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!

 

You're not alone, this is what stock does. I used to wonder about these temp swings...

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

 

Replace the thermostat.

 

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.

 

How difficult is it to change thermostats on the W212?

 

Thanks! As another member asked, how easy is it to replace it? Would doing a coolant flush also make sense, considering the age of the car?

 

You will have to drain some coolant so that you won't spill when opening the hose connections so why not just drain it all and flush the system with distilled water after thermostat has been replaced.

If you are at all a hands-on person it is not bad job at all.

 

Tstat replacement is money well saved.

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.

 

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.

 

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

 

I went to buy coolant for my 2010 E550 from the dealer and they sold me a gallon of Zerex G48 and said it is the same and only one they use for all cars. So I bought more of it from O’Reilly.

 

Just to double check, you are referring to the plastic thermostat and not the metal one? I saw some YT videos where the thermostat changed was metal, while my replacement part is plastic.

 

Update: replacement of thermostat & fan resolved the temperature fluctuations. Back to 90C without wandering.

 

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.