by Jim Wheeler
Bimmer Beat, November 1998
BMW CCA Bayou Chapter
We continue to see problems due to water pump failures on M50 and M52 engines. These engines were first introduced in 1992 in the 325s and 525s. Early engines have a plastic fan blade impeller pressed on to a metal shaft that is turned by the pulley and fan belt. The impeller pumps coolant throughout your engine and it is essential that it operates at peak efficiency.
Over time, the plastic cracks and the pump impeller freewheels and/or comes off the shaft. The result is a temperature gauge reading in the red...HOT! If this happens to you...turn off your engine and call a tow truck unless you're in danger. Otherwise, at a minimum, you will either blow a head gasket and/or warp your cylinder head. Very expensive.
Older water pumps tend to always leak when they've failed; however, these pumps don't leak very often. Unless the technician knows this, he will usually recommend a new radiator, only to find the same overheating problem still exists after the new radiator is installed. The solution is to replace the water pump with a new one with a metal impeller. Although the part number is the same for early plastic impeller pumps and the late ones (11 51 1 433 828), the newer ones have a metal impeller and are more reliable. Insist on a metal impeller if you need a pump. BMWs suggested retail price for a new pump is $84.67...small change considering a new engine is about $6000 uninstalled.