Coolant temperature going down

[loomes90]

Hi Guy,

So I have a 2020 GLC 300d, it went into Mercedes for some work due to it losing coolant.

They replaced the Temperature sensor and intercooler. I got the car back and now with a new fault.

I was no longer losing coolant however my temperature gauge seems to reach 90 degrees operating temperature, then it randomly decides to start dropping to about 70 then goes back to 90 then back down to 70, but then it can also remain stable. Seems like it depends on the mood.

It can happen on a short journey, but also on a long continuous journey.

I took it back and they replaced the thermostat, that did not solve the issue.

So returned again and they replaced the coolant pump, that did not solve the issue.

They are now telling me it is normal, which I am not convinced as it never used to do this.

What’s your thoughts?

 

[Pontoneer]

Temperature may well go up and down depending on the thermostat being open or closed , ambient temperature , air flow through the radiator ( related to speed and whether or not the radiator fan is running ) and engine load - so some variation can indeed be normal .

 

[toteimerced]

Sounds to me like the thermostat is getting weak.

 

[ALFAitalia]

Temperature may well go up and down depending on the thermostat being open or closed , ambient temperature , air flow through the radiator ( related to speed and whether or not the radiator fan is running ) and engine load - so some variation can indeed be normal .

Mine doesn't... ever.... once it's up to temp that's it...rock solid at 90, give or take a degree or two. Which was the case on all my recent cars. Which is how it should be if the stat (determining the lowest working temp) and the fan and it's switch (determining the highest temp) are working properly.

 

[loomes90]

Mine doesn't... ever.... once it's up to temp that's it...rock solid at 90. Which was the case on all my recent cars. Which is how it should be if the stat (determining the lowest working temp) and the fan and it's switch (determining the highest temp) are working properly.

I would be under the same understanding us yourself, operating temperature should be 90 and stay at it to run efficiently if it’s too high it will over heat and if too low use more fuel to try and reach operating temperature.

I have only had this car a year, but it’s never done this prior to the repair which to me suggests taking another look at the work already carried out

 

[loomes90]

Temperature may well go up and down depending on the thermostat being open or closed , ambient temperature , air flow through the radiator ( related to speed and whether or not the radiator fan is running ) and engine load - so some variation can indeed be normal .

Some variation I agree with maybe 1 or 2 degrees however this is dropping about 20 degrees then climbing back up, and it’s not happening on the odd occasion it’s happening every single journey multiple times

 

[Pontoneer]

Some variation I agree with maybe 1 or 2 degrees however this is dropping about 20 degrees then climbing back up, and it’s not happening on the odd occasion it’s happening every single journey multiple times

My current car tends to sit just above 80 , but in traffic on a warm day can go up to about 120 , and in winter just driving along can dip below 80 .

Many of my cars have had this variation so to me quite normal
. Most have a viscous coupling to drive the fan .

 

[Tonygw]

Sounds normal., they have a mapped thermostat and water pump. Temp drops after long runs or on hard acceleration for efficiency.
Both my mercs do this. last one has been like it for 6 years, no issues.

 

[loomes90]

Sounds normal., they have a mapped thermostat and water pump. Temp drops after long runs or on hard acceleration for efficiency.
Both my mercs do this. last one has been like it for 6 years, no issues.

Really? So would you suggest it might not have been working properly previously and now it is working correctly since the repair?

 

[tode]

A thermostat is called a thermostat because it's designed to keep the the temperature (relatively) constant . . . thermo +stat

Once up to temperature you should only see a variation of more than a few degrees under exceptional circumstances.

 

[Tonygw]

A thermostat is called a thermostat because it's designed to keep the the temperature (relatively) constant . . . thermo +stat

Once up to temperature you should only see a variation of more than a few degrees under exceptional circumstances.

That's the olden times.. google "mapped thermostat" and you will see. VW have used them for years and so have merc. They are ECU controlled as well as the water pump all in the name of maximum efficiency.

The temp on mine fluctuates depending on driving style and external factors.

Yes its likely it wasn't working properly before but now it is and is perfectly normal.

 

[tode]

Tonygw

I've just had a look at "mapped thermostat" and whereas they may be a little fancier than a a standard ("mechanical") thermostat, they are supposed to carry out the same job: keeping the temperature in the correct operating range and keeping it stable until the conditions change.

By "lowering the operating temperature" is noted as 85 - 95°.

To my knowledge, there is no way that the correct operating temperature for a diesel engine is as low as 70° (as in the OP's case ).

And 70° for a diesel will certainly not certainly not improve efficiency.

 

[Tonygw]

Why will 70C not improve efficiency?

 

[E55BOF]

Because the engine is running on a rich mixture until it reaches operating temperature, so using more fuel than it should be. It's nowhere near as rich as when it is cold, but it's still using more fuel than when it's up to operating temperature. This affects the emissions, and costs the driver more at the pumps as well.

 

[Tonygw]

But how do we know they didn't design the engine to run at cooler temperatures under certain load, environmental, driving conditions and warmer temps under different conditions to give an overall better long term efficiency?

Things have moved on from the simple low temp = more fuel scenario.

Both my mercs have had fluctuating temps and its all very dependent on the driving style, outside temps, distance travelled... etc etc

These days manufacturers are totally emersed in EU regulations to achieve efficiency....

 

[tode]

We know because there are certain basic principles of all diesel engines - one of which is the correct operating temperature.

One of objects of the mapped thermostats is to open when the engine is suddenly heavily solicited (sudden acceleration) - the object being to ensure that cold(er) liquid is available to offset the (predicted) rise in engine temperature. But this is not designed to lower the steady-state temperature.

 

[Tonygw]

But it does lower the temp.

Ive seen it with my own eyes! ;-)

 

[E55BOF]

Every one of my many Mercedes have run up to operating temperature (90) and stayed there, or thereabouts, at all times; three have needed replacement thermostats to achieve that. That said, though, they've all had reasonably powerful engines (mostly V8s; I've never had a four-pot) and have not, therefore, been used hard much. I've only had three diesels; they were the same.

If the OP's car held steady at 90 degrees or so (like almost every other Mercedes) before it went in for a repair, and the temperature fluctuation started only after that repair, then either it came out of the factory with a fault (unlikely), or it is faulty now. Something has changed. If it's now normal, why did the repairer only discover that after replacing the thermostat and the coolant pump? It couldn't be because they couldn't fix it, could it?

 

[E55BOF]

Sounds normal., they have a mapped thermostat and water pump. Temp drops after long runs or on hard acceleration for efficiency.
Both my mercs do this. last one has been like it for 6 years, no issues.

Have you ever seen either drop to 70 degrees?

 

[Tonygw]

Yes it often goes to 70C and fluctuates but mostly sits around 85C, only under certain circumstances will it change. Since my driving is pretty consistent I can usually predict where this would be based on my driving style. And can prevent it if I drive differently.

As I said its normal.