I have owned around 15 cars over the years and have generally replaced the water pump on an average, about every 60K miles. My 1977 Toyota Celica (20R engine) needed two: one at 80K and another at 165K.

I have replaced a number of water pumps in friends' cars, but always with RWD old-stlye engines, which is an easier chore.

I have not replaced the water pump in either of my Mercedes. Perhaps I should not publically announce this, lest Murphy hear me.

The cause is always the same: the seal leaks where the shaft goes through the body of the pump. I have yet to see a water pump with ball bearings.

I have found that one can of anti-freeze restorer/water pump lubricant (Solder Seal brand in the yellow can, at under $2.50) seems to prolong the water pump and prevents any rust in the coolant.

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Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)