According to my OBD2 reader I found my '08 528i wants to run around 210 degrees, to me, that's crazy. On my engine there's one sensor that reports to the EMU, in turn the EMU uses that info to control the thermostat and fan to maintain the coolant temp. My thought was to find a sensor that reads hotter, What I needed was a sensor that would read the same at 195 degrees as the stock one reads at 210. From Advance Auto parts the sensor was $15. I ordered 6 which got me free shipping. I then installed a thermo couple on the hose right next to the sensor and started the car letting in warm to 210 in the garage. I graphed the thermo couple reading along with the actual temp as reported by my OBD2. I then removed the stock sensor and placed it in a holder I made that held all 7 sensors (1 stock and 6 new) in a pan of water. I heated the water and recorded the resistance of all the sensors and 5 degree increments to 210 degrees. Using this method I found all but one of the after market sensors read hotter then the stock. I installed one of the sensors that read 196 degrees when the sensor was reading 210. I then double checked the reading by restarting the car and comparing the thermo couple readings to the OBD2 and sure enough, the car now runs 14 degrees cooler. When I replaced the sensor I raised the front of the car and only lost a couple of drops of coolant.

I also bought a 90's 1.4 bar (around 20 PSI) coolant cap, disassembled it, and used the pieces to make my cap a 1.4 bar instead of the 2.0 bar (around 31 PSI). The caps are different, but guts are the same.

I later replaced the coolant with this mixture - 55% water, 40% BMW antifreeze, 5% Water Wetter.

I have since made a long road trip over the last couple weeks, from Washington State to Texas, Florida, and back to Washington state. Alot of the trip the temp in the hundreds, a couple hours in New Mexico were 115 degrees. No problems what so ever and I got 33mpg on the highway going 5+ over the posted limit.

 

So you tricked it into thinking it was hotter than it actually was so it would start cooling earlier? I wonder could that have detrimental effects on anything?

 

Well I've gone 7000 miles so far, runs perfect. I thought my gas mileage might go down but when I compare to last years vacation, it actually seems better. The only other thing I've changed in the last year is going from Mobil 0w-40 to Redline 0W-40.

 

Modern cars are meant to run really hot. They get better emissions and more MPG when running hot.

I was reading about the 4 cylinder diesel BMW that varies its operating temperature according to how it's being driven. Max efficiency mode was something like 240F / 115C !!!

 

Yep depending on the mode the temps will be different. That's why there is a electronic thermostat.

Cooler for performance and hotter for commuting.

 

I have a 2011 335 as a rental while my 545 has the tranny replaced and this 335 runs at 240 cruising and will run at 260 with spirited driving (according to the gauge). I'm sure it's meant to be this way but I've never seen a car run so hot!

 

What temp would you prefer your car to run at and why?

 

I would prefer 190-195. That's hot enough to get the moisture out of the oil and cool enough to allow the use of the 1.4 bar cap.

 

Not so sure about the MPG part. The cooler the engine is running, the more timing the EMU can dial before knock. I believe it's mostly emissions.

 

Modern engines are designed to run this hot, this helps with the emmisions, I bet if you had the exhaust emmisions checked before and after your "fix" the emisions would be worse with the engine ruuning colder, when it thinks it's hot. At the risk of sounding like a smart arse, what makes you think you know better than all the engineers at BMW, who designed the engine to run like this?