What is the coolant temperature of an E39 (I6 or V8) under normal operating condition

Everything I thought I knew about thermostats is (now) wrong!

After reading the attached PDFs I now realize that the thermostat in my BMW does not act anything like a conventional thermostat (as others have noted).

Specifically, it's electrically controlled by a 0v to 12v signal that only partially is dependent upon coolant temperature.

Adding to this control-voltage method is the fact BMW engineers apparently put in a wider-range thermostat that operated in the range of 80°C (176°F) to 103°C (217.4°F).

This keeps the engine artificially hotter at low loads than it would otherwise be; and it keeps the engine artificially cooler at higher loads than it would otherwise be - all for smog reasons (which my recent smog tests seem to bear out with 0.0 emissions!).

I added KTMP to the BMW E39 glossary addendum.
- KTMP = Kühlmittel Temperatur, aka Cooling Agent Temperature

Bluebee you are correct, in that when the thermostat is open, it cannot open any more. You need to think of it as a 110 C thermostat. Under low loads its just a 110C thermostat. When the computer realizes that a high load is being demanded, it can't wait until or more accurately allow, the coolant to actually reach 110 C, the DME will heat the thermostat so it thinks it hotter than it actually is (only 80C but thermostat always thinks its 110 if its open), so it will open sooner, and more importantly to stay open, until the load demand is removed. Now on a really hot day, and if the owner is towing a heavy trailer, up a significant grade, this may not work, as the radiator capacity will be exceeded.

BTW that article is for the M62, thats the V8.

Originally Posted by pangolin

when the thermostat is open, it cannot open any more.

For cross reference, this post has a nice chart of a variety of engine fluid temperatures over time for the BMW 2001 525.

I don't know how you guys chart these things so wonderfully, but, here is another post from VitaminXX today showing engine temperature on his way to work!
-My car's engine, coolant and oil temperature, by VitaminXX

PS: How do you guys generate these wonderful charts anyway?

Just as a cross reference, to keep things together, here is an interesting set of posts discussing how the electrically operated M54 thermostat works, from doru, cn90, &

*******> ********>

Originally Posted by gtxragtop
I do not believe there are any electronics within the thermostat. There is a 12V heater that adds heat to the wax pellet used to open the thermostat. I believe all the electronics are elsewhere.

Doru responds:
This is interesting, because my t-stats for the M54 engine all failed in the stuck open position. Usually the mechanical part (wax pellet) does not fail (as often). The question now is why is the electronic t-stat for the M54 engine so much prone to failure? Is the electronic circuitry that operates these t-stats failing/ageing in time and taking the t-stat with them?
I am thoroughly confused especially reading through Zionsville blog concerning their mechanical "competition t-stat" meant to replace the electronic OEM t-stat, and which should last longer. According to your statement (which I don't doubt), in this case maybe even the mechanical part of the OEM M54 t-stat is faulty from manufacturing (or in other words, it's not very sturdy for higher temperatures?). Here is what Zionsville claims:
"We have noted more M54 engine failures due to overheating than on any of BMW's previous engines. We believe this is due to a combination of factors including the electronic thermostat and high operating temperatures. According to BMW's technical literature, the factory thermostat is computer controlled to operate from 80 ° C to 103 ° C. BMW also reports that a mechanical failsafe does not open until the coolant temperature reaches 110 ° C. That's a 230 ° F temperature band, which may also contribute to engine failures.

Here's some background on map-controlled (electronic) thermostats from DuPont:

In conventional cooling systems a wax element (this is a mechanical thermostat) keeps the coolant temperature close to about 90 °C. This control method ensures that the motor does not overheat even under extreme conditions, such as very high vehicle speeds, very high ambient temperatures and heavy loads. However, in normal conditions a petrol engine can generally be operated at 110 °C without danger. This high temperature improves tribological conditions in the motor, which in turn reduce friction losses, fuel consumption and exhaust emissions. An electronic map-controlled thermostat can satisfy such variable requirements, because it can adapt the coolant temperature to the motor's operating conditions within broader limits. With the motor at partial load it maintains the coolant temperature at a consistently high level; at full load, high engine speeds or high outside temperatures it reduces the temperature and thus ensures performance characteristics which match these conditions.

So, when the coolant reaches a certain temperature, a mechanical thermostat opens. The map-controlled thermostat is "variable" based on input from the engine computer. It runs the engines hot (which is why the radiators also fail) for emissions. However, when the electronic thermostat fails, it may continue to run the engines hot without opening up "at full load, high engine speeds or high outside temperatures." This might be satisfactory if the driver were alerted to the fact that the thermostat had failed. A trouble code is set in the engine computer, which can be read with a diagnostic tool, but nothing appears on the instrument cluster. So, a failed thermostat can take the engine with it.

Our replacement unit will set the same error code, but it does not turn on any lights."

The more we know, the better we are off. Maybe there is a way to get a more robust mechanical t-stat that fit our cars and adapt the electronic part of the OEM t-stat to it? (I am dreaming, but it bugs me how weak those M54 t-stats are)

cn90 responds:
This is an example of engineering gone wrong.

The 1997-98 M52 engine runs on conventional thermostat and works just fine.
Then BMW engineers decided to be fancy and did all kinds of changes to the thermostat in the M54 engine, but:
- They gain nothing in terms of engine performance or emission
- They risk the M54 engine to heat damage.

Anyway, glad I have the M52 motor.

For those interested, the M54 thermostat is basically "electric" and "not electronic". Basically the thermostat opens/closed based either:
- engine coolant temp
- input from the ECU

This E46 thread has good info:
http://www.e46fanatics.com/forum/sho...d.php?t=483258

1997-98 thermostat $15.
1999-2003 thermostat $60!!!

Doru responds:
So, in essence the OEM M54 t-stat has an electric heater probe, electronically controlled by the ECU, and opens even more only if needed. (ECU management sends the signal and opens it more).
If the probe goes bad, it stays open, aka current is drawn at all times heating the element and thus effectively cooling the engine too much, which in turn will make it run rich in certain environment and therefore the mpg going South. There is one problem, that it might also fail in the "stuck close" position, and then you're toast. I believe a quick "fix" in a situation like that would be to just unplug the t-stat if your temp starts climbing and there is no evidence of lost coolant. The e46 guys claim that if the electronically controlled t-stat is unplugged, it works like a "normal" mechanical t-stat, only it does not open enough when needed.
Not sure if the modified to accept OEM shape of the Zions unit opens enough in those circumstances, but they claim it does.
I might go this route when my "new" t-stat fails.

This is a very intersting topic, and maybe we can find an answer/mod to overcome this plague. Because a Zions unit at close to 200 bux a pop is to steep.

Interesting info. This explains why my OBC is almost always around 108-110 C (which i always thought was high based on this thread) and last winter when I removed the fan clutch when temps were consistently and in the teens. Then one day when we had a "heat wave" 40 d ambient temperature day, my aux fan kept kicking on-(good thing it sounds like a jet engine!)

A check of the OBC cluster showed a temp of 115C and the needle slightly past 12'oclock. I immediately pulled over and spun the fan clutch back on and OCB temp dropped back down to 110C.

So to answer the original question of this thread: my coolant temp is 110 under most conditions. M62 engine.

Originally Posted by dvsgene

my coolant temp is 110 under most conditions. M62 engine.

My M54 was around 88°C to 97°C today, at about 45°F ambient temperature (way cold!).



Interestingly, I'll have to look up what this is all about ...

If I were to take the water pump and electric thermostat out of the system, replacing it with an electric pump and a conventional thermostat, would I be getting error codes and have trouble with the engine running due to the ECU?

Originally Posted by bluebee

Interestingly, I'll have to look up what this is all about ...

Bluebee...we don't get a reading for (OIL) OEL TEMP because we have no sensor. Some of the OBC TESTs are unassigned ...and that is one of them. If we had an M5 (which actually does have an oil temp sensor)...this TEST would probably function.

In fact, I believe TEST 15-18 are unassigned. And if you have the low MID/low instrument cluster...there are some TESTs (like TEST 12) which don't function because they don't have the extended OBC feature for ARRIVAL TIME...and TEST 12 shows the data for estimating the time of arrival.

Originally Posted by QSilver7

we don't get a reading for (OIL) OEL TEMP because we have no sensor

I was wondering about that!

I'm amazed, Q, how you know all of this.

Kudos to you! Luck to the rest of us.

Originally Posted by franka

If I were to take the water pump and electric thermostat out of the system, replacing it with an electric pump and a conventional thermostat, would I be getting error codes and have trouble with the engine running due to the ECU?

Not sure about the water pump. I believe the water water pump is OK, it does not have any funky over-engineering "improvements".
The electronically controlled t-stat, on the other hand if replaced with a mechanical t-stat will always trigger a code which will be stored. And this is one of the cruxes of the problem if you want to swap the electronically controlled t-stats to a more robust mechanical t-stat (like Zionsville's unit). The v8 has a different t-stat though, Frank. There are no reports of failed t-stats on these engines AFAIK. Only for the M54 engine.

Searching on Bimmerfest, I find nobody (yet) who has retrofitted an E39 with an oil temperature sender ... but googling ... I find that Pelican Parts sells an FAE oil temperature sending unit (P/N 13-62-1-433-076) reputedly for the E39?

Does that make any sense?

Rather than hijack this cooling temperature thread further, I opened a new thread to discuss the oil temperature sensor retrofit in the E39:
- Simplest BMW E39 oil temperature sensor retrofit? Has it been done?

Bluebee:

The 1999+ inline six motors get their dash temp gauge signal from the dual temp sensor under the intake manifold (M52TU and M54 motors) - #3 in the picture below.



The dual "Double Temperature Switch" (DTS) info from Realoem.com:

03 DOUBLE TEMPERATURE SWITCH 1 09/1998 13621703993 $21.80 +core

According to Bentley manual, the DTS provides a signal for the buffered dash temp gauge and a signal to the DME for ignition/fuel management. The second signal to the DME complements the signal from the lower coolant hose coolant temp sensor.

The DTS in the side of the cylinder head (near firewall or cylinder #6) is EXTREMELY difficult to reach and replace with the intake manifold in place. I've replaced all sensors in the engine bay on my M52TU except this one.

There are threads on the forum (which I am sure you'll find) where this DTS and it's function have been discussed. One poster said he was able to change the DTS with the IM in place ... I found it difficult at best and am waiting until I replace the IM gaskets to replace the DTS.

To your original Q: my car (M52TU) runs at 204F all the time. I've driven around with an OBDII + scan tool plugged in to watch the temp as reported through the OBDII cockpit port. Some tools also allow you to see "calculated" engine load and at what % you may trigger a misfire (what I was watching for) hinting at coil packs needing to be replaced (what I did). The OBDII system will trigger a fault after X number of triggers and drive cycles ... can't see specifics as to when / what % load causes misfire. If I had an oscilloscope I would have tested the coil packs that way.

Cheers.

Originally Posted by Jase007

"Double Temperature Switch" (DTS) ... According to Bentley manual, the DTS provides a signal for the buffered dash temp gauge and a signal to the DME for ignition/fuel management. The second signal to the DME complements the signal from the lower coolant hose coolant temp sensor.

Interesting.

This seems to be the innervation:
- DTS on block --> dash temperature gauge (buffered)
- DTS on block --> coolant temperature to DME
- Thermoswitch on lower radiator hose --> DME --> to AUX fan

Interestingly, even though the BMW E39 "BMW-specific E39 acronyms (list and definition)" list of thousands of BMW terms was scoured from the Internet and added to for months, it did NOT have this DTC term.

So I'll add this single sentence (please modify as needed so all benefit):
DTS = Double Temperature Switch, block-mounted coolant temperature sensor input to buffered dash gauge & DME

Few things I have discovered about the double temperature switch/sensor:

OBC's hidden menu shows temperature from the sensor that goes to the DME only when car is running or when ignition key is in position 2. But the hidden menu switches to the same sensor that the needle gauge uses then you put key to position 1. This happens at least with high cluster. This allows you to compare the readings between these sensor, they should show about the same value.

I accidentally discovered this when I searched a reason for low temperature readings. I already had changed thermostat and that did not help. One day I noticed that temperature was 88C when my car was running but immediately changed to 95C when I switched ignition key to position 1 and went again back to 88 in position 2. I disconnected the DTS connector and now in ignition position 2. OBC showed -128C and in position 1. -30C. I think this verifies that those reading are both coming from the DTS but from different sensors.

I changed the DTS to a new one and now both readings are the same or there is a 1C difference between them. Usually value seems to be 95C with range between 94-97C.

And finally: changing the DTS was an easy job. You just need a short (or shortened by cutting a long wrench) 22mm box wrench and remove the fresh air box and tube to the firewall from drive's side. 10 minutes job now that I know how to do it.

My car is a -99 528iA with M52TU.

Originally Posted by TJJ

the hidden menu switches to the same sensor that the needle gauge uses then you put key to position 1.

That's VERY INTERESTING!

Can someone corroborate that hypothesis?

Originally Posted by bluebee

Can someone corroborate that hypothesis?

From post #22 over here ...
"Wow, I did not know about the 2 separate readings bluebee. I learn something new every day! I just got home from running errands and as soon as I read this I ran out and tested it and sure enough, 2 separate reading, although mine were only 1 degree different. Good stuff! "

Originally Posted by bluebee

That's VERY INTERESTING!

Just some extra info:
I think that it's a build-in sensor test/compare function that was not found until now... I haven't found any official documents about the hidden OBC menu. All documents are made by users and looks like Bentley's E39 manual chart is also made by them and not copied from official BMW documents.

I also checked other temperature sensors with Inpa and none showed the same value that key position 1 was showing. So I think it's safe to say that it's coming from the sensor signal that is normally used only by the needle temperature gauge. And I think that disconnecting DTS-connector verifies it since only those two hidden OBC values were effected and at the same time temperature gauge needle jumped to the edge of the scale.

After the repair my engine temperatures are now in the correct range and the difference between 2. and 1. position is still 0C or 1C. So this definately fixed the problem and should even make a little difference in fuel consumption since now DME gets the real temperature instead of much lower value. So it's not always the thermostat what causes low readings, although I think that this sensor problem is a very rare problem. But it's still something you should check since it's so easy to check.

BTW. the same dual sensor is used in all E39s and also in many other BMW models.

By way of cross reference, a similar thread was opened today:
M52 Engine Cooling Temperature

Today, a post was opened searching for a DIY for the M54 dual temperature sensor (DTS), aka the double temperature switch, in response to a "coolant temperature sensor DTC".
- How to replace Double Temperature Switch BMW E39 525i

In response, I 'started' a bestlink reference but it needs to be fleshed out by that OP who is searching for a good DIY for the R&R of that sensor:
- How to replace the M54 dual temperature sensor (DTS, aka double temperature switch) in the M54 (1) (2) (3) (4)

One aside...

Some software modifiers such as Dinan actually alter the thermostat temp when they flash your computer. So the temps may differ a little from stock if you have had that done.

I followed Q's instruction for unlocking the cluster before driving up to LA yesterday. After reading this thread, I have a few questions:

#1 Does anyone know what temperature is required to cause a response from the analog gauge?

#2 I noticed in Blue's pics the temp reads Celsius... Mine appears to display Fahrenheit. Is that merely a difference attributed to high vs low cluster?

#3 The numbers I observed remained at 175 at highway speeds, and when it fluctuated, in did so in increments of 5. A simple thing like a vehicle cutting it close and wedging themselves into my lane ahead of me would result in an increase of 10-15 "points". I'm assuming due to the disruption air flow to the grille, but that's just a guess at this point. Has anyone else tested or experienced this kind of temperature sensitivity?

#4 Idling in a back up for a few minutes saw numbers as high as 240, with analog gauge remaining solidly at noon position. That kinda scared the crap out of me. How hot it too hot under "normal" circumstances for the M52TU??? Am I even looking at the correct data, or am I looking at something else I'm just assuming is the Fahrenheit data of the coolant/engine temperature?

Good questions.

Originally Posted by TJJ

OBC's ... shows the temperature from the one sensor ... only when car is running or when the ignition key is in position 2. But [the KTMP display] switches to the [other] sensor that the needle gauge uses when you put the key in position 1. ... This allows you to compare the readings between the two DTS sensors (which should, of course, show about the same value).

I accidentally discovered this ... One day I noticed my temperature was 88°C when my car was running but immediately changed to 95°C when I switched the ignition key to position 1 and it then went again back to 88°C in position 2. I disconnected the DTS connector ... then with the ignition in position 2, the OBC showed -128°C but in position 1 it showed -30°C. I think this verifies that those reading are both coming from the DTS - but from different sensors.

I changed the DTS to a new one and now both readings are the same or there is a 1°C difference between them.

For the record, another user today noticed the temperature difference between the two halves of the DTS:
- Changed OEM coolant for Evans NPG+

Originally Posted by Whorse

Now i know why it reads 1 degree off when i turn the engine off.

BTW, did you know you can get the KTMP reading in both scales?
- How to change OBC MID IHKA KTMP temperature from Celcius to Fahrenheit (1) (2) (3) (4) (5) (6) (7) (8)