting the market or show promise for the future. More on  those alternative fuels at the end of this article.  Gasoline and diesel are liquids and  are stored in a fuel tank. A fuel pump draws the fuel from the tank through fuel lines and delivers itthrough a fuel filter to either a carburetor or fuel injection system,which is responsible for sending the exact amount of fuel required to each cylinder in order to produce the power to move the vehicle.

How Does a Fuel System Work?

The fuel system is made up of the fuel tank, the fuel pump, fuel filter, interconnecting tubes that carry the fuel from the tank to the engine, and either a carburetor or a fuel injection system.

Carburetors have been around since the dawn of the motor vehicle, but reached their limits in the mid 1980s when emission regulations forced a switch to the more precise and efficient electronic fuel injection system.  By 1991, fuel injection systems were on every car sold in the US, and by 1995, on all gasoline powered trucks.

Before jumping into fuel injection systems, let's take a look at the carburetor and see how it works.



Let's start this section by talking about the fuel itself

Gasoline is a complex blend of carbon and hydrogen compounds. Additives are then addedto improve performance. All gasoline is basically the same, but no two blends areidentical. The two most important features of gasoline are volatility and resistance toknock (octane). Volatility is a measurement of how easily the fuel vaporizes. If thegasoline does not vaporize completely, it will not burn properly (liquid fuel will notburn).

If the gasoline vaporizes too easily the mixture will be too lean to burn properly.Since high temperatures increase volatility, it is desirable to have a low volatility fuelfor warm temperatures and a high volatility fuel for cold weather. The blends will bedifferent for summer and winter fuels. Vapor lock which was a persistent problem yearsago, exists very rarely today. In today's cars the fuel is constantly circulating from thetank, through the system and back to the tank. The fuel does not stay still long enough toget so hot that it begins to vaporize. Resistance to knock or octane is simply thetemperature the gas will burn at. Higher combustion chamber temperatures require a higher octane fuel toburn properly. As compression ratio or pressure increases so does the need for higher octane fuel.Most engines today are low compression engines therefore requiring a lower octane fuel(87). Any higher octane than required is just wasting money. Other factors that affect theoctane requirements of the engine are: air/fuel ratio, ignition timing, enginetemperature, and carbon build up in the cylinder. Many automobile manufacturers haveinstalled exhaust gas recirculation systems to reduce cylinder chamber temperature. Ifthese systems are not working properly, the car will have a tendency to knock. Beforeswitching to a higher octane fuel to reduce knock, make sure to have these other causeschecked.

Diesel fuel
Diesel fuel, like gasoline is a complex blend of carbon and hydrogen compounds. It toorequires additives for maximum performance. There are two grades of diesel fuel used inautomobiles today: 1-D and 2-D. Number 2 diesel fuel has a lower volatility and is blendedfor higher loads and steady speeds, therefore works best in large truck applications.Because number 2 diesel fuel is less volatile, it tends to create hard starting in coldweather. On the other hand, number 1 diesel is more volatile, and therefore more suitablefor use in an automobile, where there is constant changes in load and speed. Since dieselfuel vaporizes at a much higher temperature than gasoline, there is no need for a fuelevaporation control system as with gasoline. Diesel fuels are rated with a cetane numberrather than an octane number. While a higher octane of gasoline indicates resistance toignition, the higher cetane rating of diesel fuel indicates the ease at which the fuelwill ignite. Most number 1 diesel fuels have a cetane rating of 50, while number 2 dieselfuel have a rating of 45. Diesel fuel emissions are higher in sulfur, and lower in carbonmonoxide and hydrocarbons than gasoline and are subject to different emission testingstandards.






Fuel Tank
Tank location and design are always a compromise with available space. Most automobileshave a single tank located in the rear of the vehicle. Fuel tanks today have internalbaffles to prevent the fuel from sloshing back and forth. If you hear noises from the rearon acceleration and deceleration the baffles could be broken. All tanks have a fuel fillerpipe, a fuel outlet line to the engine and a vent system. All catalytic converter cars areequipped with a filler pipe restrictor so that leaded fuel, which is dispensed from athicker nozzle, cannot be introduced into the fuel system. All fuel tanks must be vented.Before 1970, fuel tanks were vented to the atmosphere, emitting hydrocarbon emissions.Since 1970 all tanks are vented through a charcoal canister, into the engine to be burnedbefore being released to the atmosphere. This is called evaporative emission control andwill be discussed further in the emission control section. Federal law requires that all1976 and newer cars have vehicle rollover protection devices to prevent fuel spills.

Fuel Lines
Steel lines and flexible hoses carry the fuel from the tank to the engine. Whenservicing or replacing the steel lines, copper or aluminum must never be used. Steel linesmust be replaced with steel. When replacing flexible rubber hoses, proper hose must beused. Ordinary rubber such as used in vacuum or water hose will soften and deteriorate. Becareful to route all hoses away from the exhaust system.

Fuel Pumps
Two types of fuel pumps are used in automobiles; mechanical and electric. All fuelinjected cars today use electric fuel pumps, while most carbureted cars use mechanicalfuel pumps. Mechanical fuel pumps are diaphragm pumps, mounted on the engine and operatedby an eccentric cam usually on the camshaft. A rocker arm attached to the eccentric movesup and down flexing the diaphragm and pumping the fuel to the engine. Because electricpumps do not depend on an eccentric for operation, they can be located anywhere on thevehicle. In fact they work best when located near the fuel tank.

Many cars today, locate the fuel pump inside the fuel tank. While mechanical pumpsoperate on pressures of 4-6 psi (pounds per square inch), electric pumps can operate onpressures of 30-40 psi. Current is supplied to the pump immediately when the key isturned. This allows for constant pressure on the system for immediate starting. Electricfuel pumps can be either low pressure or high pressure. These pumps look identical, so becareful when replacing a fuel pump that the proper one is used. Fuel pumps are rated bypressure and volume. When checking fuel pump operation, both specifications must bechecked and met.

Fuel Filters
The fuel filter is the key to a properly functioning fuel delivery system. This is moretrue with fuel injection than with carbureted cars. Fuel injectors are more susceptible todamage from dirt because of their close tolerances, but also fuel injected cars useelectric fuel pumps. When the filter clogs, the electric fuel pump works so hard to pushpast the filter, that it burns itself up. Most cars use two filters. One inside the gastank and one in a line to the fuel injectors or carburetor. Unless some severe and unusualcondition occurs to cause a large amount of dirt to enter the gas tank, it is onlynecessary to replace the filter in the line.


Fuel Injection

Throttle body injection

Port injection

Direct injection






The Components of a Fuel System

The Radiator
The radiator core is usually made of flattened aluminum tubes with aluminum strips that zigzag between the tubes.  These fins transfer the heat in the tubes into the air stream to be carried away from the vehicle.  On each end of the radiator core is a tank, usually made of plastic that covers the ends of the radiator, 

On most modern radiators, the tubes run horizontally with the plastic tank on either side.  On other cars, the tubes run vertically with the tank on the top and bottom.  On older vehicles, the core was made of copper and the tanks were brass.  The new aluminum-plastic system is much more efficient, not to mention cheaper to produce.  On radiators with plastic end caps, there are gaskets between the aluminum core and the plastic tanks to seal the system and keep the fluid from leaking out.  On older copper and brass radiators, the tanks were brazed (a form of welding) in order to seal the radiator.

The tanks, whether plastic or brass, each have a large hose connection, one mounted towards the top of the radiator  to let the coolant in, the other mounted at the bottom of the radiator on the other tank to let the coolant back out.  On the top of the radiator is an additional opening that is capped off by the radiator cap.  More on this later.

Another component in the radiator for vehicles with an automatic transmission is a separate tank mounted inside one of the tanks.  Fittings connect this inner tank through steel tubes to the automatic transmission.  Transmission fluid is piped through this tank inside a tank to be cooled by the coolant flowing past it before returning the the transmission.

Radiator Fans
Mounted on the back of the radiator on the side closest to the engine is one or two electric fans inside a housing that is designed to protect fingers and to direct the air flow.  These fans are there to keep the air flow going through the radiator while the vehicle is going slow or is stopped with the engine running.  If these fans stopped working, every time you came to a stop, the engine temperature would begin rising.  On older systems, the fan was connected to the front of the water pump and would spin whenever the engine was running because it was driven by a fan belt instead of an electric motor.  In these cases, if a driver would notice the engine begin to run hot in stop and go driving, the driver might put the car in neutral and rev the engine to turn the fan faster which helped cool the engine.  Racing the engine on a car with a malfunctioning electric fan would only make things worse because you are producing more heat in the radiator with no fan to cool it off.

The electric fans are controlled by the vehicle's computer.  A temperature sensor monitors engine temperature and sends this information to the computer.  The computer determines if the fan should be turned on and actuates the fan relay if additional air flow through the radiator is necessary.

If the car has air conditioning, there is an additional radiator mounted in front of the normal radiator.  This "radiator" is called the air conditioner condenser, which also needs to be cooled by the air flow entering the engine compartment.  You can find out more about the air conditioning condenser by going to our article onAutomotive Air Conditioning.  As long as the air conditioning is turned on, the system will keep the fan running, even if the engine is not running hot.  This is because if there is no air flow through the air conditioning condenser, the air conditioner will not be able to cool the air entering the interior.

Pressure cap and reserve tank
As coolant gets hot, it expands.  Since the cooling system is sealed, this expansion causes an increase in pressure in the cooling system, which is normal and part of the design.  When coolant is under pressure, the temperature where the liquid begins to boil is considerably higher.  This pressure, coupled with the higher boiling point of ethylene glycol, allows the coolant to safely reach temperatures in excess of 250 degrees.

The radiator pressure cap is a simple device that will maintain pressure in the cooling system up to a certain point.  If the pressure builds up higher than the set pressure point, there is a spring loaded valve, calibrated to the correct Pounds per Square Inch (psi), to release the pressure.

When the cooling system pressure reaches the point where the cap needs to release this excess pressure, a small amount of coolant is bled off.  It could happen during stop and go traffic on an extremely hot day, or if the cooling system is malfunctioning.  If it does release pressure under these conditions, there is a system in place to capture the released coolant and store it in a plastic tank that is usually not pressurized.  Since there is now less coolant in the system, as the engine cools down a partial vacuum is formed.  The radiator cap on these closed systems has a secondary valve to allow the vacuum in the cooling system to draw the coolant back into the radiator from the reserve tank (like pulling the plunger back on a hypodermic needle)  There are usually markings on the side of the plastic tank marked Full-Cold, and Full Hot.  When the engine is at normal operating temperature, the coolant in the translucent reserve tank should be up to the Full-Hot line.  After the engine has been sitting for several hours and is cold to the touch, the coolant should be at the Full-Cold line.

Water Pump
A water pump is a simple device that will keep the coolant moving as long as the engine is running.  It is usually mounted on the front of the engine and turns whenever the engine is running.  The water pump is driven by the engine through one of the following:

  • A fan belt that will also be responsible for driving an additional component like an alternator or power steering pump
  • A serpentine belt, which also drives the alternator, power steering pump and AC compressor among  other things.
  • The timing belt that is also responsible for driving one or more camshafts.

The water pump is made up of a housing, usually made of cast iron or cast aluminum and an impeller mounted on a spinning shaft with a pulley attached to the shaft on the outside of the pump body.  A seal keeps fluid from leaking out of the pump housing past the spinning shaft.  The impeller uses centrifugal force to draw the coolant in from the lower radiator hose and send it under pressure into the engine block.  There is a gasket to seal the water pump to the engine block and prevent the flowing coolant from leaking out where the pump is attached to the block..

The thermostat is simply a valve that measures the temperature of the coolant and, if it is hot enough, opens to allow the coolant to flow through the radiator.  If the coolant is not hot enough, the flow to the radiator is blocked and fluid is directed to a bypass system that allows the coolant to return directly back to the engine.  The bypass system allows the coolant to keep moving through the engine to balance the temperature and avoid hot spots.  Because flow to the radiator is blocked, the engine will reach operating temperature sooner and, on a cold day, will allow the heater to begin supplying hot air to the interior more quickly.

Since the 1970s, thermostats have been calibrated to keep the temperature of the coolant above 192 to 195 degrees.  Prior to that, 180 degree thermostats were the norm.  It was found that if the engine is allowed to run at these hotter temperatures, emissions are reduced, moisture condensation inside the engine is quickly burned off extending engine life, and combustion is more complete which improves fuel economy.

The heart of a thermostat is a sealed copper cup that contains wax and a metal pellet.  As the thermostat heats up, the hot wax expands, pushing a piston against spring pressure to open the valve and allow coolant to circulate.

The thermostat is usually located in the front, top part of the engine in a water outlet housing that also serves as the connection point for the upper radiator hose.  The thermostat housing attaches to the engine, usually with two bolts and a gasket to seal it against leaks.  The gasket is usually made of a heavy paper or a rubber O ring is used.  In some applications, there is no gasket or rubber seal.  Instead, a thin bead of special silicone sealer is squeezed from a tube to form a seal.

There is a mistaken belief by some people that if they remove the thermostat, they will be able to solve hard to find overheating problems.  This couldn't be further from the truth.  Removing the thermostat will allow uncontrolled circulation of the coolant throughout the system.  It is possible for the coolant to move so fast, that it will not be properly cooled as it races through the radiator, so the engine can run even hotter than before under certain conditions.  Other times, the engine will never reach its operating temperature.  On computer controlled vehicles, the computer monitors engine temperatures and regulates fuel usage based on that temperature.  If the engine never reaches operating temperatures, fuel economy and performance will suffer considerably.

Bypass System
This is a passage that allows the coolant to bypass the radiator and return directly back to the engine.  Some engines use a rubber hose, or a fixed steel tube.  In other engines, there is a cast in passage built into the water pump or front housing.  In any case, when the thermostat is closed, coolant is directed to this bypass and channeled back to the water pump, which sends the coolant back into the engine without being cooled by the radiator.

Freeze Plugs
When an engine block is manufactured, a special sand is molded to the shape of the coolant passages in the engine block.  This sand sculpture is positioned inside a mold and molten iron or aluminum is poured to form the engine block.  When the casting is cooled, the sand is loosened and removed through holes in the engine block casting leaving the passages that the coolant flows through.    Obviously, if we don't plug up these holes, the coolant will pour right out.

Plugging these holes is the job of the freeze-out plug.  These plugs are steel discs or cups that are press fit in the holes in the side of the engine block and normally last the life of the engine with no problems.  But there is a reason they are called freeze-out plugs.  In the early days, many people used plain water in their engines, usually after replacing a burst hose or other cooling system repair.  "It is summer and I will replace the water with antifreeze when the weather starts turning". 

Needless to say, people are forgetful and many a motor suffered the fate of the water freezing inside the block.  Often, when this happened the pressure of the water freezing and expanding forced the freeze-out plugs to pop out, relieving the pressure and saving the engine block from cracking. (although, just as often the engine cracked anyway).  Another reason for these plugs to fail was the fact that they were made of steel and would easily rust through if the vehicle owner was careless about maintaining the cooling system.  Antifreeze has rust inhibitors in the formula to prevent this from happening, but those chemicals would lose their effect after 3 years, which is why antifreeze needs to be changed periodically.  The fact that some people left plain water in their engines greatly accelerated the rusting of these freeze plugs.

When a freeze plug becomes so rusty that it perforates, you have a coolant leak that must be repaired by replacing the rusted out freeze plug with a new one.  This job ranges from fairly easy to extremely difficult depending on the location of the affected freeze plug.  Freeze plugs are located on the sides of the engine, usually 3 or 4 per side.  There are also freeze plugs on the back of the engine on some models and also on the heads.

As long as you are good about maintaining the cooling system, you need never worry about these plugs failing on modern vehicles

Head Gaskets and Intake Manifold Gaskets
All internal combustion engines have an engine block and one or two cylinder heads.  The mating surfaces where the block and head meet are machined flat for a close, precision fit, but no amount of careful machining will allow them to be completely water tight or be able to hold back combustion gases from escaping past the mating surfaces. 

In order to seal the block to the heads, we use a head gasket.  The head gasket has several things it needs to seal against.  The main thing is the combustion pressure on each cylinder.  Oil and coolant must easily flow between block and head and it is the job of the head gasket to keep these fluids from leaking out or into the combustion chamber, or each other for that matter.

A typical head gasket is usually made of soft sheet metal that is stamped with ridges that surround all leak points.  When the head is placed on the block, the head gasket is sandwiched between them.  Many bolts, called head bolts are screwed in and tightened down causing the head gasket to crush and form a tight seal between the block and head.

Head gaskets usually fail if the engine overheats for a sustained period of time causing the cylinder head to warp and release pressure on the head gasket.  This is most common on engines with cast aluminum heads, which are now on just about all modern engines. 

Once coolant or combustion gases leak past the head gasket, the gasket material is usually damaged to a point where it will no longer hold the seal.  This causes leaks in several possible areas.  For example:

  • combustion gases could leak into the coolant passages causing excessive pressure in the cooling system.
  • coolant could leak into the combustion chamber causing coolant to escape through the exhaust system, often causing a white cloud of smoke at the tailpipe. 
  • Other problems such as oil mixing with the coolant or being burned out the exhaust are also possible.

Some engines are more susceptible to head gasket failure than others.  I have seen blown head gaskets on engines that just started to overheat and were running hot for less than 5 minutes.  The best advice I can give is, if the engine shows signs of overheating, find a place to pull over and shut the engine off as quickly as possible.

Head gaskets themselves are relatively cheap, but it is the labor that's the killer.  A typical head gasket replacement is a several hour job where the top part of the engine must be completely disassembled.  These jobs can easily reach $1,000 or more. 

On V type engines, there are two heads, meaning two head gaskets.  While the labor won't double if both head gaskets need to be replaced, it will probably add a good 30% more labor to replace both.  If only one head gasket has failed, it is usually not necessary to replace both, but it could be added insurance to get them both done at once.

A head gasket replacement begins with the diagnosis that the head gasket has failed.  There is no way for a technician to know for certain whether there is additional damage to the cylinder head or other components without first disassembling the engine.  All he or she knows is that fluid and/or combustion is not being contained.

One way to tell if a head gasket has failed is through a combustion leak test on the radiator.  This is a chemical test that determines if there are combustion gases in the engine coolant.  Another way is to remove the spark plugs and crank the engine while watching for water spray from one or more spark plug holes.  Once the technician has determined that a head gasket must be replaced, an estimate is given for parts and labor.  The technician will then explain that there may be additional charges after the engine is opened if more damage is found.

Heater Core
The hot coolant is also used to provide heat to the interior of the vehicle when needed.   This is a simple and straight forward system that includes a heater core, which looks like a small version of a radiator, connected to the cooling system with a pair of rubber hoses.  One hose brings hot coolant from the water pump to the heater core and the other hose returns the coolant to the top of the engine.  There is usually a heater control valve in one of the hoses to block the flow of coolant into the heater core when maximum air conditioning is called for.

A fan, called a blower, draws air through the heater core and directs it through the heater ducts to the interior of the car.  Temperature of the heat is regulated by a blend door that mixes cool outside air, or sometimes air conditioned air with the heated air coming through the heater core.  This blend door allows you to control the temperature of the air coming into the interior.  Other doors allow you to direct the warm air through the ducts on the floor, the defroster ducts at the base of the windshield, and the air conditioning ducts located in the instrument panel.

There are several rubber hoses that make up the plumbing to connect the components of the cooling system.  The main hoses are called the upper and lower radiator hoses.  These two hoses are approximately 2 inches in diameter and direct coolant between the engine and the radiator.  Two additional hoses, called heater hoses, supply hot coolant from the engine to the heater core.  These hoses are approximately 1 inch in diameter.  One of these hoses may have a heater control valve mounted in-line to block the hot coolant from entering the heater core when the air conditioner is set to max-cool.  A fifth hose, called the bypass hose, is used to circulate the coolant through the engine, bypassing the radiator, when the thermostat is closed.  Some engines do not use a rubber hose.  Instead, they might use a metal tube or have a built-in passage in the front housing. 

These hoses are designed to withstand the pressure inside the cooling system.  Because of this, they are subject to wear and tear and eventually may require replacing as part of routine maintenance.  If the rubber is beginning to look dry and cracked, or becomes soft and spongy, or you notice some ballooning at the ends, it is time to replace them.  The main radiator hoses are usually molded to a shape that is designed to rout the hose around obstacles without kinking.  When purchasing replacements, make sure that they are designed to fit the vehicle.

There is a small rubber hose that runs from the radiator neck to the reserve bottle.  This allows coolant that is released by the pressure cap to be sent to the reserve tank.  This rubber hose is about a quarter inch in diameter and is normally not part of the pressurized system.  Once the engine is cool, the coolant is drawn back to the radiator by the same hose.

Fuel System Maintenance and Repair
An engine that is overheating will quickly self destruct, so proper maintenance of the cooling system is very important to the life of the engine and the trouble free operation of the cooling system in general.

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.

Newer antifreeze formulations will last for 5 years or 150,000 miles before requiring replacement.  These antifreezes are usually red in color and are referred to as "Extended Life" or "Long Life" antifreeze.  GM has been using this type of coolant in all their vehicles since 1996.  The GM product is called "Dex-Cool".

Most antifreeze used in vehicles however, is green in color and should be replaced every two years or 30,000 miles, which ever comes first.  You can convert to the new long life coolant, but only if you completely flush out all of the old antifreeze.  If any green coolant is allowed to mix with the red coolant, you must revert to the shorter replacement cycle.

Look for a shop that can reverse-flush the cooling system.  This requires special equipment and the removal of the thermostat in order to do the job properly.  This type of flush is especially important if the old coolant looks brown or has scale or debris floating around in it.

If you remove the thermostat for a reverse flush, always replace it with a new thermostat of the proper temperature.  It is cheap insurance.

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 recommended concentration of coolant

Let's take these items one at a time.

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.

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