see also:

• car battery running flat or car not starting
• combustion leak / head gasket leaking

• over-heating your car's engine is a major threat to the engine as continued running of the car threatens blowing a head gasket and very expensive repairs / replacement of the engine
• over-heating may just occur due to driving in hilly conditions on hot days with a load, in which case you may just need to give the car a rest for a while and checking everything else is fine including sufficient coolant is present

• steam or very high temperature gauge into the red
  • immediately STOP the engine when safe to do so
  • you probably should call a tow truck and not drive it at all as it is probably already damaged
• as long as temperature gauge not too high, briefly, while the engine is still running, open the bonnet:
  • check the fan(s) are running
    • if not running, this suggests:
      • fan belt has broken
  • check the pressure in the large black radiator hoses
    • they should be under high pressure if engine is hot
      • if not, possible causes are:
        • there is no coolant
        • water pump is not functioning,
        • thermostat not functioning,
        • clogging or constriction in its run-off hose. This opens on to the coolant tank as a result of which the engine cools down. This will cause no pressure in the radiator pipe as the fluid will not return to the tank.
        • kinked hose
• turn the engine off as soon as possible and allow engine to cool down BEFORE attempting to open the radiator cap
  • check the coolant level in the plastic coolant reservoir
  • open the radiator cap once it has cooled down and ensure there is adequate coolant level in there as well as in the coolant reservoir
    • if there is no coolant visible:
      • DO NOT continue driving until it is topped up
      • top it up with coolant, or if none available, clean water
      • if further driving results in a drop in the coolant level - go to section below of diagnosing coolant loss
  • if adequate coolant but temp gauge goes very high within 15 minutes of driving:
    • probably faulty thermostat or water pump
  • if coolant level in reservoir is adequate, check if the lower coolant hose is cool when engine is VERY hot as this suggests a faulty thermostat
    • engines have a valve-based thermostat that determines and regulates coolant inflow so that the engine operates at an optimal temperature and does not overheat. The thermostat should be closed when the engine is cold and not running. Regulation of water by the thermostat prevents engine hot spots. Once the optimal temperature has been reached, the coolant water will be let into the radiator for energy dissipation. If the thermostat is not opening at this time, it may cause the lower radiator hose to stay cold, and at the same time, overheat the engine.
• if coolant level in reservoir is over-full but radiator is empty
  • this suggests a significant coolant leak such as head gasket issues
• air pockets in the coolant may impair the water pump performance
  • prevent air pockets by:
    • ensuring coolant levels do not fall too low
    • properly bleeding the system after changing coolant fluid
  • coolant air may also be due to faulty head gasket
• clogged / corroded coolant system
  • this is uncommon but may occur and impair coolant flow and is mainly due to using water instead of coolant for long periods, but can also be caused by a leaking head gasket

• this could just be from:
  • not having topped up the coolant for a long time
  • the engine has over-heated from prolonged excessive driving loads in hot weather
  • over-filling the coolant reservoir - it should be within the markers when cold
• coolant in reservoir but not in radiator
  • consider faulty thermostat especially if the coolant loss only occurs on longer drives when engine is hot
    • thermostat usually opens at 70degC and this then allows the water to circulate and cool the engine
    • if the thermostat is stuck closed or not opening until a much higher temperature, the water pump will increase pressure with this pressure only being released by the pressure valve on the thermostat which allows some minimal water flow through it even if closed, and then via the reservoir cap which has a hole to allow release of pressure - hence the reservoir will be over-full in this scenario and steam will escape via the reservoir cap (and possible via the radiator cap)
  • possible water leak which stops the vacuum suction building up when coolant cools when engine off - this vacuum is needed to suck coolant from the reservoir via the top hose
• check for signs of external coolant leakage
  • coolant leakage may be difficult to see directly as it may be a fine spray when under high pressure when engine is running
  • look for coolant stains with a torch
    • you may need to wash everything down then run the car and re-check
  • look for intact radiator hoses and connections
  • check the reservoir for leaks or a leaking cap
  • check the top of the radiator for evidence of fine cracks or leakage
  • check inside the car for evidence of coolant loss / dampness from the heater
• check for signs of internal coolant leakage into engine and mixing with engine oil
  • check oil stick for white smearing of the oil on the stick
  • open oil cap and check for evidence in there - you will need a torch
  • internal engine leakage is a serious problem and needs to be rectified immediately to avoid serious engine damage - coolant anti-freeze is not good mixed with oil and will damage the engine
• check for signs the coolant has entered engine piston area and being lost as steam in exhaust
  • steam coming from exhaust after engine has been running for a while
  • advanced options include combustion tests of coolant to ascertain if exhaust gases have made their way into the coolant
  • this could be from:
    • a small crack in the intake manifold gasket or the head gasket,
    • a defective/cracked Exhaust Gas Recirculation (EGR) valve (this uses coolant to cool down the exhaust gases before sending some back into the engine for better fuel economy
    • cracked head
    • cracked block (or cylinder liner)
  • a sealant additive to the coolant may fix this
• pressure test the radiator
  • this requires a special air pump attached to radiator inlet and pumped to the pressure as indicated on the radiator cap
  • if no leaks found, consider pressure testing or replacing the radiator cap
• still no cause found
  • may be:
    • a faulty water pump
    • faulty heater core
    • other causes of coolant loss

• the car's water pump is the key to ensuring the engine runs properly
• it is an impeller pump driven by the engine's drive belt, with the turning blades of the pump forcing coolant to flow through the engine and back to the radiator to be cooled by a forced air cooling fan
• the wrong coolant will cause deposit-buildup inside the pump, which slows the ideal process of engine cooling.
• the pump's impeller fin blades may corrode if either:
  • wrong coolant chemistry or contaminated is used
  • faulty radiator cap allowing air into the coolant
• signs of leaking water pump from worn out gaskets and seals
  • coolant leakage at front centre of engine
  • rust or deposit build up around the pump
  • these can often be repaired before it gets worse
• water pump making high pitched sound
  • usually caused by a loose belt often due to the pulley becoming loose
  • may be caused by the bearings inside the pump wearing out
• catastrophic failure of the water pump is possible but unlikely and will cause a RAPID rise in engine temperature
  • pump impeller coming off the shaft
  • pump blades chipping or falling off
  • acute shaft fracture
  • belt breakage

• thermostat valves can become stuck
• remove thermostat and test it's function in hot water

• the coolant should act to reduce corrosion and rust
• use a multimeter set to DC voltage
• while the engine is cold, take radiator cap off
• start engine and run it at idle of 1500rpm to warm up
• place negative probe of battery's negative terminal
• place positive probe into the radiator coolant
• if the meter reads 0.4V or less, the coolant is good; however, if it reads more than 0.4V, then the additives in the coolant that prevent electrolysis have broken down, which means the coolant should be replaced.

• this may be due to:
  • inadequate coolant
    • the coolant is brought over from the engine to the heater core that then blows warm air into your car.
  • problems with the heater core
    • heater cores are typically situated directly in the back of dashboards
    • coolant may not be traveling through the heater core properly
    • the air from the blower motor is not reaching it
    • there’s a clog in the small tubing of the heater core
    • symptoms of this include:
      • fog inside the car
      • a fruity, sweet smelling odor
      • car using coolant very quickly
      • engine overheating
  • clogged or broken heater controls
    • may need to replace some of the control buttons or your heater control valve.
    • The heater control valve is underneath your hood and acts as the switch that turns the heat on and off. If that piece is not working right, your car could get stuck blowing cool air into the cabin.
  • dysfunctional thermostats
    • If the thermostat gauge stays on the “C” even after the engine has time to heat up, you may have a broken thermostat. If the thermostat can’t signal to the car that the engine is warm, the coolant won’t be sent over to provide heat to your heater core and the air will stay cool. Thermostats are a relatively easy and inexpensive fix, so installing a new one can get your heater working again quickly.
  • water leaks

• the car engine needs to be kept at an optimum operating temperature (usually around 100degC) which should not be too cold nor too hot and this is the prime role of the cooling system
• coolant (either water or preferably an ethylene glycol based coolant which does not freeze in very cold conditions and will have a lower boiling point and less tendency to corrode than water) circulates the engine pushed by the water pump which runs continuously when the engine is running driven by a belt from the engine
• the coolant is pushed to the top hoses of the radiator and then flows down through the radiator where the wind cools the coolant back down before it travels via the lower radiator hose to the thermostat and then back through the engine.
• the thermostat is designed to prevent or substantially reduce water circulating when the engine is cold so that it attains optimum operating temperature faster
  • the thermostat has an internal sensing system on the engine side which at 70degC opens the spring-loaded valve
  • as a safeguard, the thermostat also has a small hole with a mechanical valve to reduce pressure build up if the thermostat fails to open
  • while the thermostat remains closed the lower hose from the radiator will be cooler than the top radiator hoses, once it opens, this difference reduces significantly
• the radiator cap is a pressurised cap to intentionally build up pressure in the system as this increases the coolant's boiling point
• the reservoir acts as two roles:
  • provides a mechanism to top up the coolant level
    • this occurs once the engine cools down and the pressure falls in the cooling system and sucks coolant from the reservoir into the top of the radiator
  • provides a pressure relief system if the thermostat fails to open although this will result in loss of coolant through the cap of the reservoir