Case Studies

BMW Electric Water Pump Failure – How, Why, and What To Prevent


This BMW 540 has was diagnosed with a defective engine coolant pump. Symptoms were overheating and damaged drive belt system. Disassembly of the front cover and removal of the water pump allowed for inspection of the pump.

The mode of failure with this pump appears to be simple age and miles, likely exasperated by a lack of coolant changes. The bearing failed on the pump, allowing the plastic impeller to contact the aluminum housing, you can see the growing on the surface. Finally the plastic impeller itself failed, the vehicle overheated, and came to a stop. As the water pump impeller is made from a plastic material, the plastic becomes brittle with age and extended immersion in the hot engine coolant, sometimes simply cracks apart, even without a bearing failure. The defective pump, and pieces of the failed impeller, are on the left side of the photo, the new pump on the right.

On these water pumps, we suggest replacement at the 80,000 mile interval. Failure usually occurs very soon after the car hits this milage mark. There is a better pump available, we use one with a metal, rather than plastic impeller. When replacing any cooling system part for an overheating concern, it is very important to replace the engine thermostat at the same time, the thermostats are almost certainly damaged from the overheating, and it inexpensive insurance to do so. As you can see from the photos, it is a bit of a process to access the pump, so be sure to always replace the drive belt and tensioner at the same time.

Aside from protecting from freezing, antifreeze also contains lubricants for the water pump bearing, as well as corrosion inhibitors, unfortunately both of these fade with time, and must be renewed with a coolant flush. As this car is an aluminum alloy engine, it is always a good policy to flush the cooling system at least every three years, and to use a quality antifreeze with corrosion inhibiters.

Not a simple repair, yes, but one, if done correctly, will last another 100,000 miles. That’s our goal with BMW, and other autos here, fix it right the first time, and prevent problems from happening in the first place. 30 years of service experience have well taught us that “an ounce of prevention is worth a pound of cure.” Be cured, once and for all, and give us a call, we’re happy to answer any service questions you might have on your BMW, Audi, Mercedes, Lexus, Volvo or Volkswagen.

Earning your trust, every time you turn the car…that’s what we do…every day…for the last 30 years. Click here see what our happy customers have to say about us and our service at AMC Customer Reviews.

If you have questions, or if we can be of further assistance, just call us at (207) 882-969, we’d love to meet you, and your car!

Bruce and the AMC Service Team