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NB This article is unfinished but there's enough here to be useful. I will be adding more text and photos. Please email me if you have any queries. Updated April 2017

A complete illustrated guide to automotive overheating, diagnosis and repair.


Water and steam can come out of a overheating car's radiator with great force when the radiator cap is removed. If you really have to take the radiator cap off when the engine is hot fold a full sized towel into four, place it over the cap and turn the cap slowly and rock the cap gently from side to side as you turn it. Have a bucket cold water handy in case of burns.

Water boils at a higher temperature with higher pressure which a good radiator cap provides. When the radiator cap is removed the presure drops. Water which was not boiling when the cap was on can instantly turn to highly pressurised steam and hot water when the cap is removed.

An electric radiator fan can turn on without warning both when the engine is running and also soon after the motor is turned off.(the water is no longer moving through the motor so it has more time to heat up inside the motor which turns on the sensor that allows electric current to flow through the cooling fan.)

Don't cause further damage to your car

Driving an overheated car can blow your head gasket, crack your motor and increase the cost of repairs . If your car over heats stop driving and allow it to cool down, check the fan belt isn't broken and the radiator is full of water. Either leave the motor idling with the heater on hot and the heater fan on high or leave the key on, the heater on and the motor off. The radiator and heater fans will run and help cool the water in the radiator and engine. Leaving the bonnet open allows air to circulate around the motor and be moved faster through the radiator by the fan. With the motor off it takes a bit over an hour for a motor to cool from hot to air temperature. Reducing speed, keeping the air conditioning off and running the heater while driving can sometimes provide enough additional cooling to stop a borderline system overheating.

How Your Engine Cools

The engine transfers some of its heat to the water inside the engine. When the water in the engine reaches the opening temperature of the thermostat (anywhere from 71 to 92 degrees C) the thermostat opens and allows the water pump to pump the water through the radiator. The water is cooled by the radiator before returning to the engine. The water in the radiator is cooled by the air that is either pushed or pulled through the radiater by the fan (or forced through by the cars movement)

The fan rotates when the viscous coupling locks up or in the case of an electric fan when the temperature sensor in the engine reaches a certain temperature.

The radiator cap increases the pressure in the cooling system which raises the temperature the water boils at as does the addition of coolant/antifreeze to the water.

A sound heat gasket in the motor stops combustion gases entering the cooling water.

Find why your car overheats - a typical approach

  • This section is a quick guide to finding why a car is overheating. Go to the detailed sections for more detail.
  • Check the fan belts are reasonably tight. Allow the car to cool a little. Read the safety section above and remove the radiator cap.
  • Fill the radiator slowly with water and then while the engine is running give the car a quick rev. If the water shoots in the air you probably have a blown head gasket. Otherwise once the engine has reached running temperature the water should flow rapidly across the top of the radiator indicating that the thermostat is open
  • Turn the engine off and apply pressure to the cooling system. Look for leaks.
  • Pull out the dipstick and remove the rocker cover cap. Look for a whitish oil and water mix which indicates a blown head gasket. A little white mix stuck to the bottom of the rocker cover cap is ok but check your engine ventilation system later.
  • Run the engine and see if the fan comes on before the engine boils. If the car has a viscous coupling roll up some newspaper and stuff the newspaper into the fan. It should push the paper out of the way. However if this stops the fan turning the viscous coupling is not working.
  • Lower the coolant level and look through the radiator cap hole to see if the few radiator tubes you can see are partially blocked. many an overheating car has been fixed by cleaning out the radiator tubes. To do this you remove the top tankand push rods through the cores. Normally a job for the radiator shop.
  • Remove the spark plugs, rotate the engine to the correct position and apply compressed air to the cylinders one by one. See if the air bubbles out of the radiator indicating a blown head gasket.
  • A few bubbles while the engine is running do not necessarily indicate a blown head gasket.
  • Much more detailed information below.

Check for water leaks in all the places listed below

  • A pressure tester and a good spot light are almost essential for finding small leaks. See the next section for how to make and use a simple radiator pressure tester.
  • Remove the overflow pipe from the radiator and slide on the pressure tester hose. Pump up the pressure to ten psi, turn off the tap so the air can't leak back through the pump. The pressure may drop a little initially as the hoses swell but then should remain steady for perhaps five minutes. If the needle drops you have a leak. The radiator cap has a return valve that allows the air pressure to get past the cap and pressurise the radiator.
  • All hoses
  • NB Hoses can get rather stuck. To remove a hose slide a small screw driver under and parallel to the hose and gently wriggle. Then gently rotate the hose backwards and forwards while also trying to pull it off. Be particually careful and gentle with the hoses that go through the firewall and on to the heater. Its easy to damage the heater pipes when you pull them off and a very big job to get the heater radiator out and back in.
  • Radiator A cracked top tank is a common leaking place.
  • Radiator caps ned to be pressure tested to make sure they allow coolant to flow to the overflow bottle at the correct pressure. Inspect the rubber gaskets. At higher pressures water boils at a higher temperature. Although the coolant should never reach 100 degrees C it definetely helps if the water doesn,t boil if it does.
  • Water Pump leaks are usually a steady drip from the bottom of the pump. Some water pumps are behind the cambelt cover in which case the water may appear at the bottom of the cam cover.
  • Frost Plugs are usually on the sides of the motor under the manifold, the back of the head and the back of the block under the gearbox bellhousing (look for water coming out between the motor and gearbox). Anyone who's had to pull out a gearbox to replace a frost plug thinks both that brass frost plugs are better than steel ones (they don't rust) and that a good antifreeze with corrosion inhibitor in it is well worth the money. The temporary get you home solution is to fill the hole with panelbeaters filler usually called bog.
  • Blown Head Gaskets can leak water into the combustion chamber or to the outside of the head between the head and the block. The biggest giveway is if the oil is a milky color and/or if the engine oil level is very high. Pull out the dipstick and check the colour and level of the oil. Take the radiator cap off and look for a stream of bubbles in the radiator. Give the engine a couple of quick revs and if the water shoots up into the air you probably have a blown head gasket. See the seperate head gasket section for more information on blown head gaskets.
  • Cracked Head and very rarely a cracked block. As above check the oil and refer to the head gasket section.
  • The Heater Radiatoris usually found under the dash. They can take all day to remove and replace. Then the leak has to be fixed or perhaps $200 spoent on a new heater radiator core. An alternative is to bypass the heater by joining the two heater hoses together and doing without the heater.
  • Inlet Manifold Gasket The inlet manifold often has water circulating through it and if the gasket material between the water way and the airway is damaged or if the manifold to head bolts are loose water will either run down the block or get sucked into the motor where it would normally evapourate. Some V8 engines are notorious for leaking inlet manifold gaskets.
  • Into the autotransmission via where the trans oil flows through the radiator. See here. A pressure test on the radiator should pick this up or possibly a rising oil (and water) level of the trans fluid. The Nissan Pathfinder referred to in the link already has an external oil cooler so in a mild climate putting the trans oil through the radiator is perhap unnecessary. And why wait until your trans is stuffed before acting.
  • Overflow Bottle. Water forced out of a cooling system by a blown head gasket will go into the overflow bottle. Is the level rising or even overflowing when the engine is running ?
  • Warning It is normal for the air conditioner to expel some condensed water vapour after it has been running.
  • Some people say that coolant has a sweet smell at the exhaust when it burns in the engine. If this smell is present it would indicate a blown head gasket.

Pressure Tester

  • A pressure tester helps find leaks in a cars water system . Apply pressure, wait, know where to look, have a good light and place a clean piece of cardboard under the engine to see where the drips are coming from.
  • Make a simple pressure tester from a bicycle pump, a few brass fittings, a small tap and a pressure guage. Apply perhaps four turns of thread tape to the male threads and screw the parts together.
  • To test your pressure tester for leaks block the end of the hose that will later go on the radiator overflow pipe, pump up the pressure and make sure the needle on the guage is steady which indicates that there are no leaks in the tool.
  • The radiator cap has a return valve that allows water to return to the radiator from the overflow bottle. Remove the overflow hose from the radiator, attach the tester hose to the overflow pipe and pump up the pressure to the higher of 10psi or the radiator cap pressure and quickly close the tap. If there is no leak the pressure will not drop over 5 minutes.(there may be a small initial drop if the hoses swell a little or if air in the system compresses)
  • To find a leak if the guage indicates drpping pressure get someone to keep pumping while you look with a light for the leak. (n.b. if your radiator is on its last legs you may create a leak by raising the pressure too high)
  • Lay a clean sheet of cardboard or similar under the car to make it easy to see any water dripping down.
  • It's normal for the air conditioning to drip a little water into the engine bay after it has been running.

Cooling Fans

  • There are three types of automotive cooling fans. There is a fan that is fixed permanently to the water pump and always turns at the same speed as the pump. this sort almost never breaks but wastes energy by turning even when the coolant does not need cooling.
  • The next sort is an electric fan which is turned on by a signal from a temperature sensor (usually) triggering a relay that then allows electricity to flow to the cooling fan.
  • The third sort has a fan that is attached to a viscous coupling. The first sort of viscous coupling is speed controlled and the second type is controlled by the temperature of the air coming out of the radiator. The speed of the fan is controlled by the temperature of the air coming out of the radiator.

Viscous couplings

  • The less common speed controlled coupling responds to speed only. The speed of the fan is controlled by the coupling slipping to a lessor or greater degree.
  • Temperature controlled or thermal couplings have a temperature-sensitive bimetal coil spring on the front that responds (or is it the plate?) to the temperature of the air coming through the radiator. When the air coming through the radiator is hot, the spring expands and rotates a shaft which then opens an internal valve that allows the silicone oil inside the coupling to flow through a hole.
  • When the silicone fluid has flown out of the resevoir into the main part of the coupling the rotating rear section of the coupling is able to move the fluid that then turns the front part of the coupling and hence the fan.
  • There is always some slippage and the fan should never turn quite as fast as the pulley and rear half of the viscous coupling.
  • As the air cools, the spring contracts and closes the valve and eventually the fluid returns to the resevoir. The rear half of the viscous coupling no longer has a conection to the front half via the fluid which means that the fan is no longer turned with any significant force.

Diagnosing fan viscous couplings

  • Hold the fan and wriggle it to determine whether the unit is mechanically sound. A few millimeters of movement at the fan tip is ok.
  • A comon sign of a viscous fan coupling not working is the air conditioning not working at low speeds when the engine is hot.
  • Look for silicone leaks
  • If the viscous coupling is locked on all the time there is often constant fan noise. A little noise at start up is normal due to fluid settling while the car is at rest
  • The next step is to determine that the fan slips (hardly turns) when cold and grips when hot (turns quickly)
  • Turn the the fan when the engine is cold and not after it has recently been hot. It should feel sticky but turn easily with gentle pressure. Start the car and the fan may turn very slowly but stop when you push a rolled up newspaper in to it. At a certain temperature that varies from car to car the fan will slowly speed up. It will not turn as fast as the pulley it is attached to. When the engine is hot and you push the news paper into the fan the fan will push the newspaper out of the way.
  • Different fans will vary in their behaviour
  • The thickness of the oil inside the viscous couplings determines the amount of slippage in the fan and when it comes on. There are comonly 3 different grades of silicone fluid available. Putting in a thicker silicone fluid will mean the fan comes on sooner and or turns faster.

Repairing a viscous coupling

    Getting home tricks

  • Sometime the shaft at the centre of the bimetallic spring is stuck. Try a quick blast with the hot air gun. Extra heat may get the bimetallic spring to put extra pressure on the shaft going into the unit. Otherwise Try gently rotating it backwards and forwards. This could save you dismantling the whole setup.
  • Sometimes you can find another slot to put the end of the spring into. If this turns the shaft antclockwise the valve will be held open and the fan turn always. This temporary repair enables you drive home without overheating
  • Try removing the spring and turning the shaft anticlockwise with a screwdriver. Then glue the shaft in place to get you home
  • As a last resort drill a hole through the viscouss coupling from front to back and lock up with a bolt. This means that your viscous coupling is wrecked but the fan will turn all the time.
  • To dismantle see here
  • there is an adjustment inside the viscous coupling that affects the temperature the coupling locks up at. Adding more fluid if there is not enough or changing to a thicker fluid will help the coupling lock up more strongly
  • Porshe 928 see here
  • Electric fans

  • An electric cooling fan can come on at any time - in some cases even when the key is off so keep hands away.
  • Testing Electric Fan Circuit

  • The first cooling fan test is to idle the motor until it's hot and see if the fan turns on. This is typically when the needle is about 3mm past the mid point in the temperature guage and definetely before the water in the radiator boils.
  • Fans can be turned on by the air conditioner or the temperature switch so if getting the motor hot doesn't turn on the fan try turning on the air conditioning. This may show that the fan is working but would not show that the cooling fan switch is working. The final fan checkis to disconect the plug from the fan motor which is usually near by. If it has 2 wires conect 1 to one side of the battery and the other to the other side of the battery. If it has 3 wires one is an earth and the other 2 are for slow and fast motor speed.
  • The cooling fan switch is easy to confuse with the temperature guage switch. Pull the wire off the cooling fan switch while the motor is WARM OR HOT and check that the temperature guage doesn't move. If it does you have the temperature guage sender not the fan cooling switch.
  • Next test the cooling fan switch by taking a wire from the live(usually +ve) side of the battery to a bulb and then to the cooling fan switch. When the engine coolant is hot enough the bulb should light. If it does't the cooling fan switch is faulty.(otherSWITCHES may be wired DIFFERENTLY?
  • To test for power to the cooling fan switch, pull the wire off the cooling fan switch and connect it through a headlight bulb to earth. If the bulb lights you have power to the cooling fan switch. (remember to turn the key on). If it doesn't (and you remembered to check the fuses) find the relay.
  • Go to here to learn all about relays.
  • One of the reasons for using a bulb is to protect the computer if you have chosen the wrong wire.
  • Refer to the wiring diagram to the right. This is a very simple circuit but it illustrates the basic circuitary of many fan electrics.
  • You have to understand how they work in order to fix problems in electric cooling fan circuits:
  • Check that the fan is pulling or pushing the air through the radiator from the front towards the rear.
  • Vicous couplings are easy to repair by adding a special fluid


  • The thermostat allows the engine to warm up quickly which stops excessive wear caused by the engine running at too lower a temperature. When it is closed it stops most of the water flow through the radiator. When it is open it controls the volume of water flowing through the radiator.
  • The thermostat will open when the water reaches it's opening temperature (often 82 degrees C) It closes when the water cools . This happens as the water heated by the engine can't flow through the radiator and cool before it re enters the engine to absorb more heat.
  • Warming the engine quickly allows the heater to start working earlier and avoids gradual damage that occurs when the engine runs at a colder temperature. On many cars the cylinder nearest the water pump will habeingve more wear as that cylinder runs cooler than the others.
  • The thermostat is usually inside a housing on the motor immediately before the top radiator hose, sometimes where the lower hose enters the engine(eg 1996 Daihatsu Charade) and no doubt occasionally elsewhere.
  • The main problem with thermostats is that they fail to open. If the thermostat is next to the top hose, a closed thermostat means that hose can be cooler than usual when you touch it. However some mechanics drill an extra small hole to a thermostat which allows a small amount of water to circulate even when the thermostat is closed so you can be fooled.
  • Another way to see if the thermostat is working is to take the radiator cap off and give the motor a quick rev. if the thermostat is open you will be able to see the water will surge across the radiator by looking through the radiator cap hole
  • Test the thermostat by removing it from the car and tying a piece of wire around it and lowering it into a cup of boiled water. A good thermostat may not open as the thermostat and cup absorb much of the heat from the boiling water. Quickly empty the cup and pour in a second lot of boiling water. A good thermostat will usually open with the second lot of boiling water.
  • The thermostat needs to be installed the right around so that the bit containing the wax that expands is nearest the engine. In many cars it is not possible to put the thermostat in the wrong way around.
  • It is possible a thermostat will not open if a air bubble is caught below it. The jiggle pin should let the air through but I prefer to drill a 2mm hole through the flat part of the thermostat to allow an additional escape for any air. Look around the engine for a bleed nipple to open and let the air out.
  • Some Chrysler 2.2 and 2.5 litre engines have the thermostat higher than the radiator cap and are vunerable to this. Thier bleed nipple is on the engine near the top hose. When filling the radiator another way to remove air is to jack the front of the car up high so that the radiator cap is higher than the thermostat. Subaru legacys have the bleed nipple on the radiator near the top hose(rhs).
  • Have the heater on hot when filling a radiator to assist with getting the air out. Idle the car for a while (especialy with cross flow radiators) and check again after a short drive.
  • If you leave the thermostat out the engine will usually run cooler. I removed the working thermostat from a Nissan Cefiro and it went from overheating on the motorway to running below normal temperature
  • Failsafe thermostats fail in the open position. However many criticize them for failing when the car gets just a little hot and staying open. This gets expensive.
  • Idiot of the year award goes to the designer of the 1.6 8v 1999 mk4 Vauxhall Astra which has it's thermostat behind the cambelt. I suggest making up a thermostat holder to fit in the top radiator hose to avoid this stupidity. Not all Astras have this pathetic positioning of the thermostat.
  • A high flow thermostat will help your car cool more efficiently.
  • More on thermostats at here


  • A head gasket sits between the engine's head and the block (the bottom 2/3 of the engine). It stops gases escaping from the cylinder where the petrol burns into the water holes that surround the cylinder and continue up into the head. The head gasket also stops water leaking from the waterways into the cylinders or to the outside of the engine.
  • Pull out the dipstick. If the oil is a milky colour or if the engine oil level is very high you almost certainly have a blown head gasket. Also take off the rocker cover cap. A small amount of white oil/water mix may be just from condensation but if there is more whiteish oil/water mix over the rockers then the head gasket is almost certainly blown.
  • Take the radiator cap off and give the engine a good rev - does water shoot up in the air?
  • Remove the spark plugs. Place the car in park if an automatic or neutral if a manual. Disconect the wiring to the distributer, pull the petrol pump fuse and turn over the motor with the key.
  • Alternatively leave the key off and take a wire from the small spade terminal on the starter and touch it on the positive battery terminal.
  • If water comes out of the spark plug holes while turning the motor over the head gasket is blown. You can place a piece of paper over the spark plug holes and look /smell for anti freeze on the piece of paper after turning the motor over.
  • Give the engine a compression test. If the compression is way down the head has to come off anyway, so not much point in carrying on with the diagnosis. Low compression on 2 adjacent cylinders points to a blown head gasket.
  • Make sure the radiator is full and bleed all the air out. Get the engine hot so that the thermostat is open or remove it. Fit 1 spark plug and remove the rest. Turn the motor over and look for bubbles in the top of the radiator. Repeat for the other cylinders. Confirm with the next test if necessary.
  • Make a compressed air to spark plug hole adaptor by breaking off and drilling out the porcelin from a spark plug so that the thread remains with a hole down the center. Braze a male air hose fitting to the remains of the spark plug.
  • Bring the piston to top dead center on the firing stroke by getting someone to flick the starter switch for a split second at a time while u hold a screwed up rag over the spark plug hole. When u feel air pressure pushing against the rag the piston is moving from the bottom to the top. Stop turning the motor with the starter and put a screw driver through the spark plug hole. Turn the motor slowly with a spanner on the crankshaft pulley nut until the screw driver stops rising. You have the piston very close to top dead center. Screw in the tool and attach a clear plastic hose to the fitting. Loop the hose up then down and then up again. Put a small amount of water in the hose (see pic) so that it's near the middle of the hose and not flowing in either direction. Turn the motor by putting a spanner on the crankshaft pulley and rocking it backwards and forwards. The water will move towards the engine when the piston is going down and away from the motor when the piston is comming up. Get the piston exactly at the top.
  • Lock the crankshaft pulley with a socket and power bar. Remove the plastic hose and connect a compressor hose to the tool. If the piston is not at top dead center the motor will turn uncontrollably possibly injuring u or flying thru the radiator. Avoid this by either putting the piston at top dead center or draining the compressor of air before connecting the air hose to the tool and then turning it on. This will limit the damage if you got it wrong.
  • With the compressor supplying air to the cylinder at full pressure if the head gasket is blown air will bubble out of the radiator water or escape into the adjacent cylinder if the head gasket is blown betwen cylinders. If the air pours out of the exhaust or the air filter and not out of another cylinder the piston may be at top dead centre following the exhaust stroke not the compression stroke.
  • If your confused and have the piston at top dead center on the compression stroke the distributer rotor will point to the spark plug lead that goes to the cylinder your working on.
  • Mechanics have a tool that sucks fumes from the top of the radiator (with the cap off and engine running). The blue liquid in the tester turns yellow if the head gasket is blown. If the liquid changes colour the gasket will always be blown. Unfortunately, (in fact it's a complete piss off} the the liquid is not sensitive enough. If the liquid does not change color the head gasket may still be blown. The test has more chance of working if the engine is hot and has been running for a while which gives the fumes more time to escape into the water. This type of test also does not show a head gasket blown between two cylinders or a crack into the inlet or exhaust ports. Testing for exhaust gases in the coolant with an exhaust gas analyser is reputed to be more accurate, but be careful if you have recently flushed your radiatror with a peterolwum based product.
  • If a compression test shows two adjacent cylinders with low compressons the head gasket could be blown.
  • Put a small blob of engine oil on a piece of oil and heat the underside of the foil. If there is no water in the oil it will smoke if there is it will sizzle. Good video here
  • See in the above video how to use a leakdown tester to do what I described above with the air hose to spark plug hole adaptor
  • Another technique is to get the engine hot and connect a pressure tester to the the radiator. Take out the spark plugs and look inside the cylinders with a camera for water leaks. In really difficult cases you need to keep the pressure on overnight. Run a compressor until the pressure in the tank is 15 psi. Use a pressure reduction guage to protect your cooling sytem and connect the compressor to the cooling system overnight and look through the spark plug holes in the morning. I think its better to use 120 psi from an air hose into the cylinder as described above than 15 psi on the water system. You could put a flouresent dye in the radiator water to make it easier to see.
  • Drain your oil and see if any water comes out. Oil can float on top of water.
  • Remember to change the oil and filter if water has been in the oil. It is sometimes hard to get all the white muck out of an engine after a blown head gasket. Make sure the sump slopes towards the sump plug hole and leave the engine for an hour or two so all the muck can run out. You may have to change the oil twice running the engine very briefly between changes to stir up any "muck".
  • A cracked head and very rarely a cracked block will behave in a similar manner to a blown head gasket.
  • Very rarely (well I've never seen it) a head bolt will pass throught the cooling system. (wtf surely not.)
  • Some people believe that if white smoke comes out of the exhaust the head gasket is blown - I don't. I also do not believe bubbles in the radiator while the engine is running indicate a blown head gasket. Many cars that do not have blown head gaskets have small bubbles in the radiator. Some people say they can smell antifreeze comming out of the exhaust if it's got into the cylinders.
  • As water heats up disolved air will escape as bubbles. When a liquid boils, it literally turns into a gas. (steam). As the water at the top of the pot evaporates (boils), the gas can go straight into the air. But when water evaporates (boils) at the bottom of the pot, it forms bubbles that have to float up to the top. The same thing happens in a radiator.
  • You can buy chemicals to add to your radiator that sometimes will repair a blown head gasket or a cracked head or block. Follow the manufacturers instructions and then leave the car overnight. Sometimes it's like magic many times it's money down the drain.
  • Head gasket can blow if the head bolts hit the bottom of the hole before they are fully tight. if this happens the head of the bolt is not pushing down on the head hard enough. This typically happens after head work when the head has been planed.
  • If the gasket is leaking to the outside of the block which is unusual then add pressure to the system and look with a spot light and a mirror.
  • You need to be clear whether the blown head gasket is the cause of overheating or the result. EG Your heater springs a leak and the carpet soaks it up and consequently you don't notice. This (along with your husband not watching the temperature guage) could have lead to the engine overheating and the head gasket blowing.
  • To prevent such things happening run a small hose from the radiator overflow bottle overflow hose (the hose that's not the one from the radiator) and stick it out the front grill just below the bonnet. If the car starts boiling steam will escape into the air in front of the drivers field of vision.


    Getting radiators repaired in New Zealand

  • Be careful spending too much money at the radiator shop or dismantlers. You can find new radiators at very competitive prices these days. Trademe is your friend. New goods are protected by the consumer guarantee act. For example Listing #: 1580751690 is for a new corolla radiator for $157 delivered within Auckland with na 12 month guarantee.

    Refilling radiators

  • Fill the system with water, run for 5 minutes and dump the water and repeat until clean. Do not put cold water into a hot engine unless you do it very slowly.
  • Fill the cooling system with a mix of water and antifreeze/corrosion inhibitor
  • When refilling the radiator with coolant you need to bleed the air out of the system
  • Turn the heater to hot so water can circulate through the heater and push any air out of the heater . Open any air bleed nipples. Many cars have none. The air can be removed from the cooling system when radiators with vertical tubes but radiators with horizontal tubes may need to idle for 5 minutes after the coolant is hot enough to open the thermostat. The air will bleed quicker if you have drilled an extra 5mm hole in the thermostat
  • Some cars need special measures to bleed all of the air out of the system. This happens for a variety of reasons including the water ways in the engine being higher than the radiator cap. In this case parking the car on a slope or jacking it up to raise the radiator cap above the engine helps. If your car has a bleed nipple open it until water comes out. On a Subaru legacy there is a plastic plug on the top of the radiator near where the top hose comes in. Loosen that until water comes out. Some cars including some Subaru legacys have a plastic plug to let out the air. Do not overtighten the plug as it's easy to strip the thread. Sometimes it is necessary to loosen a hose clip and even slide a small screw driver under the hose to let out the air.
  • Fill the overflow bottle to the manufacturers mark or about 60% full.
  • Then take the car for a drive on a hilly road to work out any remaining air and keep an eye on the temperature guage. Finally allow the engine to cool to nair temperature and top up again. Leave the coolant system under pressure for 15 minutes and then check for leaks and your finished.
  • A sure sigh of air in the system is the heater blowing cold when the engine is hot.

Unrealistic Use

Your cars been cooling all year, come the summer holidays, it's the hottest day of the year, you overload it , thrash it all the way, run the air conditioning and it boils. It shouldn't and the problem is that everything is working about 87 percent of what it should be.

Allow the engine to cool, drive sensibly when the air temperature has dropped (early evening?), perhaps with the heater on when going up hills and it may not overheat. The hot season passes, cars uunloaded and all is ok.

OR get impatient , keep going as before and blow your head gasket. Yes tow trucks are expensive.