A bit more info.
The thermostat is in the cold water return from the radiator not the hot water out to the radiator. The thermostat has a heater which is controlled by the engine DME. The engine temp is kept hotter under light load and speed conditions. This is for emission reduction purposes. The M52TU and M54 engines have completely different cooling system flows compared to the earlier conventional cooling system M52 engines. They have minimal water flow through the block. Most of the water flow is through the head and is closely regulated by the engine DME controlling the heater in the thermostat. Under high load high speed conditions most of the water flow is through the radiator. This necessitates the need for an auxilliary water pump to ensure adequate water flow through the heater.
See these pdfs for more info:
edit: added this pdf see pages 16/17 for cooling system details http://www.bmwtech.ru/pdf/e46/ST034/9%20Engines.pdf
Hope this helps
99 528iT M52TU
Last edited by Aussie528iT; 02-28-2013 at 04:11 AM.