Be sure to use the Mercedes-Benz coolant MB 325.1, as it is only slightly more expensive, but it is worth it. Dilute 50/50 with distilled water. Don’t use tap water if you can avoid it as water has dissolved ions, and hard water can cause precipitates with corrosion inhibitors such as phosphates. These are commonly used in many antifreeze products, causing deposits and decreased flow and heat transfer. Mercedes coolant is formulated without phosphates so hardness is less of a problem. Deionized water has sodium in it, while using distilled its preferable because it has negligible ion content. Antifreeze corrosion inhibitors packages are not all alike and considering the cost of a radiator or engine it is cheap insurance to use the OEM formulation. Mercedes uses silicates and is formulated to protect the metals in the system, particularly aluminum. Additionally, brazes, plastics, and gaskets are also a consideration. If you mix two types of coolants equally, even if the second is a universal coolant, you just halved each corrosion inhibitor concentration causing neither to be adequate to protect the system. Universal coolants can be mixed with any type of coolant without immediate damage to the cooling system, but don't ignore the fact that you are reducing the long term protective benefits of both coolants if they are formulated differently, inviting long term damaging effects.
For the most part, vehicles 2002 and later specify a cooling flush every 15 years or 150,000 miles. Mercedes 2001 and earlier are every 3 years. There are exceptions in the transition years. Later production vehicles with 15/150 service interval have a silica gel packet in the coolant reservoir that constantly replenishes the silicate concentration in the Mercedes Benz coolant helping to promote the long life. If you top off with another type of coolant such as OAT (organic acid) or phosphate based you are contaminating the factory coolant system. Ignore color when trying to differentiate coolants, as these are added dyes and don't indicate anything specific about the components. Coolants compatible with MB coolant would be typically labeled G-05. Consider how unfortunate it would be to have to replace the aluminum heater core if it started leaking. Although uncommon, when it does occur it is usually the result of poor maintenance or ignoring the use of the correct coolants. The repair is costly and involves complete removal of the instrument panel to the firewall. The silicate formulation in the MB coolant offers very quick protection of the water pump from cavitations’ damage, while OAT coolants are much slower to protect. When cavitations occur at high temperatures under load the fluid boils in low pressure areas generated by the pump blades. When these bubbles collapse under pressure they cause tiny shock waves that blow the oxide coating off the metal. The metal can then erode and corrode since it is unprotected, causing the silicates to quickly form a coating that protects the metal and heals quickly if damaged by cavitations. The benefits of using the factory coolant make a long list; it is worth the small added cost. Regardless of the vehicle brand, it is always best to use the OE specified coolant and follow the manufacturer guidelines for dilution.
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