That is one of the reasons (apart from just regular running costs!) that puts me off the S4's. You can get them for a bargain now, but when they go wrong, they really go wrong! Secondary water pumps are another pain, they are located just above the "V" so get subjected to massive heat, and over time become brittle. Thats a good £500 job in itself IIRC. However they are amazing cars for their money now.