Scale Formation in Diesel Engine Cooling Systems
by David Doyle, CLS, OMA I, OMA II
General Manager, Tribology

A heavy-duty diesel engine transfers only about one-third of the heat it generates into mechanical energy. This results in a high amount of heat generated that needs to be dissipated from an operating engine. Excess heat generated that does not dissipate through exhaust or to the surrounding atmosphere requires a healthy cooling system to prevent engine failure. Proper maintenance of a cooling system in a heavy-duty diesel engine is critical. One aspect of this is the prevention and control of scale formation in a cooling system.

Normal tap water can naturally contain minerals, primarily calcium and magnesium, which cause scale to develop. Water that contains over 100 ppm of these minerals is considered hard water. Scale is formed in a cooling system when minerals present precipitate after an engine at operating temperatures is shut down.

Scale takes place around engine hot spots, such as liners and the cylinder heads. Engine heating and cooling allows minerals to precipitate from cooling water, leading to the formation scale on cooling system heat exchange surfaces. Scale becomes a major insulator, not allowing heat to dissipate as easily from the engine, thus amplifying the temperature of the engine hotspots. Scale build-up results in excessive heat retention in the operating engine and engine oil. There is a greater tendency for scale formation to occur when air bubbles around a hot source develop and in areas of low flow. Scale formation in these engine hot spots can also potentially accelerate surface corrosion.

Excessive scale formation can also lead to engine failure. Abnormal operating temperatures caused by scale build-up can lead to cracked heads and warped engine blocks. Damage to water pump seals and water pump bearings can occur when scale build up occurs. Phosphates used in some conventional coolants will combine with minerals to create calcium phosphate and magnesium phosphate scale, which is especially harmful to water pump seal surfaces. Scale build up can cause thermostats to start sticking, which can prevent cooling systems from regulating properly.

Controlling scale formation helps to optimize a cooling system’s performance by helping the cooling system heat exchange surfaces remain clean. Polymeric additives in coolants inhibit the deposit of minerals by coating these particles so they cannot attach themselves to engine surfaces. Other additives protect by coating engine heat exchange surfaces which inhibits scale buildup.

Prevention by using a suitable water source for engine cooling systems is the easiest management approach to this problem. Industry standards and OEMs provide quality requirements for the water used to blend for use in cooling systems. Deionized water is recommended, which has gone through a process to remove the naturally occurring minerals that contribute to scale formation. While deionized water is recommended, most repair facilities do not have a DI water station, and is not typically available at retail, unlike distilled water, which is an option to use. It is recommended that if source water is a potential issue, then pre-mixed 50/50 should be used and not concentrate,

Flushing products are available to remove scale buildup, depending on the severity of the problem. Some flushing products use an acidic chelating cleaner which binds with metal scale material and remove deposited scale during flushing. Other flushing products are non-acidic which work to dissolve scale particles.

It is recommended that the diesel engine coolant is tested when engine oil samples are taken for testing. Testing engine coolant for total hardness provides an indication of scale deposit formation. Total hardness test results provides calculated quantity of calcium carbonate and magnesium carbonate in parts per million (ppm).

As the leader in in-service coolant, oil, and fuels analysis, ALS Tribology has affordable and extensive engine coolant testing services that can meet your maintenance and equipment reliability requirements. Please feel free to contact any of our laboratories within our global network for help in complementing your asset reliability practices, or contact us through