Servicing the Engine Cooling System
 

Did you know that up to a third of the heat energy produced by an internal combustion engine ends up as waste heat in the cooling system? A gallon of gasoline produces about 19,000 to 20,000 BTUs of heat energy when it is burned, which is enough to boil over 120 gallons of water! So the two or so gallons of coolant that circulate within the typical automotive cooling system have to carry away a lot of heat. The radiator also has to be fairly efficient at getting rid of the heat, too, otherwise the BTUs will start to back up and make the engine overheat. An efficient cooling system, therefore, requires several things: an adequate supply of coolant, an efficient heat exchanger, a fan to pull air through the radiator at low speeds, a water pump to keep the coolant moving, and a thermostat to regulate the operating temperature of the engine for good performance, fuel economy and emissions. The coolant must also have the right mix of water and antifreeze to provide adequate freezing and boiling protection, and the proper amount of corrosion inhibitors to protect against rust, oxidation and electrolysis. To keep the cooling system in good operating condition, it is important to check the level, strength and condition of the coolant on a regular basis - and to replace or recycle the coolant before the protective additives are entirely depleted.
According to the U.S. Department of Transportation, cooling system failure is the leading cause of mechanical breakdowns on the highway. And according to numerous aftermarket surveys that have been performed over the years, coolant neglect is one of the leading causes of cooling system breakdowns. Check The Level. One reason for checking the coolant level regularly is to detect leaks that can lead to overheating. The level should be checked at the coolant reservoir, not the radiator, because the radiator will siphon coolant from the reservoir when it is needed.
Most vehicles will lose a little coolant over time due to evaporation from the reservoir. But a significant loss of coolant in a relatively short period of time usually signals a leak, a radiator cap that is not holding pressure or a cooling system that is running too hot. Visually inspect the radiator, water pump, hoses, freeze plugs, etc. for external leaks, and then pressure test the radiator and cap to find out where the coolant is going. A tight system should hold the maximum rated pressure for at least two minutes with no drop in the gauge reading.
If you do not see any visible leaks and the system holds pressure, make sure the cap is good and has the correct pressure rating for the application (somebody may have replaced it with the wrong cap). Still cannot find where the coolant is going? Check the automatic transmission dipstick. A leaky ATF oil cooler loop in the radiator may be allowing ATF fluid and coolant to intermingle.
If the system does not hold pressure, you have found an internal leak. Now you have to figure out where. Check the level and appearance of the oil on the dipstick for coolant contamination in the crankcase. A higher-than-normal oil level and/or a foamy appearance to the oil or droplets of coolant on the dipstick would tell you the engine has a leaky head gasket or cracked block. Coolant leaking into a combustion chamber past the head gasket or through a crack in the cylinder head will often foul the spark plug and contaminate the oxygen sensor. The silicate corrosion inhibitors in conventional antifreeze will poison the O2 sensor, so plan on replacing the sensor(s) if this has happened.
If no leaks are found, the loss of coolant may be due to long-term neglect or a temporary episode of overheating. Has your engine overheated recently? A defective cooling fan, slipping drive belt, exhaust restriction (plugged converter) or even overloading the engine may have caused the system to get too hot and boil over.