The BMW E46 is the fourth generation of the 3 Series. It is reliable if the maintenance on it is performed properly. One of the most important aspects of a car's engine is the cooling system. If the cooling system fails, damage can be caused to the engine. The coolant expansion tank on an E46 is the tank that the coolant is routed to when it becomes hot during driving conditions.
Lift the front of the BMW with a floor jack and support it on jack stands. Alternatively you can drive the front of the car up onto ramps to access the underside of the car. Disconnect the coolant sensor that goes to the bottom of the expansion tank. Disconnect the metal clip at the bottom of the expansion tank that holds the tank to the engine bay.
Place a container under the engine that is large enough to catch the coolant that comes out of the cooling system. On manual transmission cars, unscrew the blue plug on the bottom of the tank and let the coolant drain out into the container. On E46s with automatic transmissions, the transmission cooler must be disconnected from the radiator. Pull the clip up and out that is located where the trans cooler hose goes to the radiator. The trans cooler should hang free, allowing it to drain into the container.
From the engine bay, disconnect the two large hoses that go from the top of the expansion tank to the radiator by pulling the clips on the hoses off with a pair of needle nose pliers. Pull the smaller hose that goes into the lower side of the expansion tank off as well. This can be pulled off, but will take some effort. Pull the expansion tank straight up and out. The tank is held in very firmly. You may need to pry the tank up with a screwdriver while pulling on it.
Install the new expansion tank by pushing it firmly down and into place until it is seated at the proper level. Reconnect the upper hoses and install the clips that hold them in place. Reconnect the smaller hose on the side of the tank. Refill the cooling system with coolant and bleed the system of air.
