Thermostat
 

Last fall while in for other service I didn't feel comfortable doing myself, the mechanic read an error code (sorry I don't know the code number) described as the "second" stage of the thermostat on my M62 engine. He indicated the thermostat will only fail at this stage when the vehicle is driven hard- high RPM's. Since that time, driving normally I have noticed no issues related to cooling- heater works fine, temp gauge is normal, no drivability or starting issues. So I'm struggling with the need to replace it, the water pump was replaced for failure in 2016 and the thermostat was replaced prophylactically at that time and the thermostat was replaced again in 2016 because it failed. I flush the coolant about every 5-6 years. Appears my vehicle eats thermostats like tic tacs, does anyone have a similar experience and can anyone shed some light on this second stage of the thermostat's function?

Search and read up on the MAP cooling function of the DME. The thermostat has a wax pellet in it that the DME can melt and open the thermostat more than under normal thermostat operation. It’s supposed to help balance the need for power, and fuel economy, pre-detonation, etc. because the motor runs so hot (I think), but it I can’t really understand it in actual operation, and I read it isn’t really necessary.

I watch the function in real time and it seems to activate after I’ve gone up the big hill, or passed the car on the highway, floored it up the on ramp, etc. - too late to help with power. On the other hand , I’m cruising on a flat highway at 75 and it activates every 3 to 5 minutes endlessly. Makes no sense.

As far as thermostats, I’ve gone through two in 20 years.

Wow, very interesting info here. :thumbup: I had known most of it already, but found a couple of items near the end that may be relevant to the OP:

"If the coolant temperature exceeds 113oC at the engine outlet, the heating of the characteristic map thermostat is activated by the DME irrespective of the other parameters."

Basically the design is of a standard wax thermostat (as used in all cars for about a hundred years), but with a setpoint of 110*C (ridiculously high for emissions and efficiency reasons, at the expense of engine durability). Then they have an ECU controlled heating element to give the wax capsule in the thermostat some extra heat, causing it to open as low as 80*C when the ECU thinks it needs some extra cooling (high power periods, etc.).

So if the engine has a cooling problem that makes it run hotter (the thermostat does its thing based on the actual temp at the thermostat wax capsule, but the temp for ECU decision making is measured at the engine exit) than 113*C, the ECU would command that extra heat pumped in to the thermostat all the time. That is not a normal thing, and I can see how the constant current flow could burn out the heating element prematurely, leading to your car eating thermostats like tic tacs.

"The pointer of the coolant temperature gauge in the instrument cluster is located in the mid-position at coolant temperatures of 75oC - 113 o C"

This is a fairly well known issue, but I did not realize the break points were designed to match with the thermostat control points. Makes some sense. I have modified my own temp gauge to make it more linear (many posts on this forum about this), but for the OP, it could be that your car is constantly running hot (113 or a little higher) and your gauge is not showing any difference. You could use an OBDII reader to read the actual number for confirmation on what the real temp is, without needing to program new gauge breakpoints into your ECU.


Checking my notes from when my own thermostat failed on my 3.0i M54, the car seemed to be running fine, but I got a code with my Foxwell: 1-DME, 7B DME: Activation, map cooling. The code would come back immediately after clearing and restarting. The thermostat was OE, dealer replaced 44k miles earlier. Heater element measured at 3.7 Ohms, which xoutpost sources said was lower than the ~10 Ohms it should be. Replaced the Wahler-made Genuine BMW one with a Mahle, with the heater element measuring at 15.8 Ohms.

Great info, thanks to all. Appears in a MAP thermostat the heating element receives power from the DME before 110C allowing it to open, which is good for faster heater operation. And a MAP thermostat heating element also receives power above 113C to allow the engine to run cooler, apparently by opening the thermostat further allowing more coolant flow. (A very complex and expensive solution to achieve incremental emission and gas mileage gains over a simple 85C thermostat).

As the previous poster noted, this thing can be intermittently receiving power and cycling multiple times on any trip, but particularly so when occasionally running the engine harder and with more frequent short trips. I don’t run the engine hard but I do have frequent short trips, making the early demise of the secondary function make sense. Now I don’t feel bad about having it replaced and in fact want to replace it, and moreover I don’t think I need to replace the water pump. And it’s also worthwhile to ensure something isn’t causing the engine to run hotter, although my thought is an old vehicle with corroded channels in the engine, heater and radiator is the most likely culprit. Can anything be done about that except replace thermostat and flush coolant which I plan to do? I would not use any additives, but do the dealers use a flush that can dissolve some of the corrosion?

I don’t understand the German logic: Oddly, I find my map cooling turning on and off on highway speeds on flattish roads with almost clockwork regularity….every 5 or so minutes the map cooling will come on and the temp will drop to 90 (!) degrees. Why? I’m not asking for extra schlitz at all…cruise control on at 73. The hidden menu doesn’t show the radiator outlet temp, but I know from plugging my foxwell in that it soars to 160 to 170 or so during the time my map cools is active and the engine outlet temp drops to 90 degrees, which doesn’t seem like a lot of spread between motor and radiator outlet temps.

I’ve got some unfortunate experience with overheating and have come around to the belief that even 10 degrees cooler would be better. My oil cooler mod keeps the oil at 93 degrees and I think that’s what saved my motor when it overheated.

AFAIK the main idea behind the controlled thermostat is to lower combustion temps in order to meet Nox emissions specs without using an EGR valve(bonus for cost and reliability). So what ever time the DME drops the temp is probably what is needed to average the Nox emissions to meet spec.

I wonder if flashing an EU2 tune would fix that (as well as getting rid of the secondary air)

It's well established by know that the map controled thermostat is great in theory and improves fuel economy but at the cost of increasing pressure on cooling system (more heat = more pressure).

Also, if component in cooling system fails (and it will), you're much closer to potential serious problem of overheating vs running on cooler thermostat.

I've ran this on my E53 4.6is and have sold few dozen to E39 forum members couple years ago. I've had good connection in Europe and got this for much better price. This is OEM thermostat for E38 740D never sold in US.
https://www.ecstuning.com/b-genuine-...CABEgKFZ_D_BwE

Direct fit other than installing simple resistor into disconnected wiring connector to avoid CEL.

Can you watch the map cooling in a foxwell real time data chart?