BMW cooling systems have been known for being troublesome for quite some time. One of the principle areas of failure are the thermostat and water pump. Some of the old style water pumps had a plastic impeller that has a tendency to become brittle and break off after many years of service. BMW has replaced the plastic impeller in recent years with a metal one. If you know that you have a plastic impeller in your car, then you should replace it as soon as possible. Overheating problems are common on these cars, and if your engine overheats, you may find yourself having to replace your head gasket, which is not cheap.
Begin the process by gaining access to your water pump. This requires the removal of the fan and belts, as well as the removal of all coolant from the system. See the following technical articles for instructions on how to get to this point:
- Removal of all coolant from the system
- Removal of the accessory drive belts
- Removal of the radiator fan
With all of your equipment removed, the front of your engine should resemble Figure 1. Remove the fan pulley from the water pump - it is held on with four small nuts. Your water pump should now resemble Figure 2. Loosen the four nuts that hold the water pump to the engine block. Then, using a rubber mallet, softly tap on the side of the water pump (Figure 3). It should separate from the block after a few taps with the rubber mallet (Figure 4). Once the water pump is loose, you should be able to pull it out of the engine block.
Figure 5 shows a brand new water pump with the metal impeller. Figure 6, Figure 7, and Figure 8 show the differences between the plastic one and the metal one. There is no functional difference - they will both pump the same amount of coolant through the system.
With the pump removed, check the inside bore where the water pump fits for debris or corrosion (Figure 9). With a wire brush, remove any corrosion or debris that may have built up there (Figure 10). Install the new water pump using a new o-ring. Place a little bit of white lithium grease around the o-ring to ensure that you have a good seal, and to ease installation of the pump into the engine block (Figure 11). Install the new pump and tighten down the four bolts that attach it to the block. Torque them to 8 ft-lbs - no tighter.
Figure 12 shows your new water pump installed. Reattach the fan pulley, the fan, and all of your belts. Refill and bleed the system, and you should be good to go! While replacing your water pump, it's also a good time to replace your thermostat, while you're in there.
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