by Jim Wheeler
Bimmer Beat, December 1998
BMW CCA Bayou Chapter
Last month, we wrote about the continuing problems we see due to water pump failures on M50 and M52 engines since 1992 in the 325s and 528s. BMW uses a plastic fan blade/impeller pressed onto a metal shaft which is turned by the pulley and fan belt. Over time, the plastic cracks and the pump impeller freewheels and/or comes off the shaft. Your engine will overheat, and if you don't attend to the problem, you will require expensive repair.
One of the classic symptoms of a bad impeller is that your engine will overheat on the highway, but no necessarily at idle. Eventually another problem may occur...the impeller may break up and its piecescan (and usually will) lodge in the worst possible places inside your engine's coolant passageways. The remedy requires removal of the cylinder head and a careful back flush and inspection of those passages to remove any pieces of the impeller as can be found. The head gasket set is usually about $300, while the labor will vary depending upon how badly disintegrated the impeller is and whether or not the engine has been cooked too much. We have seen some cases that required a complete engine replacement.
There is another cooling problem we see regularly on the same engines...namely a broken thermostat which manifests itself by requiring an extended warm-up time. Most of the time, the engine never reaches proper operating temperature. Upon removal from the engine, we usually find the straps that hold the thermostat together have separated from the body of the thermostat and the thermostat is always open.
This condition can cause plug fouling due to the fact that the coolant temperature sensor senses the coolant as being in a start-up mode (cold) and richens the fuel/air mixture. Eventually, damage to and replacement of your spark plugs and oxygen sensor will be inevitable.
Now for the worst news...BMW continues to install plastic impeller water pumps in their new cars, despite a high failure rate. So if you have a 318, 325, 328, 525 or 528 built since 1992, your water pump may be faulty. Curiously, all of the replacement water pumps which BMW sells are made with metal impellers which tend to be very durable.
So what should you do? Have your car checked if you experience any cooling system problems, whether overheating or running too cool. As a rule of thumb, we recommend that a water pump be replaced if it is an original with over 35,000 miles. We would rather be safe than sorry.