The thermostat on my 2011 E550 failed and was replaced shortly after I bought the car. NBD, but I obviously want to keep an eye on it until it has a chance to gain my trust. I don't know what the state of failure was when I got it, or how long it took to fully and completely fail, but I noticed it was having an issue long before the check engine light ever came on (even started a thread on it a day or two before the warning finally lit). I know what a "normal" looked like pre-thermostat replacement, but have no idea what actual normal should look like. Could someone with an E550 with the same powertrain let me know where on the gauge the needle falls once it's fully up to temp and operating normally? Thanks.

 

It's going to be hard to gain that trust:

 

Hard to say exactly as the gauge is so coarse, but mine is at about 95C, perhaps 97C. It has never been over 100C other than when the thermostat failed. I replaced it myself ages ago now. Has been fine.

 

Why do you think your thermostat is not working well ???

 

Exactly what I was looking for! More complex behavior than I thought. Before the new thermostat, it would read one tick above 80°C pretty consistently once it was warmed up. To the point that I once saw it read something like 95°, and was concerned (not for the temperature necessarily, but because it was unusual). Now it's moving around as one would expect given that behavioral description.

As for why I thought it wasn't right, it would take a LONG time to warm up. Didn't help that it's winter, and I live in the mountains above Denver and drive down to the flats most mornings. Even with that, it was definitely not behaving as one would expect. I even started a thread because it seemed like it was just taking way too long. Then a day or two after that, it threw a check engine light for a bum thermostat.

 

This smart Tstat is map based. It does not directly react to the temperature sensor.

The coolant temp is not even representative of engine temperature.

The stock engine stores extreme heat in dry pistons.

When you drive down to Denver flats your coolant can hardly get warmed up while your pistons hardly see any cooling. The heat is not circulated out.

Notice that when you stop 10mn to get ice cream, the engine fan is screaming after restart, even in Winter?

You need to rev up your engine pass 3500.Rpm to activate piston cooling. So drive in the appropriate gear to run above 3500.Rpm.

 

Mine sits at what I presume based on that info above to be 90C once warmed up, though it looks like 91 or so but that might just parallax. I've never seen it go higher.

 

Get OBD2 based gauge, it will have the honest engine coolant temperature data which you may freak out



.

 

I still remember back in the days people complained about something is wrong with their vehicle when the coolant gauge kept fluctuating, it is actually not a bad idea for the average customer to see a doctored gauge
: ) but we are no average consumer : )

 

I'm keeping the car rock stock, so more detailed information isn't necessary. I've done more than my fair share of engine builds and tuning (to include engineering parts for aftermarket applications for major aftermarket manufacturers). This ain't that. It went from one behavior that may or may not have been part of the failure mode to a different behavior after someone else wrenched on it. I haven't had a car in a shop prior to this one in nearly 20 years, and my trust level is very low. I just need to know what "normal" looks like on this car. I haven't had many modern vehicles go through the stable, and those that have have had gauges that warm up to "normal" and sit there rock steady unless something is wrong. That's pretty much how this behaved until it really started going wrong.

The next question that follows, now that I know what the "temperature" gauge is saying, is what is the normal operating temperature range from the above ranges? It used to always be in the 80-115° range, and now it goes back and forth between that and the 115-120° range. This is on a cold morning with mountain up and down, but after it's had a reasonable chance to warm up. 80-115° is what I would expect, but that higher range, which is a tellingly much narrower band, seems a bit high. I know modern cars try to run as hot as they can without damage for emissions purposes, but I don't know what the norms are in anything this modern and high tech.

 

We can't learn anything about our coolant temperature when the white lie value is up to 25 Celcius at certain region of needle value.
90C needle means actual is 80 to 115C , at least 90C is indeed betwen 80 to 115C
95C needle means actual 115-120C , where does 95C belongs to ? 95C is 20C lower than 115C ...Holy Moly , the white lie shifting its angle ...LOL.
Only when a true 120C or higher is reached, the needle will show honest 120C too....duggh, the cylinder head would warp by next 2-3 minutes if we are unlucky where indeed
a true overheating is actually in progress.

Having a true honest coolant gauge ( via OBD ) is the only way you can undertand your engine true coolant temperature at various operating condition and climate.
Having a true honest coolant gauge ( via OBD ) give us ample warning when and if cooling system is declining in its performance, be it by neglect or by bad luck.

One more thing about Mercedes engines with heater in its thermostat, aka MAP thermostat.
It will want to hit 105C at some point in its engine operation when driven. Mine is MED177 ECM, M276.8xx my engine, I would think M278 would be the same strategy
M272 / M273 I do not know.

The ECM is an opportunistic bas-tard when it comes to hotter is better ( up to a point ) for fuel efficiency.
Get a Xentry and see the engine ECM live data, there it shows the temperature target the ECM wants the engine to achieve for some point in time.
Your will see 105C as one of the target. Just in case you are non-metric, 100C is pure water boiling temperature.


Today's log. See the 102.9 Celsius


.
102.9 Celsius





With speed data.



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ECM info using Xentry

105C target



.
90C Target



80C target




The cooler the ambient temperature is or during rain, the more opportunistic bas-tard the ECM will be to hit 105C.

Before this video time frame, it was not raining and coolant was 92C there about, when the rain comes , you can see the intake air ( aftercooler ) drop from 45C to 35C at image #4 below.
ECM then decided abundant cooling capacity is AVAILABLE when rain happen, and let's do 105C !!!
Since the video is 300% speed up, each video minute counter multiply it by 3x.



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Even at 34C ambient temperature, for me to see a true coolant temp ( via OBD ) maintained at 115C to 120C is because I was at the track having fun.
No way in normal driving I could hit above 115C sustained.
But at the least I know what's happening towards what kind of load I do to my engine at the current ambient temp.... thanks to OBD




If I did not use OBD at this point in time, I would not slow down after lap 2 and perhaps my engine may suffer or be damaged when and if overheat warning is only to appear at 130C .



Ignore the TRANS temp, it is a wrong data PID.


.

 

I have in my S550 with the 278 engine a AMG special cluster that allows me to see engine coolant and oil temperatures. When fully warmed up the engine coolant temp settles to 198 deg (92C). With constant speed driving the oil temp comes to the same.

My cluster temperature gauge shows very close to the same 90C or slightly above it.

My E550 with the 273 engine runs a little higher at around the 95C mark. I do not have any other temp display in that one.

 

@S-Prihadi , That is some outstanding data! I have a 273, so it doesn't help me much (though that in no way diminishes the value of the data), but hopefully anyone coming along behind with a 278 and the same question will find everything they need in that single post!

The 95° indicated corresponds to 115-120°, which seems awfully high. I guess it's good enough for Mercedes's goals on warranties and reputations of reliability... I'd prefer it to run a bit cooler still... Seems like an ECU level modification though...

 

Yes, the 95C needle is scarry : The 95° indicated corresponds to 115-120°
Our radiator/pressure cap is a 1.4 BAR rated, thus MB has the ba-lls to allow 130C as OVERHEAT warning temperature, based on poper 50/50 coolant mix.

I can't find the table anymore, but 50/50 coolant and at 1.4 BAR is about 136C boiling temperature if I remember right.
In the old days and even now, my Japanese cars is using 1 BAR pressure cap, thus the coolant boling temperature is less.

BMW uses 2 BAR pressure cap, wow !!

I replaced my pressure cap per 10 years ( done ) and also the coolant recovery bottle (done ) where internally it held the additive where it allows MB to claim coolant life that crazy long .
Coolant is a 5 year max for me, I love my engine

 

Mine, too. This is my normal reading. Occasionally, the oil temp hits 212°F (100°C), but rarely. Maybe the coolant temp might break 200°F, but I don't remember seeing that except for the time my coolant hose came off. I've never seen the tranny temp get above 180°F.

Steady driving gives me the following:

 

Oil = Coolant is a nice temp reference.

The key for oil temp is too have the Rpm necessary to spray-cool pistons at the driving Rpm.

For MOS-0 stock that's about 3000.Rpm
For MOD-1 0W40 thats about 2500.Rpm
For MOD-4 5W50 that's about 1300.Rpm

-- Without effective cooling the heat stored in pistons accumulates and balance increases to create extreme heatsoaks.

-- The only way to go is to remove heat as it is being produced because heat transfer is a slow process.

When measuring numbers without effective spraying, Temps are not correlated to stored engine heat.

+++ PISTONS COOLING...
When you're out driving, ​​​​​​imagine pistons being cooled On/Off.

This is exactly what effective Rpm does.
Below Rpm = over heated pistons
Above Rpm = controlled pistons heat

Choices are many.