Coolant Symbol Appeared Briefly, but not hot.
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Junior Member
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From: Southern US
2010 GLK350 4Matic
Coolant Symbol Appeared Briefly, but not hot.
Recently I was driving in about 70f ambient temp, and after about 100 miles the Coolant Symbol only appeared and went away. It popped up and was gone so fast I couldn't even make it out the first time. A minute or so later it happened again and I was able to see the symbol only because I was paying attention.
It wasn't a red symbol, it didn't say Coolant Low, and the car was not over heating at all.
As to cause, I was running my defroster for awhile. I pulled up my wireless OBD2 and noticed the temp was about 185.
I have noticed in the past that when I have my heater on the temp will drop from its normal 210 all the way down to about 185, sometimes less. As soon as I turn the heater off it goes right back to normal temp around 205-210.
Would this light illuminate due to insufficient temp? Both times it happened I was doing 75 mph and car was running great.
I have no check engine lights.
Thoughts?
It wasn't a red symbol, it didn't say Coolant Low, and the car was not over heating at all.
As to cause, I was running my defroster for awhile. I pulled up my wireless OBD2 and noticed the temp was about 185.
I have noticed in the past that when I have my heater on the temp will drop from its normal 210 all the way down to about 185, sometimes less. As soon as I turn the heater off it goes right back to normal temp around 205-210.
Would this light illuminate due to insufficient temp? Both times it happened I was doing 75 mph and car was running great.
I have no check engine lights.
Thoughts?
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2013 glk 350
A sticking old thermostat? I’ve had that happen where it drops . Is coolant level the same the next day ? If it keeps acting strange replace it. Most people would not pay attention to that or temperature.
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From: Lac Ste. Marie, Quebec
2006 Alfa Romeo Brera, 2010 GLK350, 2018 BMW 640i GT, 1997 Subaru SVX, 2012 Moto Guzzi Norge GT8V
It's perfectly normal for the temperature to drop when you turn on the heater. You are drawing heat out of the system and blowing it into the car. You can often nurse a car down the road for many miles when it is overheating by turning the heat on full. My wife was able to drive my Sierra truck about 50 miles when the fan relay blew on the Flex-A-Lite fan by running the heater/def on full blast.
I would check the coolant level.
I would check the coolant level.
Last edited by Silver Shadow; Nov 22, 2025 at 05:05 PM.
Thread Starter
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From: Southern US
2010 GLK350 4Matic
Because it happened for 1/4 of a second each time I wonder if the connection may be loose.
It was also raining when this happened.
Last edited by droptopchevy; Nov 22, 2025 at 11:57 PM.
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From: Southern US
2010 GLK350 4Matic
https://imgur.com/a/BmYlULv
The second the symbol came up I pulled the temperature up on my phone and it was around 185.
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'13 s212 63 p30. '06 LX470
After you let the car cool down to a completely cold state, did you check the coolant? You did not mention this step. Make sure the coolant is at the proper level when dead cold so you can remove that variable.
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2010 GLK350 4Matic
Sorry, yes. Coolant level was about a third up the thing in the overflow.
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From: Baltimore County, MD
'13 s212 63 p30. '06 LX470
MBWorld Fanatic!
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2013 glk 350
My coolant low light goes on when it’s below the seam of white and dark , yes that’s the fill line I believe but I’ve not read up on it. Keep it at that line cold. (Now I’m using dye to locate an intermittent leak) maybe it’s all normal as others have said . But your car is getting older!
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I wonder if the coolant system may have ingested some air, due to low coolant level in the expansion tank, while it sloshed around.
Air pockets, if any, might have moved around in the system when the cabin heat radiator flow was opened for heating.
Don't actually know if the GLK has a valve that shuts off and opens coolant flow to the cabin heat radiator or if just air flow is simply blocked off to and from the radiator. I've seen both types of setups in different cars.
Air pockets, if any, might have moved around in the system when the cabin heat radiator flow was opened for heating.
Don't actually know if the GLK has a valve that shuts off and opens coolant flow to the cabin heat radiator or if just air flow is simply blocked off to and from the radiator. I've seen both types of setups in different cars.
Last edited by MBKLUE; Nov 23, 2025 at 07:59 AM.
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From: Southern US
2010 GLK350 4Matic
I couldn't see the seam, so it may have been right. I added about 300 ml distilled water and it should be perfect now. I'll report back if anything else happens.
I don't think it could have been low enough to ingest air but I suppose it's possible.
I've always noticed a significant (30+ F) drop when using the heater at highway speed so I didn't know how "normal" that was.
I don't think it could have been low enough to ingest air but I suppose it's possible.
I've always noticed a significant (30+ F) drop when using the heater at highway speed so I didn't know how "normal" that was.
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I couldn't see the seam, so it may have been right. I added about 300 ml distilled water and it should be perfect now. I'll report back if anything else happens.
I don't think it could have been low enough to ingest air but I suppose it's possible.
I've always noticed a significant (30+ F) drop when using the heater at highway speed so I didn't know how "normal" that was.
I don't think it could have been low enough to ingest air but I suppose it's possible.
I've always noticed a significant (30+ F) drop when using the heater at highway speed so I didn't know how "normal" that was.
As Silver Shadow mentioned, and as you probably already know, turning on cabin heating is going to remove some of the heat from the engine coolant circuit. That said, under normal circumstances, the engine coolant thermostat should compensate and reduce the coolant flow through the main radiator so that temperature is increased back into the standard temperature operating range. It seems like that is not happening.
Last edited by MBKLUE; Nov 23, 2025 at 04:09 PM.
Thread Starter
Junior Member
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From: Southern US
2010 GLK350 4Matic
Yes, correct. With the heater off, or AC on, I can monitor the temp while driving and watch the thermostat open at about 208-212. Once I turn the heater on I can instantly watch it drop, and at least on the highway doesn't seem to ever get back to thermostat open temp. I'm not sure if this persists during city driving but I will monitor.
May be time for a new thermostat.
May be time for a new thermostat.
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2013 glk 350
If temp doesn’t recover after you turn on heat on highway your thermostat is bad and sticking open. Replace it before it gets worse, it can stick closed ! FCP euro sells good products with lifetime warranty . Don’t buy an autos store special made ? It may work fine like this forever but who knows .
