Fuel Cooling System in Diesel Engines

Fuel Cooling System in Diesel Engines
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The cooling system of a diesel engine must be capable of continuously removing approximately 30% of the heat generated by the combustion of its fuel without overheating. Assuming that you have a reasonably clean cooling system, this is usually not a problem. 

In addition to the refrigerant level, there are several points that should be reviewed, let’s see …

Refrigerant level

The level of the refrigerant is critical for the proper functioning of a cooling system. If the coolant level drops to a point where air is drawn into the cooling jackets, the cooling capacity will be reduced, resulting in serious mechanical damage, including corrosion of the cavitation. Always check the coolant level.

Composition of the Coolant

Engine coolant is usually a mixture of ethylene or antifreeze with propylene glycol base and water. The freezing point of the mixture will depend on the relative amount of glycol used. It is important to use the best available water mixed with no more than 60% antifreeze based on ethylene glycol, or no more than 50% antifreeze based on propylene glycol.

Diesel Engines

It is equally important never to use water exclusively as engine coolant. (Water is corrosive to engine operating temperatures). The quality of water is important. The following table shows the recommended minimum acceptable characteristics for cooling system water, according to a manufacturer (Caterpillar, Inc.) …

Minimum Water Quality for Use as Refrigerant



ASTM Tests

Chloride (Cl) g / gal (ppm)

2.4 (40) max

D512B, D512d, D4327

Sulfate (SO4) gr / gal (ppm)

5.9 (100) max

D516b, D516d, D4327

Total Hardness, gr / gal (ppm)

10 (170) max


Total solids, gr / gal (ppm)

20 (340) max



5.5 – 9.0


Recommended Additives and Inhibitors

Engine manufacturers will usually provide you with detailed guides about the additives needed to prevent corrosion or provide supplementary lubrication to the components of the cooling system. It is very important not to exceed the recommended concentrations of these additives. High concentrations can cause precipitation of solids, and can cause damage from seals and other internal parts. On the other hand, the low concentration of cooling additives can cause perforations in the cavitation of the coating of the cylinders that is in contact with the water.

Occasionally, engine manufacturers will provide special filters that distribute additives in the refrigerant (controlled discharge filters). If an engine is equipped with these filters, then it is very important NOT to add additional additives separately to the refrigerant. On the contrary, if the chemistry is controlled by specific additives then NO controlled discharge filters should be used.

Integrity of the Cooling System

No revision of the cooling system will be complete unless it is fairly clear that the system is sealed and free of air. The induction of air in a cooling system for any reason is a serious matter, since it can cause internal cavitation and corrosion stains on water jackets, especially in higher temperature parts, such as cylinder liners.

As you know, the fuel in a Common Rail system reaches high temperatures in the return of the high pressure pump and the injectors, due to the high operating pressures. To alleviate the negative consequences that the diesel oil had when arriving at the fuel tank, a gasoil cooler is inserted in the return pipe. In this way we guarantee the useful life of the tank as well as the pressure increase in it with the high temperature. There are several cooling systems. The simplest ones incorporate a radiator (air-gasoil exchanger) in the return circuit, located in different places according to the manufacturer.

The fuel cooler is in cars a fin radiator located on the floor of the vehicle, and in the Sprinter, a cooling coil located behind the tank.

Radiator fins on the vehicle floor Cooling coil behind the tank. In the first generation of CDI engines, that is, in OM 611.960, the cooling of the fuel was not carried out by the air current caused by the vehicle’s running, but by a heat exchanger by means of the cooling agent. In the CDI engine of the Class A, the OM 668, does not require a fuel cooler in the return, since it has a steel tank. For Opel Omega O25DT engine, and BMW M57 3.0L, it is located in the underside of the vehicle.

Particular care should be given to engines that have hoses with silicone gaskets because the hoses will not adhere to the engine connections. Only constant tension spring clamps should be used to hold these hoses and their integrity should be checked routinely. If you have any doubts about the integrity of the system you should consult the engine manufacturer, and additional tests should be performed until such doubt is resolved.


The lubrication of the motor is perhaps the single most important element of a good maintenance program. Engine oil lubricates moving parts; provides protection against corrosion; absorbs and neutralizes contaminants; it serves as a refrigerant; and it’s a sealer. Through regular oil and filter changes, the oil removes foreign matter from the engine, while contributing to internal cleaning and minimizing wear.

Engine lubrication oils are prepared with petroleum or synthetic bases, and are formulated with different additives that provide or modify certain characteristics of the base ingredient. Among these are detergents, alkalinity agents, oxidation inhibitors, dispersants, and anti-wear agents. These additives, or the additive package, is what gives the desirable qualities to the engine oil, and the depletion of the additives, as well as the accumulation of products of combustion, are the elements that limit the life of the oil load.

Re-refined oils are acceptable as long as they comply with SAE viscosity and API specifications for new oils.

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