A coolant leak may develop from the water pump. When replacing the water pump, the drive belt tensioner should be inspected and most likely replaced as well.Google+
Car Problem Reports
Mercedes-Benz 300E Check Belt Tensioner When Replacing Water Pump
Mercedes-Benz 300E Problem
Average Mileage: 182,500 mi (130,000 mi - 235,000 mi)
Belt became loose & inspection revealed rubber bushing detached from tensioner. Full replacement fixed problem.
Visitor, 1990 Mercedes-Benz 300E, 130,000 mi
I've had to have the water pump replaced 3 times beginning at 130,000 miles every other year. After the water pump was replaced the tensioner had to be replaced.