It is recommended to change the timing belt every 60,000 or 90,000 miles for that car (my book wasn't clear which mileage interval should be used). If the timing belt is original on your car, now would be a good time to replace it, while it's in service for the water pump. I'm not familiar with that engine but there may be some significant labor overlap between the two parts.

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95 E320 Cabriolet, 159K