I recently changed out the water pump on my m104 engined 1993 300E. It looks like I get to have the joy of doing it again. It appears to be leaking at the water pump to block connection. This joint is sealed by an o-ring in the pump.

When installing I used a new o-ring, lubed it just bit, cleaned up the block, and tightened the water pump to torque specs. It stays dry when the engine is cold, but leaks under pressure.

Any thoughts on how to make sure this doesn't happen again? There was a bit of rust on the engine block around the water pump fitting, perhaps that was the problem? If so, how should the engine block be cleaned and prepped? I'm assuming the use of RTV style sealants is not a good solution. Frankly, I'm a bit baffled, as there's not much to go wrong here.

- JimY