BMW: 1998 BMW Z3 1.9 Overheating, style impeller, bmw z3

Question
I have a 1998 BMW Z3 1.9 that overheats. The first symptom occured while at idle in a futal attempt to charge the battery. The car overheated and the radiator cracked open so I repalced it, the water pump, the thermostat and housing, the thermostat temp switch that controls the fan and also tested the relays for both high and low fan speeds by jumpering out the plug at the thermostat. With all this new stuff and relay checks the car still overheats. One thing I noticed is that the heater core for the inside temp does not seem to get very warm even on max heat. I vented the radiator and reved the engine as indicated in the manual with no change. Could this be a system airbound problem? Also the thermosat that controls the fan speed does not seem to reach adequate temperature to control the cooling fan. I also tested the switch in hot and boiling water OK with an ohm meter. Any other suggestions of where to go from here would be appreciated? Thanks!

Answer
Sounds like the water pump to me. It is very easy to pull. Some of the early production water pumps had a plastic style impeller and they will come apart. The replacement water pump will be steel (from B.M.W.) I have seen some aftermarket ones that still have the plastic style impeller, so be careful what you buy. It is rare that you will get an air pocket, but it does happen. When it does I will let the vehicle sit over night with the radiator cap and bleed screw out and it will "burp" itself. If the thermostat was stuck open that would be the only way you would not get good heat from you heater core, and if that was the case the engine would almost never get to normal temp on you gauge. The hardest part of getting the water pump off it the fan. You will need a 32mm wrench and something to hold the fan pulley in place,B.M.W. uses a special tool for this. The threads are reverse threads, so don't use the old "lefty loosey" method or you will be there all day. Let me know how it goes for you. Hope this helps.