W212 Petrol Coolant Temp Fluctuations

[cheapshots]

Hi all,

W212 Petrol, M274 Engine, 2013, 60k mileage

Coolant temp in town is stable at just under 90 which is fine........when travelling high speeds it's mostly at 80, however it will dip to 60's if I'm coasting.

Had a MB software update recently (for the gearbox) but apart from that, the car has been fine.

It's almost as though this dip of temp on the motorway is intentional.......any idea's?

 

[Riva811]

Hi all,

W212 Petrol, M274 Engine, 2013, 60k mileage

Coolant temp in town is stable at just under 90 which is fine........when travelling high speeds it's mostly at 80, however it will dip to 60's if I'm coasting.

Had a MB software update recently (for the gearbox) but apart from that, the car has been fine.

It's almost as though this dip of temp on the motorway is intentional.......any idea's?

Check your coolant level, or might have air pockets. It also might be the thermostat sensor

 

[cheapshots]

Tried to check the coolant level but the coolant reservoir is badly designed and was hard to tell where the fluid level is. Will check again today.

As for air lock – hmm maybe! Any idea if there’s a way to bleed the air out without draining the coolant from (I assume) the radiator?

Is the thermostatic sensor inbuilt into the coolant thermostat itself?

 

[MrGreedy]

It sounds very much like symptoms you would get if your thermostat is stuck open.
Having done the free check on the coolant level as per Riva811's advice, the usual advice would be a new thermostat, and one sourced from Mercedes as Mercs can unfortunately a bit sensitive to aftermarket thermostats.
If you can buy the original manufacturer's thermostat as an aftermarket, that's another safe option.

It wouldn't be usual to have a sensor built into a thermostat. One the V6 diesel, some have an immersion element to give better control of thermostat operation.
The temp sensors are located elsewhere (usually).

 

[cheapshots]

Thanks guys. It's odd because if the thermostat is stuck open why is the temp a constant 90 when driving in town…

I've googled ways to bleed the coolant system to try and remove any potential airlocks. Seems fairly straightforward. Will give that a go today and take it from there.

If it is the thermostat that needs replacing, I'm wondering if it's a DIY job.... I'm OK with DIY jobs on the car, have some basic tools etc...

Here's hoping it's an airlock!

 

[cheapshots]

Hi guys, having bled the coolant system, the issue persists. New thermostat it is - anybody managed to change it themselves? Below is a link to the part I think I need (will confirm this with MB this morning).

NEW Genuine Mercedes-Benz OM274 Petrol Engine Cooling Thermostat A2742003300 | eBay

Suitable For Selected OM274 Petrol Engines. Unsure on Suitability?. Useful Links. Can’t find what you're looking for?.

www.ebay.co.uk

 

[E55BOF]

Sunds like a thermostat problem to me. The coolant temp will rise in town driving because you are getting much less airflow through the radiator.

 

[cheapshots]

Thermostat is part number A2742000615 from Merc £120.

I'm going to try and do it myself; if it looks too complicated I'll take it to my local garage.

 

[Riva811]

Yeah my money was on the thermostat as well. Seems to give a diagnostic error that the coolant temperature is higher than the thermostat or something like that.

 

[cheapshots]

Thermostat changed and problem fixed.

Thermostat was located in an inaccessible area so the mechanic had to do it. Took him over 3 hours! £200 Labour.