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I have the same problem too on my 2013 C250. I have flushed the coolant, changed the fan, changed the thermostat, replaced old coolant pipes and still nothing. The dealer said it was normal and after around 5K kms the problem is less evident (i.e. not fluctuating as much anymore) but the engine constantly runs at 95 instead of 90 (with minor fluctuations down and up between 95 and 85) now and the top up coolant warning appears randomly every now and then. Not sure how much more money I am willing to spend to resolve the issue but the next step would be to replace the radiator and replace the temp sensor.
 

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I have the same problem too on my 2013 C250. I have flushed the coolant, changed the fan, changed the thermostat, replaced old coolant pipes and still nothing. The dealer said it was normal and after around 5K kms the problem is less evident (i.e. not fluctuating as much anymore) but the engine constantly runs at 95 instead of 90 (with minor fluctuations down and up between 95 and 85) now and the top up coolant warning appears randomly every now and then. Not sure how much more money I am willing to spend to resolve the issue but the next step would be to replace the radiator and replace the temp sensor.
Same problem and it's nonsense because:
1- the temperature of a diesel engine must be maintained between 195 and 205 f to achieve optimal combustion.
2- if the temperature does not reach 190-195f there will be sludge formation in the engine oil and this can slow down the return of the valves in & out but especially those of the intake because they are continuously cooled by incoming air.
Im still looking for the problem, yesterday I did a test that left me speechless; I gently crushed with a flat plastic vise the rubber pipe outlet of the thermostat going to the radiator inlet and I went for a ride convinced that the temperature would rise above 195f... but not at all after reaching 195f the temperature still fell to 178F WTF...
It is very rare that I don't know where to look anymore and today I decided to change the thermostat... after a 50km test the temperature went up 7 times to 195f and went down 7 times to 178f I know because I installed a scangauge on my obd2 plug and the reading of the computer is not wrong.... then I start to wonder if there wouldn't be an electric valve somewhere... because it's very precise like opening 195f sudden opening then in 2 seconds the temperature drops to 178f. Tomorrow I will investigate this path. If someone has the solution, I thank them in advance, I go blind.
 

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Hello, i know this is ab old threat but ill share how my problem was fixed. First of all my car runs fine normal gauge to 90 and then when release gas pedal temperature start to drop to 80 sometimes below that. After couple of monts of checking every component of the car, its simply fix with just reinstall instrumental cluster software. It apear that after some time its starts to bug out and this problem appear. Just go to dealer it cost me 35 euro to reinstall it. And 10 days later problems gone. No more anoying drops. The car is c250 2012. I drove with that problem 6 monts. And last time in the shop one of mechanisc says he is shure its just software problem so he reinstall it and no more problem. Well i hope you have luck like me. Thank you. Sorry for bad english.
 

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Hello! Just bought my first MB c350 cgi and I love it. But I've noticed some strange behaviour. When I start the car and wait some minutes before driving the temp gauge is normally moving up to a warmer temp.
This is normal temp as I understand;
View attachment 1015713

But after driving slowly for I while it goes down?!
View attachment 1015721


Can someone explain this?


Sent from AutoGuide.com Free App
I am a diesel mechanic but not a specialist... a thermostat controls the temperature of the engine coolant however this type of thermostat is not precise and when the operating temperature is reached it opens fully because it assumes that the engine is working at full capacity. This type of thermostat is safe device for the engine but it will operate close or wide open depending on the temperature.
 

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Hello, i know this is ab old threat but ill share how my problem was fixed. First of all my car runs fine normal gauge to 90 and then when release gas pedal temperature start to drop to 80 sometimes below that. After couple of monts of checking every component of the car, its simply fix with just reinstall instrumental cluster software. It apear that after some time its starts to bug out and this problem appear. Just go to dealer it cost me 35 euro to reinstall it. And 10 days later problems gone. No more anoying drops. The car is c250 2012. I drove with that problem 6 monts. And last time in the shop one of mechanisc says he is shure its just software problem so he reinstall it and no more problem. Well i hope you have luck like me. Thank you. Sorry for bad english.
 

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Thank you very much for the info... but I'm skeptical as to whether the software had anything to do with it... it shouldn't be because the temperature of these motors is controlled by a mechanical thermostat with an expansion trigger. But thanks. I'll call Benz tomorrow to check it out... you never know.
 

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Well like you said never know. My father car has same problem. Already changed termostat, water pump, fan and more. Today his car is for software reinstall. His car is c220 2010 om561 engine. Same bihevior but his car sometimes stay at 70 degrees for couple of minutes then goes to 80 stay there then goes to 90 and after some time driving again goes to 70. Will update after couple ot hours. Mechanic says that if there are changes in temperature you shoudnt notice then inside. Thats how its programed to work.
And still on my car i didnt change pump and temostat and problem is gone. Today is day 11 now my car goes to 85 degrees when the pump switchs it drops to 82 and after couple of sec goes to 90 and dont move anymore. For a 35 bucks im happy with result.
 

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If you're telling the truth then he must have an auxiliary solenoid valve that controls the temperature... do you know?
 

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Its same engine so if my car has his must has it too. The car will be ready in a hour and will update after some test drive
 

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I will check the cooling diagram because if the temperature is controlled by programming then there must be an electric valve that opens and closes somewhere on the engine cooling system.
My vehicle is a GLK250 diesel from memory the engine is an om654 or something like that.
 

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I know that om651 has switchable water pump controlled by solenoid. Atleast this is what i was been told by mercedes mechanics and this info is at internet. But still today im at 150 km with my car without temperature drops. Before sometimes even at highway with speed 140km i notice drops from 90 to 80. Now its gone.
 

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Little update after testing my fathers car yesterday and today its seems problem with temperature gauge drops is fixed. His car doing thi every day and after reinstall of instrumental cluster problem is gone. He is happy that there is no more such anoying thing. Just to say its very hard to make mercedes mechanics to convince there is a problem and even to make such reinstall. But after some times of arguing they did it and it cost only 35 evro and 2 hours of their time. Now 2 cars finaly has their problem solved. I dont know will this work for you but well its cheap solution so trie it if you want. Thank you.
 

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This is my car now and after the update it reach 90 much faster than before. This is with heater always on. With heater off its much faster.
 
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