Coolent temp fluctuating while driving
Thread Starter
Newbie
Joined: Mar 2017
Posts: 12
Likes: 4
W212 E350
Coolent temp fluctuating while driving
I have a 2014 E350 with about 92k miles on it.
For as long as I've had the car (8+ years), the normal behavior when driving is for the coolant temp to climb after starting the car to about 90C and then stay there until I turn off the engine.
About a year ago, I noticed the temp would start to fluctuate between 80 and 90c, where the temp would drop for anywhere from 2-20 minutes and then come back to 90C. The temp will remain at 90C for a period of time and then the pattern repeats. etc
I have a xentry scanner and no error codes on the car and no check engine light currently.
I'm going to be doing a long road trip in hot weather in a month, so looking for advice. I'm worried the thermostat could be failing and could run into over-heating.
Thank you!
For as long as I've had the car (8+ years), the normal behavior when driving is for the coolant temp to climb after starting the car to about 90C and then stay there until I turn off the engine.
About a year ago, I noticed the temp would start to fluctuate between 80 and 90c, where the temp would drop for anywhere from 2-20 minutes and then come back to 90C. The temp will remain at 90C for a period of time and then the pattern repeats. etc
I have a xentry scanner and no error codes on the car and no check engine light currently.
I'm going to be doing a long road trip in hot weather in a month, so looking for advice. I'm worried the thermostat could be failing and could run into over-heating.
Thank you!
Last edited by dawgfan; Jul 7, 2025 at 02:00 PM. Reason: Changing typo in the title
Out Of Control!!
Joined: Apr 2019
Posts: 11,922
Likes: 6,775
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
MANAGED ENGINE HEAT
I have a 2014 E350 with about 92k miles on it.
For as long as I've had the car (8+ years), the normal behavior when driving is for the coolant temp to climb after starting the car to about 90C and then stay there until I turn off the engine.
About a year ago, I noticed the temp would start to fluctuate between 80 and 90c, where the temp would drop for anywhere from 2-20 minutes and then come back to 90C. The temp will remain at 90C for a period of time and then the pattern repeats. etc
I have a xentry scanner and no error codes on the car and no check engine light currently.
I'm going to be doing a long road trip in hot weather in a month, so looking for advice. I'm worried the thermostat could be failing and could run into over-heating.
Thank you!
For as long as I've had the car (8+ years), the normal behavior when driving is for the coolant temp to climb after starting the car to about 90C and then stay there until I turn off the engine.
About a year ago, I noticed the temp would start to fluctuate between 80 and 90c, where the temp would drop for anywhere from 2-20 minutes and then come back to 90C. The temp will remain at 90C for a period of time and then the pattern repeats. etc
I have a xentry scanner and no error codes on the car and no check engine light currently.
I'm going to be doing a long road trip in hot weather in a month, so looking for advice. I'm worried the thermostat could be failing and could run into over-heating.
Thank you!
Actually this is smart Tstat over colling engine after it accumalting high heat.
In slow traffic extreme heat is unable to circulate out of engine core.
As a result ECU set Tstat for max opening
When higher Rpm + effective wind speed resume heat gets lowered slowly over 10mn
To lower heat on stock M276-NA keep Rpm's up around 2kRpm to help spin the "gas-saving" water pump.
Do not idle up stationary! A slow 1000Rpm engine runs hotter than above 2000Rpm.
Older engines gain increased blow-by heat thats hardly circulated out.
Get fresh oil service before your long trip.
Replace cabin air filter for the occupant's comfort creatures
Last edited by CaliBenzDriver; Jul 7, 2025 at 03:05 PM.
MBWorld Fanatic!
Joined: Feb 2003
Posts: 1,414
Likes: 339
From: Charlotte, NC
Had: 1987 300TD, Had: 2004 C230 Sport Sedan, Have: 2014 E350 Sport, Have: 2019 S450
Our ‘14 E350 is in the shop at the moment for this exact problem.
Since new, the car warmed up to 90C, and never wavered - summer/winter/cruising/stop-go/etc; always 90C.
Last week started seeing yellow thermometer warning - thought it was coolant low. Temp ran around 90, but dipped to 80 for a short time on the highway, and rose to 100 immediately after exiting highway and waiting at light. Shop just diagnosed thermostat and intermittent cooling fan issue.
I imagine if the thermostat was sluggish, this could cause what we observed. Hopefully car will be ready tomorrow after changing out thermostat and cooling fan.
Since new, the car warmed up to 90C, and never wavered - summer/winter/cruising/stop-go/etc; always 90C.
Last week started seeing yellow thermometer warning - thought it was coolant low. Temp ran around 90, but dipped to 80 for a short time on the highway, and rose to 100 immediately after exiting highway and waiting at light. Shop just diagnosed thermostat and intermittent cooling fan issue.
I imagine if the thermostat was sluggish, this could cause what we observed. Hopefully car will be ready tomorrow after changing out thermostat and cooling fan.
Last edited by dmatre; Jul 7, 2025 at 05:53 PM.
Thread Starter
Newbie
Joined: Mar 2017
Posts: 12
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W212 E350
MBWorld Fanatic!
Joined: Oct 2010
Posts: 5,141
Likes: 1,293
From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
If you are at all a hands-on person it is not bad job at all.
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Out Of Control!!
Joined: Apr 2019
Posts: 11,922
Likes: 6,775
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
80kMi MAINTENANCE
Tstat + 1Gal. of Zerex + O-Rings.
They fail-safe open with fault code related to cold temp.
High temps are not related to Tstat function... but related to heat accumulation.
Few ppl swear "if cooling is not right, it must be Tstat" - I used to think that way so I swapped MB Tstat 60kMi... New tstat: unchanged heat.
The engine is controlled by ECU including its cooling.
The display gauge going up/down is the result of accumulated heat, not failed tstat.
Circulating heat out of this Mercedes is not trivial.
Last edited by CaliBenzDriver; Jul 7, 2025 at 10:46 PM.
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From: Jakarta-Indonesia
2014 - W212.065 - E400 ( M276.820, 3 liter Turbo) RWD not Hybrid
If you are still on blue coolant, now you can not buy the blue one anymore from MB.
You can get only PINK one.
Blue and pink can not be mixed.
You must flush properly the old blue a few times with very clean water and then use the PINK.
Good luck.
You can get only PINK one.
Blue and pink can not be mixed.
You must flush properly the old blue a few times with very clean water and then use the PINK.
Good luck.
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Joined: Apr 2021
Posts: 846
Likes: 559
From: San Jose, CA
11 E550, 16 AMG GTS, 13 S550
if youre on the OE coolant it has a lifespan of "10yrs" but it starts to degrade in performance. If draining and flushing cooalnt isn't your bag right now theres a band-aid solution:
Link has my dealer-transferring number so I'll get credited on your sales. It's a surfactant meaning it helps the bonding of the coolant to the metal which improves cooling performance tremendously. Similar products on the market are "Water Wetter" but obviously this is a high performance version of that.
https://www.amsoil.com/p/amsoil-domi...cb/?zo=7236674
You can't buy the blue stuff anymore but Zerex by Valvoline still has it:
and in case you can't find it I also have this stuff from Amsoil which is a universal, but obviously much higher quality and won't eat at your radiator/end-tanks due to its formulation and mixes wirth everything
https://www.amsoil.com/p/amsoil-pass...pc/?zo=7236674
as for how easy the thermostat is. It's like a 1 hour job with zero experience just following instructions. You dont quite have a reason to suspect its going out until you get the code for it and the OE unit is pretty high quality to start out with. I recommend the Mahle Behr replacement and I would stock a couple gaskets for future use because they do go out much faster than the thermostats do and you will need to remove the thermostat for a couple other jobs, like the oil filter housing, and cam magnets on the right side of the engine. Although you can cheat the last one with just a set of pliers
Link has my dealer-transferring number so I'll get credited on your sales. It's a surfactant meaning it helps the bonding of the coolant to the metal which improves cooling performance tremendously. Similar products on the market are "Water Wetter" but obviously this is a high performance version of that.
https://www.amsoil.com/p/amsoil-domi...cb/?zo=7236674
You can't buy the blue stuff anymore but Zerex by Valvoline still has it:
and in case you can't find it I also have this stuff from Amsoil which is a universal, but obviously much higher quality and won't eat at your radiator/end-tanks due to its formulation and mixes wirth everything
https://www.amsoil.com/p/amsoil-pass...pc/?zo=7236674
as for how easy the thermostat is. It's like a 1 hour job with zero experience just following instructions. You dont quite have a reason to suspect its going out until you get the code for it and the OE unit is pretty high quality to start out with. I recommend the Mahle Behr replacement and I would stock a couple gaskets for future use because they do go out much faster than the thermostats do and you will need to remove the thermostat for a couple other jobs, like the oil filter housing, and cam magnets on the right side of the engine. Although you can cheat the last one with just a set of pliers
__________________
Alpha European Autotech
Purchase Amsoil at 25% off from me
Chris Tran, Retired Alpha European Autotech Owner
Amsoil Independent Dealer #7236674
Alpha European Autotech
Purchase Amsoil at 25% off from me
Chris Tran, Retired Alpha European Autotech Owner
Amsoil Independent Dealer #7236674
MBWorld Fanatic!
Joined: Oct 2010
Posts: 5,141
Likes: 1,293
From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
if youre on the OE coolant it has a lifespan of "10yrs" but it starts to degrade in performance. If draining and flushing cooalnt isn't your bag right now theres a band-aid solution:
Link has my dealer-transferring number so I'll get credited on your sales. It's a surfactant meaning it helps the bonding of the coolant to the metal which improves cooling performance tremendously. Similar products on the market are "Water Wetter" but obviously this is a high performance version of that.
https://www.amsoil.com/p/amsoil-domi...cb/?zo=7236674
You can't buy the blue stuff anymore but Zerex by Valvoline still has it:
and in case you can't find it I also have this stuff from Amsoil which is a universal, but obviously much higher quality and won't eat at your radiator/end-tanks due to its formulation and mixes wirth everything
https://www.amsoil.com/p/amsoil-pass...pc/?zo=7236674
as for how easy the thermostat is. It's like a 1 hour job with zero experience just following instructions. You dont quite have a reason to suspect its going out until you get the code for it and the OE unit is pretty high quality to start out with. I recommend the Mahle Behr replacement and I would stock a couple gaskets for future use because they do go out much faster than the thermostats do and you will need to remove the thermostat for a couple other jobs, like the oil filter housing, and cam magnets on the right side of the engine. Although you can cheat the last one with just a set of pliers
Link has my dealer-transferring number so I'll get credited on your sales. It's a surfactant meaning it helps the bonding of the coolant to the metal which improves cooling performance tremendously. Similar products on the market are "Water Wetter" but obviously this is a high performance version of that.
https://www.amsoil.com/p/amsoil-domi...cb/?zo=7236674
You can't buy the blue stuff anymore but Zerex by Valvoline still has it:
and in case you can't find it I also have this stuff from Amsoil which is a universal, but obviously much higher quality and won't eat at your radiator/end-tanks due to its formulation and mixes wirth everything
https://www.amsoil.com/p/amsoil-pass...pc/?zo=7236674
as for how easy the thermostat is. It's like a 1 hour job with zero experience just following instructions. You dont quite have a reason to suspect its going out until you get the code for it and the OE unit is pretty high quality to start out with. I recommend the Mahle Behr replacement and I would stock a couple gaskets for future use because they do go out much faster than the thermostats do and you will need to remove the thermostat for a couple other jobs, like the oil filter housing, and cam magnets on the right side of the engine. Although you can cheat the last one with just a set of pliers
Thread Starter
Newbie
Joined: Mar 2017
Posts: 12
Likes: 4
W212 E350
if youre on the OE coolant it has a lifespan of "10yrs" but it starts to degrade in performance. If draining and flushing cooalnt isn't your bag right now theres a band-aid solution:
Link has my dealer-transferring number so I'll get credited on your sales. It's a surfactant meaning it helps the bonding of the coolant to the metal which improves cooling performance tremendously. Similar products on the market are "Water Wetter" but obviously this is a high performance version of that.
https://www.amsoil.com/p/amsoil-domi...cb/?zo=7236674
You can't buy the blue stuff anymore but Zerex by Valvoline still has it:
and in case you can't find it I also have this stuff from Amsoil which is a universal, but obviously much higher quality and won't eat at your radiator/end-tanks due to its formulation and mixes wirth everything
https://www.amsoil.com/p/amsoil-pass...pc/?zo=7236674
as for how easy the thermostat is. It's like a 1 hour job with zero experience just following instructions. You dont quite have a reason to suspect its going out until you get the code for it and the OE unit is pretty high quality to start out with. I recommend the Mahle Behr replacement and I would stock a couple gaskets for future use because they do go out much faster than the thermostats do and you will need to remove the thermostat for a couple other jobs, like the oil filter housing, and cam magnets on the right side of the engine. Although you can cheat the last one with just a set of pliers
Link has my dealer-transferring number so I'll get credited on your sales. It's a surfactant meaning it helps the bonding of the coolant to the metal which improves cooling performance tremendously. Similar products on the market are "Water Wetter" but obviously this is a high performance version of that.
https://www.amsoil.com/p/amsoil-domi...cb/?zo=7236674
You can't buy the blue stuff anymore but Zerex by Valvoline still has it:
and in case you can't find it I also have this stuff from Amsoil which is a universal, but obviously much higher quality and won't eat at your radiator/end-tanks due to its formulation and mixes wirth everything
https://www.amsoil.com/p/amsoil-pass...pc/?zo=7236674
as for how easy the thermostat is. It's like a 1 hour job with zero experience just following instructions. You dont quite have a reason to suspect its going out until you get the code for it and the OE unit is pretty high quality to start out with. I recommend the Mahle Behr replacement and I would stock a couple gaskets for future use because they do go out much faster than the thermostats do and you will need to remove the thermostat for a couple other jobs, like the oil filter housing, and cam magnets on the right side of the engine. Although you can cheat the last one with just a set of pliers
Last edited by dawgfan; Jul 8, 2025 at 04:07 PM.
MBWorld Fanatic!
Joined: Feb 2003
Posts: 1,414
Likes: 339
From: Charlotte, NC
Had: 1987 300TD, Had: 2004 C230 Sport Sedan, Have: 2014 E350 Sport, Have: 2019 S450
Our ‘14 E350 is in the shop at the moment for this exact problem.
Since new, the car warmed up to 90C, and never wavered - summer/winter/cruising/stop-go/etc; always 90C.
Last week started seeing yellow thermometer warning - thought it was coolant low. Temp ran around 90, but dipped to 80 for a short time on the highway, and rose to 100 immediately after exiting highway and waiting at light. Shop just diagnosed thermostat and intermittent cooling fan issue.
I imagine if the thermostat was sluggish, this could cause what we observed. Hopefully car will be ready tomorrow after changing out thermostat and cooling fan.
Since new, the car warmed up to 90C, and never wavered - summer/winter/cruising/stop-go/etc; always 90C.
Last week started seeing yellow thermometer warning - thought it was coolant low. Temp ran around 90, but dipped to 80 for a short time on the highway, and rose to 100 immediately after exiting highway and waiting at light. Shop just diagnosed thermostat and intermittent cooling fan issue.
I imagine if the thermostat was sluggish, this could cause what we observed. Hopefully car will be ready tomorrow after changing out thermostat and cooling fan.
Thread Starter
Newbie
Joined: Mar 2017
Posts: 12
Likes: 4
W212 E350
Thanks for the update. I guess I'm leaning toward not replacing the thermostat, since doing just normal daily driving it seems fine and there aren't any error codes yet.
