I have learned that one of the most important services on any water cooled engine is to drain and refill the coolant system with a quality anti freeze. With age, the lubrication qualities of the anti freeze are depleted, however, the properties that keep the cooling system from freezing haven't diminished much. Most people take that attitude that if it is still protecting the engine from freezing and the "color" looks good, then it is still good. I believe that changing anti freeze every two years is preventative maintenance for the engine and cooling system. I can honestly say that I can never remember replacing a water pump in any of my MB's and I usually drove them to 175 - 200K miles before retiring them. I learned early in my driving life that you can either pay for maintenance or you can pay for repairs. Usually, the repair costs far out weigh the cost of preventative maintenance. Dusty......