Hello, Bimmerphiles! If you are having trouble sleeping because you are worrying about how the thermostat in your Bimmer works, read on.

Around the model-year 2000, BMW began using electrically-heated engine-cooling thermostats, called MAP-controlled thermostats. “But wait!” you say, “Why would they want to heat the dang thermostat with an electric heater, wouldn’t that tend to confuse it?” Precisely, dear Alphonse, precisely.

Water-cooled motor vehicle engines have had thermostats for many, many years. Indeed, I cannot recall ever working on one that had not originally been equipped with a thermostat; and my experience goes back, let’s just say, “a ways.” We used to remove thermostats in an effort to keep hot rod engines from overheating, but that is a story for another Philes’ Forum.

The function of the thermostat is pretty straightforward: It controls the minimum coolant temperature of the engine. The thermostat does this by remaining closed, and not permitting coolant to circulate through the radiator, until a certain coolant temperature is achieved. While the thermostat is closed and the engine is running, coolant does circulate through the block and cylinder head(s), and coolant is available for heating the passenger compartment. Typical opening temperatures for the older style, non-electrically heated thermostats are in the 80 – 90 °C range (180 – 195 °F). American cars of the pre-emissions era were typically fitted with 180 °F thermostats while “smoggers” ran 195 °F stats.

The mechanism that causes a conventional thermostat to open is usually a wax pellet that expands as it is heated by the coolant, opposing the thermostat’s spring. When the pellet is sufficiently heated, it overcomes the spring force and the thermostat opens. As coolant temperature continues to increase, say as the engine is placed under load, or as ambient temperature increases, the thermostat will open further until it is fully open. In a properly sized and maintained cooling system, the thermostat should never have to open fully.

Running an engine at higher coolant temperature provides benefits in the areas of emissions and fuel economy. This is because the “quenching” effect of the relatively cool (as compared to the combustion gasses) cylinder head and block surfaces is reduced, so less heat is lost to the coolant – hence, more is converted to mechanical energy.

Pretty simple if you think about it, especially if you have a framed copy of the First Law of Thermodynamics on your fireplace mantle.

The downside of running an engine at higher temperature is that there can be a reduction in maximum power, and there is an increased tendency of detonation (“spark knock” or pinging). The engine in my Ford hot rod has a 160 °F thermostat. You would not want to drive that baby in the cold weather, due to lack of cockpit heat and carburetor icing, and emissions are, thankfully, not a consideration on a historic vehicle. But man, is she FAST! So, in an ideal world, an engine would be run at higher temperature until increased power is needed or until excessive detonation is detected, at which time the coolant temperature would be reduced to an increased power/detonation-suppressing level. And that, in a nutshell, is exactly why there came to be electrically-heated thermostats!

Photo #1 depicts a typical electrically-heated thermostat assembly, this one from an E46. You can see what looks like a conventional mechanical thermostat sticking out the engine side of the thermostat. And that is exactly what it is, a conventional mechanical thermostat. The only difference is that this thermostat has an opening temperature about of 97 °C (207 °F). You can also see the connector for the heater wires on the radiator side of the thermostat.

Things get more interesting when the thermostat assembly is disassembled. See Photo #2.

On the right is the familiar good ol’ thermostat, but what is that on the left of the photo? That is the heating element that fits inside the center of the thermostat.

How cool is that? The heating element is connected to the engine control computer (DME in BMWspeak). Anybody know what DME stands for?

During “normal” engine operation, the DME keeps the thermostat heater pretty much turned off, and the thermostat controls coolant temperature at about 97 °C in exactly the same manner as thermostats have been doing since antediluvian times (well, maybe not THAT long). When the DME, which is monitoring coolant temperature, radiator-return temperature, intake-air temperature, engine load, gas-pedal position and detonation along with a bunch of other things, sees a benefit to running the engine at a reduced coolant temperature, it turns on the heating element inside the thermostat, thereby artificially heating it and “fooling” it into controlling at a lower temperature. Pretty good idea if you ask me.

Now if I could only figure out how to install one of these trick thermostats into the ol’ hot rod, I could probably drive her in the winter!

Copyright 2020; V.M. Lucariello, P.E.

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