M156 Coolant temp fluctuations
05-27-2022, 05:03 AM
#1
Newbie
Thread Starter
M156 Coolant temp fluctuations
I need advice and a fresh perspective.
I replaced the thermostat two weeks ago. The plastic in the old one was already at its limit.
Since then I have been having problems with the coolant temperature. The engine warms up properly. When it reaches 98 degrees Celsius, the temperature starts to drop. It drops exactly every one degree until it reaches 76/75 (SD logs). It lasts several seconds and then the temperature starts rising again.
This happens only when I drive and when I take my foot off the gas.
So I bought a second thermostat from another dealer. I looked at the water pump, installed the thermostat. No change. I replaced the temperature sensor. No change.
SD shows no errors. No leaks etc.
At a standstill, the engine reaches 105 degrees then the fan comes on and cools it down to 91.
Is it possible that the fan comes on at that 98 degrees and cools the fluid to 75? It could also be air in the system. What puzzles me though is that the temperature range is always the same.
Any ideas?
I replaced the thermostat two weeks ago. The plastic in the old one was already at its limit.
Since then I have been having problems with the coolant temperature. The engine warms up properly. When it reaches 98 degrees Celsius, the temperature starts to drop. It drops exactly every one degree until it reaches 76/75 (SD logs). It lasts several seconds and then the temperature starts rising again.
This happens only when I drive and when I take my foot off the gas.
So I bought a second thermostat from another dealer. I looked at the water pump, installed the thermostat. No change. I replaced the temperature sensor. No change.
SD shows no errors. No leaks etc.
At a standstill, the engine reaches 105 degrees then the fan comes on and cools it down to 91.
Is it possible that the fan comes on at that 98 degrees and cools the fluid to 75? It could also be air in the system. What puzzles me though is that the temperature range is always the same.
Any ideas?
Last edited by Hokanato; 05-28-2022 at 05:52 PM. Reason: Grammar
05-28-2022, 05:51 PM
#3
Newbie
Thread Starter
Thanks! I'm not too worried, but it's not normal IMO. On the old thermostat the temperature was stable. Now, I'm focusing too much on it, and it annoys me 
11-14-2023, 07:22 PM
#4
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MY'14 W212 M276 3.5NA @60kMi
11-15-2023, 01:27 PM
#5
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11 E550, 16 AMG GTS, 13 S550
Coolant temp switch isnt fun what part brand did you use?
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Purchase Amsoil at 25% off from me
Chris Tran, Service Manager/Owner
Amsoil Independent Dealer #7236674
11-15-2023, 01:41 PM
#6
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Thread Starter
Not really. Since my first post on this topic, I've driven about 5tkm. Oil clean (I changed it twice), coolant with no loss. I stopped worrying about it. Of course, I would like to know where they (temperature drops) come from. Especially since I see them on the display and SD.
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11-15-2023, 03:17 PM
#8
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MY'14 W212 M276 3.5NA @60kMi
all right...
I need advice and a fresh perspective.
I replaced the thermostat two weeks ago. The plastic in the old one was already at its limit.
Since then I have been having problems with the coolant temperature. The engine warms up properly. When it reaches 98 degrees Celsius, the temperature starts to drop. It drops exactly every one degree until it reaches 76/75 (SD logs). It lasts several seconds and then the temperature starts rising again.
This happens only when I drive and when I take my foot off the gas.
So I bought a second thermostat from another dealer. I looked at the water pump, installed the thermostat. No change. I replaced the temperature sensor. No change.
SD shows no errors. No leaks etc.
At a standstill, the engine reaches 105 degrees then the fan comes on and cools it down to 91.
Is it possible that the fan comes on at that 98 degrees and cools the fluid to 75? It could also be air in the system. What puzzles me though is that the temperature range is always the same.
Any ideas?
I replaced the thermostat two weeks ago. The plastic in the old one was already at its limit.
Since then I have been having problems with the coolant temperature. The engine warms up properly. When it reaches 98 degrees Celsius, the temperature starts to drop. It drops exactly every one degree until it reaches 76/75 (SD logs). It lasts several seconds and then the temperature starts rising again.
This happens only when I drive and when I take my foot off the gas.
So I bought a second thermostat from another dealer. I looked at the water pump, installed the thermostat. No change. I replaced the temperature sensor. No change.
SD shows no errors. No leaks etc.
At a standstill, the engine reaches 105 degrees then the fan comes on and cools it down to 91.
Is it possible that the fan comes on at that 98 degrees and cools the fluid to 75? It could also be air in the system. What puzzles me though is that the temperature range is always the same.
Any ideas?
You using good parts was a smart move else everyone would blame junk parts as responsible for a problem that may not be one.
Temps are good!
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Hokanato (11-16-2023)
