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2010 Mercedes glk 350 temperature varying 10 degrees while driving
When coasting my engine temperature will drop to 80 or 82 and when accelerating will go up to 90 and then it will drop again - does not happen every time I drive but 50% of the time. This is relatively new and I’m unsure what could be causing the fluctuation like this and why it never exceeds 90 but drops. Potentially too much coolant of the thermostat is stuck open so when coasting with more airflow it’s running cooler? Not sure any help is appreciated. I have a video of it that it won’t let me upload. Within a 2 minute drive it fluctuates from 80/82-90
Maybe it has already failed and is stuck half open. This happened to me and I didn’t realize what was going on until the next winter, warm ups became far too long.
In my W204 I have noticed the temp go down to 80c when I was on the highway cruising around 70mph. This stood out because im always seeing it at 90c once it gets up to temp.
You dont mention when this happens, backroads or highways?
As we know these cars do not show actual coolant temp once it gets to operating temps. From 96c-116c the gauge will actually show 95c (mine seems to be around 90c), Mercedes reasoning is to not worry the driver about the car over heating.
To see if the thermostat is operating I will watch the temp gauge and look for it to hit 95c, then I will notice it drop down to 90c. To me this tells me the thermostat is opening.
I've never seen my GLK gauge fluctuate like that, BUT if you monitor the temps while driving with a scan tool you will see the temp. fluctuate anywhere from 205 to 212, based on engine speed, load, and thermostat position.
Disregard the temp gauge on the dash and plug in a scan tool to get the actual values. That will tell you the actual range of fluctuation but I would suspect the thermostat is sticking or water pump is on the way out if you determine it's not at proper operating temp with the scan tool.
Last edited by droptopchevy; Sep 1, 2025 at 11:53 PM.
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