The water pump is a crucial component of a Mercedes cooling system. Its job is to keep coolant circulating from the radiator through the block and head so that the motor is cooled properly. If the water pump fails or begins to leak badly, it can lead to an overheated motor, which in turn can cause the head to warp, or even an engine failure if the car is driven in this state. Replacing the water pump is relatively complicated, but the accomplished home mechanic can do it a day or so.
Make sure you have the proper water pump for your car. The best way to ensure this is by using the chassis number of the car when you buy the water pump from the parts distributor. A water pump for a C280 usually runs around $250. Before trying to loosen the water pump bolts, it is also a good idea to spray some type of lubricant on the bolts to make them easier to remove.
Jack the car up and place it securely on jack stands. Drain the radiator with the drain valve at the bottom of the radiator.
Open the hood and remove the fan shroud, fan belt, power steering pump and hoses that block the water pump bolts. Leave the power steering pump attached to the hoses and push it out of the way.
Remove the pulley on the power steering pump. Use a strap wrench to hold the pulley while you unbolt the pulley bolt with a socket and wrench. Remove the pulley on the water pump with the same method.
Undo the rigid pipe in the side of the water pump that goes to the front of the engine. Undo this at the front of the motor and leave on the pump, or remove at both ends and set aside.
Remove the plastic leak tube, if one is present, on the bottom of the pump.
Remove the four 6-mm Allen head bolts that hold the water pump on. To get to the front ones, use a ball-head attachment for the Allen wrench that allows the end to pivot around. Be very careful not to strip the bolts when removing them.
Install a new pump with a new O-ring. Reinstall everything in the reverse order of removal.
Place the car on level ground and fill the radiator with the motor running.
