I happened onto this article just searching the net. Very interesting info, especially about the water pump and why it should be replaced. Seems he runs a repair shop.
The BMW M50 engine series is a very durable powerplant. The frequency of actual failure is very low considering the number of these cars that we service. Even with repeated bouts of overheating, they seem to survive well. The least common cooling system failure is the head gasket, or at least leakage in the head-to-block interface. In this article, Iíll cover the diagnosis, repair and prevention of these failures, but it will be up to you to convince your customers that regular maintenance and inspection is the only way to prevent expensive failures.
The events leading up to a head gasket failure can almost be plotted on a graph of typical events. The customer complains of overheating, but the coolant level doesnít drop. The fan, belt and hoses all appear OK, and thermostat function tests fine with a non-contact temp probe. The engine temperature spikes at times, but then is normal. The engine does or doesnít overheat at idle, but the temperature climbs quickly at higher engine speeds.
On most M50 engines, the water pump impeller installed at the factory is plastic. For whatever reason, the impeller separates from the shaft or disintegrates and the water pump no longer pumps. In the first case, with the impeller loose, especially with the engine at operating temperature, diagnosis can be especially difficult. Just when you think there is no flow, some coolant starts to move and everything tests OK, temporarily.
Another problem along this same line is that the auxiliary coolant pump for the heater system can circulate enough coolant to provide cooling at idle, but not at higher engine speeds.
Then the problem increases in frequency and intensity, coolant is lost, a leak starts at the plastic thermostat housing or, in a worst case scenario, the upper radiator hose blows off, due to failure of the plastic outlet neck (sometimes caused by over-tightening). Then, a series of overheating events culminate with a severe loss of coolant, a misfire and a customer with a very red face.
At this point you canít pressure-test the system, a block test may or may not have been conclusive and there are still the questions about the water pump, fan clutch, auxiliary fan and switch and a stuck thermostat. Itís usually only after all of these other possible overheat sources are eliminated that a head gasket failure is suspected or indicated.
So now I know why it was overheating. This guy just saved me some bucks.