A Guide to Coolants and System Maintenance

July 26, 2016 - 9:00 AM by Samantha Pudney


Diesel engine specialists say the most frequent source of premature engine failure is the cooling system. Improper coolants and inadequate cooling system maintenance can result not only in overheating, but also in corrosion damage. Here’s what you need to know to keep your diesel engine cool and corrosion-free.

Coolants normally consist of a mix of water, antifreeze, and a conditioner or inhibitor. As a mixture, they remove heat, prevent freezing, inhibit corrosion and scale buildup, prevent cavitation and lubricate components like water pumps.

A 50-50 mixture of ethylene glycol antifreeze and water provides good heat transfer and freeze protection. The mix will stay in fluid-form to minus 36⁰C and will prevent freeze damage to a somewhat lower temperature.

Straight antifreeze should never be used as a coolant because it doesn’t transfer heat as well as a mixture, nor does it protect against freezing as well. Minor overheating sometimes can be remedied by diluting the coolant with more water, or by switching to an antifreeze with different heat transfer properties.

Using deionised or distilled water is best. Water with high oxygen and chlorine content, or other mineral content can contribute to corrosion and scale buildup.

However, you can’t rely on pure water and antifreeze alone to protect your cooling system. It’s a good idea to use a coolant conditioner, also known as inhibitor or supplemental coolant additive, formulated to protect surfaces from both galvanic corrosion and cavitation erosion, and prevent scale buildup.

Diesel cooling systems are an ideal breeding place for galvanic corrosion. Vibration inside the block causes tiny pockets of reduced pressure, where heated coolant vaporizes, forming bubbles of steam. As these bubbles collapse against internal surfaces they physically erode the metal. This “cavitation erosion” can eat pinholes right through liners and cooling jacket walls, allowing coolant into the cylinder.

Scale, a hard coating of mineral solids and other impurities in the water, may clog passages and diminish the transfer of heat through the water jacket walls into the coolant. Suspended solids also grind away seals, causing water pumps to leak.

Conditioners become depleted with use, and coolant should be tested twice a year. You can get test strip kits to check coolant yourself for conditioner strength, combustion gas presence, and electrical current; or take a sample of coolant to a lab for a report on dissolved solids, acidity, glycol content and freezing point. As long as it is reasonably clean and the conditioner potency is maintained, antifreeze it is good for several years. Follow your engine manufacturer’s recommendation.


System Maintenance

Even with the right coolant, cooling systems require some maintenance. When the antifreeze is drained, the system should be flushed with a radiator-cleaning compound or a de-solvent consisting of one part muriatic acid to two parts water. Reverse-flush the system afterwards using air pressure to blow hot water backwards through the plumbing.

If you have a heat exchanger, regularly check the zinc plugs for corrosion by banging them with a hard object or scrub with a wire brush. If they are more than half diminished in size, replace them. If there is a coolant problem, disassemble the heat exchanger, removing the core, and inspect the tubes. Minor deposits can be pushed out with a rod of soft material. Don’t use a hard rod that could damage the thin copper tubes. If the deposits can’t be pushed out, take the core to a radiator shop for a chemical bath.

If coolant level in the expansion tank drops, check the “weep hole” at the bottom of the freshwater circulating pump. If it’s weeping it may be an indication that the water pump seal is going. Replace it to avoid coolant loss, overheating and engine failure. However, ceramic seal types are designed to weep a little.

Coolant can also escape through faulty seals elsewhere in the cooling system, including the cylinder head water seal rings or the head gasket, and through leaks in oil coolers, fuel coolers, gear oil coolers, cabin heaters, faulty pressure caps and pinholes caused by corrosion. If one gasket is leaking the others will soon follow so replace them all. Don’t use antifreeze with methyl alcohol as it can dissolve seals and hoses. You can use a pressure tester to detect leaks in the system. Inspect hoses and hose clamps at least once a year and replace them where necessary.