Engine coolant is a mix of chemicals and water that performs a number of different functions.  First and foremost, the coolant serves to carry heat away from hot engine parts and transfer that heat to the heat exchanger element.  Other commonly known characteristics are: to provide an antifreeze capability (protects the cooling fluid from turning to a solid in ambient temperatures that are below freezing), to provide internal engine parts with protection from corrosion, to provide anti-boil protection in extreme heat, and to provide lubrication to moving cooling system components such as the water seal.

Another job of the coolant is to provide anti-cavitation protection in diesel engines.  Diesel engines create very high pressures very quickly during combustion.  At these high pressures the liner flexes outward and then returns to normal as the pressure recedes.  This outward/inward motion creates pressure fluctuations in the coolant at the liner wall.  When there is low pressure, a micro-bubble is formed, only to implode immediately when the pressure goes back up.  These implosions, occurring over and over, blast away at the surface of the liner in a process called cavitation erosion.  Over time, this can create “pin holes” all the way through the liner allowing coolant into the cylinder.  It is for this reason that a diesel engine should always be filled with a coolant that is specifically designed for use in heavy duty diesel engines.

Diesel engine coolant contains “SCAs” (Special Coolant Additives) that create a thin solid protective layer of film on the liners which absorbs the energy from the imploding micro-bubbles.  Each time a micro bubble implodes, it blasts away the protective SCA coating.  That missing bit of coating is then replaced by some of the unconverted liquid SCA still suspended in the coolant.  Due to the fact that there is a limited supply of SCA in the coolant, observing the recommended interval for coolant replacement is important.

Replacing the coolant serves two functions.  First, it is important to have enough unconverted SCA in the coolant to continue to protect the liners from cavitation erosion.  Second, the spent SCA suspended in the coolant acts as an abrasive on all the components of the cooling system, such as the water pump seal, hoses, etc.  Flushing and replacing the coolant removes the spent SCA particles as well as restores the unused SCA to required levels, thereby increasing the life of the cooling system components.

Another type of heavy duty, diesel approved, coolant is Extended Life Coolant.  Extended life coolants also provide protection against cavitation, but instead of using SCAs, these coolants employ Organic Acid Technology (OATs).  In extended life coolants, a smaller number of additives are used and they are used more efficiently.  It is because of this that they are effective for a longer period of time.  It is important to note that when using extended life coolant, it is best not to top off with conventional coolant, as this negates the extended life benefits.


Beyond performing the daily coolant level check, coolant maintenance is, for the most part, a matter of replacement at the recommended interval.  Some manufacturers utilize a coolant filter, with additional SCA tablets contained within, and recommend an annual replacement of the filter in between coolant flushes.  Other manufacturers do not use a filter and simply recommend a coolant flush at a certain interval.  Weather using extended life coolant, or conventional, the additive packages will eventually break down, and the coolant will need to be replaced.

It makes sense to perform the coolant replacement in conjunction with another cooling system service or repair, such as the heat exchanger service.  If the coolant is going to be changed, it is important to drain every location in order to remove as much old coolant as possible.  On a MAN 12 cylinder, an additional 5 gallons (or more) of coolant per engine can be removed beyond what comes out of the quick connect drain fitting on the J-pipe.  This does require more effort, because there are 6 different places, per engine, to drain the coolant, but it doesn't make any sense to replace some coolant while leaving 25 percent of the old coolant in the engine.



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