Car_Almond

2007 530xi E60: I recently got the pesky P0597 "Service Engine Soon" OBDII code (Thermostat Heater Control Circuit Open) fault which means the heating element in the thermostat or the wiring to it is "open", as in broken electrical path.

Why does a heating element exist inside the thermostat? When adding heat, it makes the thermostat open sooner and more, cooling the engine down more. Are they just trying to take the lag out of response? Other cars don't do this.

Only explanation I could find was "By controlling the coolant flow or engine temperature electronically, part throttle increases fuel economy and reduces emissions, whereas reducing the temperature under load increases power."

Not sure that makes sense completely. I've read discussions on the internet where people seem to think having a busted thermostat heating element makes the car warm up slower, but we know that adding heat next to a thermostat opens it, not closes it down more.

twh

The thermostat is basically mechanical. It opens only when the coolant reaches a temperature that operates the mechanism in the thermostat. The heater circuit is so the car computer can cause it to open sooner than the coolant temperature if it wants.

Car_Almond

Gotcha yes.
Another forum gave me this: http://www.meeknet.co.uk/e38/E38_Map_Thermostat.pdf
.... which explains WHEN the thermostat is "fooled" into opening at cooler engine temperatures. It appears the small heater inside the thermostat tricks the thermostat into opening when the car is suddenly driven hard (heavy right foot) in anticipation of greater engine heat generation.

That sound right to everybody? I've never heard of this on any other vehicle. You'd think the thermostat would naturally react to greater engine heat by opening up fully, and that's enough. But noooooo, BMW engineers want to complicate our lives unnecessarily.

BimmerFan52

Yes, a great safety mechanism to protect your engine.

Let's say your 16 year old borrows your BMW (you may not have a 16 year old in your stable but I do). He has 330 lb-ft of Dingolfing torque at his finger tips.

Let's say the engine is stone cold and after 20 seconds of driving he decides to floor the accelerator and hold it. The coolant is no where near to opening the thermostat but he is cranking out everything the engine has and in a matter of 10 seconds the energy being delivered in the cylinders can super heat the block which has no water flowing through it (remember the thermostat is closed based upon coolant temperature).

After a second or two the ECU senses full throttle and sends 12V to the heater coil in the thermostat to force it to open, allowing water to flow and begin cooling the block.

I learned of this feature when i replaced my water pump and thermostat as preventive maintenance. After one week I got a check engine light and tracked it down to a burned out heater coil in the thermostat (measure the resistance across the contacts with the connector plug removed from the thermostat). I made the mistake of buying a Chinese made Beck Arnley thermostat. The ECU tests the presence of the heater coil and if the circuit is open it throws a fault light.

Don't try to save a few bucks like I did and buy junk. Buy the German OE thermostat (Wahler).

Car_Almond

BimmerFan52, roger that. Its what they are doing to us here.

When I first got this 530xi N52 engine, I did notice it had the novelty of an Electric WaterPump. I immediately thought the engineers at BMW did away with a thermostat entirely, since they could just read the temperature (thermocouple sensor) and adjust the speed of the water pump electric motor as needed to maintain the right temperature.

I was surprised to find an old-style wax element thermostat, and then found out it also has a heating element inside it, another surprise. By controlling the electric motor, they can flow whatever amount of coolant they want, from 5 RPM to 1,000 RPM say, so why do we need a mechanical thermostat too?

For those who think the old-style wax thermostat is a back-up device, its not. Note it can fail closed, which causes overheating of course. And, if it fails open, then its the same as not having a wax thermostat at all, which they could have done. I'm saying they should have just throttled the electric water pump motor as needed to control coolant temperature directly.

Even though I'm a mechanical engineer myself, I don't assume the BMW engineers were idiots here. There must be a reason why they gave us an old fashioned wax thermostat complete with the "cheat device" heating element AND a speed-variable electric pump.

Just to explain myself, Keith Tanner, MegaDork, on another forum, uses the https://www.pegasusautoracing.com/pr...asp?RecID=5854 and says: "We've been using the Davis Craig on one of our supercharged Miatas recently. It works like a charm and has increased the cooling capability of the vehicle considerably. No thermostat, it's all done with pump duty cycle."

Allen

I have a related question. Why is there a open/close valve (thermostat) at all? Isn't the water pump electronic? Why not just eliminate the thermostat and control the water pump on/off from the computer...based on if the water needs to flow or not flow? So if the pump is turned off...as in the valve closed. Then when the temperature of the engine/water rises...turn on the water pump...as in the valve open. I'm sure I'm missing something?

LOL...after typing all that....I just read the entire post above, which asks the same question. Sorry about that!

BimmerFan52

Given our locations, Allen (AL) and I (AZ) probably wouldn't care much if the electric water pump controlled all movement of coolant throughout the car.

But Car_almond, your tootsies (depending upon where you are located in CO) could get mighty cold in the winter if you cease all coolant movement through the radiator. The thermostat allows bypass flow to other areas (heater core) while cutting off flow to the engine block. Also, totally cutting off flow through the radiator could cause temperature stratification in the radiator where internal radiators (transmission fluid) are located.

Of course a thermostat never totally closes. Minimal coolant flow at the thermostat is required to accurately sense changes in "coolant system temperature".

My N62B44 is old school so I haven't needed to learn the ins and outs of the electric water pump, but there is obviously a multitude of sensor measurements feeding the DME such as throttle position, ambient temperature, coolant temperature (and if I know BMW another 100 or so bits of data) that the DME uses to decide when and how fast turn the electric water pump. But it has to turn at least occasionally to keep coolant flowing at the thermostat in order to keep the coolant temperature sensor accurately measuring the average system coolant temperature.

Car_Almond

Page 8 of http://www.clemson.edu/ces/crb/publi..._2_15_07_1.pdf explains what BMW could have done.
The only reason I can see for BMW using a thermostat would be to keep a larger coolant mass flow rate over the cylinders during warm-up when the thermostat blocks off flow to the radiator.

Thats likely the answer here. Remember some flow is needed to the cylinders to prevent hot spots, spreading around the heat generated in there. (Thermostats cut off flow to the radiator when cold.)

Its certainly true that cabin heater operation in cold weather is faster responding when the thermostat forces flow away from the radiator (bypass radiator mode) too.

The answer to why BMW uses that small heater element next to the wax thermostat, aka "waxstat", is because it wants to make sure the thermostat opens fully when the engine is being driven hard, resulting in a lower target temperature during hard running. OK, its finally making sense to me why they wanted to keep those thermostats with heating elements! Seems weird at first.

haanz

Hello All,
I have an overheating issue . yesterday, The red alarm appeared and i stopped until the engine cooled then drove home with radiator fan at full speed . I checked the water pump and looked like there was no coolant circulation. I tried to run the water pump by method of bleeding . It did not work and i just heard the sound of thermostats . Today, in the morning , I made three cycles of bleeding and every thing worked well. then i drove it for 15 minute and overheating issue appeared . What is the reason is it the thermostats or the water pump ?
Please help ,

zoppp

Well if you haven't replaced your water pump, that's probably it. The water pump going out is a common occurrence on these cars once you are over 80k miles. Luckily it is easier to get to on the e60 than it is on the e90 so a DIY should be fine.