Engine Running Cool
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Joined: Oct 2010
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From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
Engine Running Cool
A couple of days ago noticed my car engine temperature running at 80 degrees. It has been running at around the first mark above 80 degrees, which should be about 87-88 degrees.
Then, yesterday got the CEL. Connected my iCarsoft and it gives code explaining engine coolant temperature at below control range.
Looked at engine coolant temperature when driving. If I first let car idle the temperature climbs up to 81 degrees but when I drive long enough at highway speed it drops down to 72 degrees. Outside temperature yesterday was 67 degrees.
During this time the temperature gauge in the cluster shows 80 degrees when iCarsoft says 72 degrees. I get the normal 87 degrees in the cluster when iCarsoft shows 81 degrees.
iCarsoft shows how temperature constantly changes depending on driving speed and engine revolution.
I think my thermostat is stuck in too much open position and does not allow engine temperature raise up to normal level.
What do you all think about this?
Then, yesterday got the CEL. Connected my iCarsoft and it gives code explaining engine coolant temperature at below control range.
Looked at engine coolant temperature when driving. If I first let car idle the temperature climbs up to 81 degrees but when I drive long enough at highway speed it drops down to 72 degrees. Outside temperature yesterday was 67 degrees.
During this time the temperature gauge in the cluster shows 80 degrees when iCarsoft says 72 degrees. I get the normal 87 degrees in the cluster when iCarsoft shows 81 degrees.
iCarsoft shows how temperature constantly changes depending on driving speed and engine revolution.
I think my thermostat is stuck in too much open position and does not allow engine temperature raise up to normal level.
What do you all think about this?
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From: Cincinnati
2010 E350 4Matic
Sounds like mine when the thermostat went bad. I replaced it and coolant at the same time. Wasn't a big deal to do. Could be wrong (its been a while) but I thought I ended up getting a code also.
Last edited by MBNUT1; Jan 14, 2020 at 12:30 PM.
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I suspect the thermostat as well as they normally fail in the open position. The CEL will generally come and go until you replace the thermostat. This is exactly how my failed thermostat acted until I replaced it. My temp has always run just a hair above 90c.
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MBWorld Fanatic!
Joined: Oct 2010
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From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
iCarsoft actually gives a helpful hint for this. It explains how my issue is normally caused by an open stuck thermostat.
I made a 3 hour highway drive today and temperature went down to 70 degrees as a lowest. This was the iCarsoft reading. Car showed right below 80 degrees. CEL did not come back after I reset the code. It did retain the permanent code for it though but the light is off.
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MB’s of all kinds.
I bought a new thermostat today from my MB dealer for $195 including tax. Pretty pricey as normally you can buy a thermostat for about $30 but this one comes with the housing. Thermostat is built in the housing and cannot be bought separately. I’ll be changing it next weekend I hope.
iCarsoft actually gives a helpful hint for this. It explains how my issue is normally caused by an open stuck thermostat.
I made a 3 hour highway drive today and temperature went down to 70 degrees as a lowest. This was the iCarsoft reading. Car showed right below 80 degrees. CEL did not come back after I reset the code. It did retain the permanent code for it though but the light is off.
iCarsoft actually gives a helpful hint for this. It explains how my issue is normally caused by an open stuck thermostat.
I made a 3 hour highway drive today and temperature went down to 70 degrees as a lowest. This was the iCarsoft reading. Car showed right below 80 degrees. CEL did not come back after I reset the code. It did retain the permanent code for it though but the light is off.
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Joined: Jan 2009
Posts: 9,225
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From: V E G A S
1922 Ford Model T / no OBD
The cluster gauges are dummy gauges, so they don't show few degrees changes till something big happen.
On W211 I had climatronic who would display temperature that ECU is seeing and it was quite different what cluster would show.
Now 72 C thermostat opening is close to normal and on cold days the engine will stay at opening temperature till you put heavy load on it.
Some engines have thermostat with heating element in it, so that might take more troubleshooting.
My routine test is observing heating time. Engine should come to operating temperature around 80C on 2 miles drive for gasser or 4 miles drive on diesel when you are above freezing temperatures.
When it takes much longer - you need new thermostat.
On W211 I had climatronic who would display temperature that ECU is seeing and it was quite different what cluster would show.
Now 72 C thermostat opening is close to normal and on cold days the engine will stay at opening temperature till you put heavy load on it.
Some engines have thermostat with heating element in it, so that might take more troubleshooting.
My routine test is observing heating time. Engine should come to operating temperature around 80C on 2 miles drive for gasser or 4 miles drive on diesel when you are above freezing temperatures.
When it takes much longer - you need new thermostat.
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Thread Starter
MBWorld Fanatic!
Joined: Oct 2010
Posts: 5,142
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From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
The cluster gauges are dummy gauges, so they don't show few degrees changes till something big happen.
On W211 I had climatronic who would display temperature that ECU is seeing and it was quite different what cluster would show.
Now 72 C thermostat opening is close to normal and on cold days the engine will stay at opening temperature till you put heavy load on it.
Some engines have thermostat with heating element in it, so that might take more troubleshooting.
My routine test is observing heating time. Engine should come to operating temperature around 80C on 2 miles drive for gasser or 4 miles drive on diesel when you are above freezing temperatures.
When it takes much longer - you need new thermostat.
On W211 I had climatronic who would display temperature that ECU is seeing and it was quite different what cluster would show.
Now 72 C thermostat opening is close to normal and on cold days the engine will stay at opening temperature till you put heavy load on it.
Some engines have thermostat with heating element in it, so that might take more troubleshooting.
My routine test is observing heating time. Engine should come to operating temperature around 80C on 2 miles drive for gasser or 4 miles drive on diesel when you are above freezing temperatures.
When it takes much longer - you need new thermostat.
Also the CEL came back on.
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Senior Member
Joined: Jul 2017
Posts: 377
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From: Savannah, GA
2010 W212 E550 4matic
Try replacing the coolant temp sensor in the back of the engine on the driver's side cylinder bank. I have a post about it somewhere here. Mine was doing something similar last winter, replacing the sensor cured it. Retails for under 20$ from MB.
Check the pump in front of the expansion tank, make sure it's not stuck open bypassing some of the coolant into the rad.
Check the pump in front of the expansion tank, make sure it's not stuck open bypassing some of the coolant into the rad.
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Joined: Jan 2009
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From: V E G A S
1922 Ford Model T / no OBD
Most of my diesels have thermostat opening at 78C and with steady driving it would keep that even with ambiedient at 100F
Thread Starter
MBWorld Fanatic!
Joined: Oct 2010
Posts: 5,142
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From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
Replaced thermostat today and found the old one closed so I’m not sure how it failed to make engine cool as closed means hot engine.
But, after change now engine heats up fast as it always did and now the cluster temp gauge shows between 2nd and 3rd mark so it wants to run at about90 C. Interestingly iCarsoft shows the same temperature. Also, now iCarsoft shows up to 100 C and down to 84 C when changing engine load quickly but it soon returns to around that 90 C.
I think it is fixed now.
MBWorld Fanatic!
Joined: Jan 2009
Posts: 9,225
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From: V E G A S
1922 Ford Model T / no OBD
Did your thermostat had heating coil?
Proper test of old thermostat would be boiling it in pot of water.
Thread Starter
MBWorld Fanatic!
Joined: Oct 2010
Posts: 5,142
Likes: 1,295
From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
The main thing is it works now and the engine definitely runs hotter now. I can feel a big difference when I set cabin temperature up for heating. With cool engine was not blowing much hot air but now it is again.
