I have been watching this post with interest.
I have been searching online and have come up with this piece of information:
Thermostats have a "rated" temperature such as 180F or 195F. This is the temperature the thermostat WILL START TO OPEN, give or take 3 degrees.
Usually located within a metal or plastic housing where the upper radiator hose connects to the engine, most of today's thermostats utilize the "reverse poppet" design, which opens against the flow of the coolant. Thermostats have a wax filled copper housing or cup called a "heat motor" that pushes the thermostat open against spring pressure.
As the engine's coolant warms up, the increase in heat causes the wax to melt and expand. The wax pushes against a piston inside a rubber boot. This forces the piston outward to open the thermostat. Within 3 or 4 degrees F. of the thermostat preset/rated temperature which is usually marked on the thermostat, the thermostat begins to unseat so coolant can start to circulate between the engine and radiator. It continues to open until engine cooling requirements are satisfied. IT IS FULLY OPEN ABOUT 15-20 DEGREES ABOVE ITS RATED TEMPERATURE. If the temperature of the circulating coolant begins to drop, the wax element contracts, allowing spring tension to close the thermostat, thus decreasing coolant flow through the radiator.
My thoughts are, (and correct me if I am wrong).
The spring tension must play a part in how quickly the thermostat opens. If the spring was replaced with one with less tension, then the expansion of the wax against the lesser tension spring must open the thermostat more quickly. This would then allow the coolant to flow sooner through the radiator. As the engine cools, the lesser tensioned spring would be slower to close the thermostat which should keep the flow through the radiator longer. Therefore keeping the engine cooler.
If that is the case perhaps the spring on a 95 degree thermostat (of any make) is preset to the tension we need for the E65/E66 engine.
Maybe it is as simple as finding a 95 degree thermostat and taking it apart and using the spring on the BMW thermostat?
Last edited by CJ745NZ; 01-17-2013 at 03:11 AM.