C63 W204: Water temp fluctuating

[SLSWE]

Hey all looking for advice here,

I've noticing that my water temperature is fluctuating and are wondering if this is common or something I should have looked at. I am fully aware that engine oil temperature fluctuates between 100-120+ C depending on driving style etc, however the water is not mentioned too often.
When driving on the highway today in 60 - 80mph the water temperature struggles to stay on 92C which I understand is the standard temp. Today it went down to 84C and then moved back up to 92C and then down again to 86C or so and then keeps fluctuating but rarely sits at 92C. When I then left the highway it went to 92C and then increased to 94C when I drove on roads around 25MPH to my garage.

Is this normal and does anyone else experience the same? Oil temp is around 105C and I guess that is the most important temperature so it's not overheating or being to cold wearing the engine.

If not, any ideas what this are symptoms of and what that needs to be replaced to solve it? I'm thinking it is either the thermostat or water pump if this is not as it should be. When walking from the garage I was not sure if the radiator fan (or what fan it is) actually sounded before I turned the engine off.

 

[badgx]

If the fans are working and not loosing any coolant, I would invest in a new thermostat.

Its a service component, will take approx 30 minutes to change. You will need some spare coolant to top up due to some spillage.

Also clean the mating surface with an old rag before installing the new thermostat, it comes with built in rubber gasket.

The part number is A1562030475 for the M156 engine.

 

[SLSWE]

Thanks @badgx! Should I be worrying that it is damaging the car e.g not drive it or is it more considered 'not optimal'?

 

[Steve84N]

It does fluctuate a bit but I've found that once it's up to 90+ it doesn't go back down to mid eighties so it could be overcooling, indicating a dicky thermostat.

 

[badgx]

Thanks @badgx! Should I be worrying that it is damaging the car e.g not drive it or is it more considered 'not optimal'?

This really depends on whether you have a coolant loss. If you can confirm, will be great.

I would only worry, if there was a coolant loss. Sometimes a minor coolant loss on M156 is usually down to a leak to the plastic feed between the expansion tank and thermostat housing. When this happens, people think of the worse i.e. head bolt issues. See photo below of my old tstat. The hole is where the plastic feed connects to the housing. It's plastic connectors, which go brittle over time.

So don't worry, for as long as you haven't got coolant loss and assuming your fans are working, I would tackle the thermostat ASAP.

Also I would avoid driving it till its replaced

 

[SLSWE]

Went for a short ride today to confirm if the fan was working or not, and it does. However, it does not go into full blow mode when needed (around 90C+) which then leads to water temp starts moving up to 94C before I parked the park.
Does this confirm that I need to change the thermostat? I'll combine that with changing the belt and pulleys for preventative maintenance. I hope this is the last bit of trouble of the M156 for this season
I didn't see any leaks around the expansion tank or the thermostat and no drops under the car as well.

 

[badgx]

I've added a drawing how the belt wraps round the pulleys. Lol.... not the best drawing but its a life saver when reinstalling. No head scratching

 

[SLSWE]

Update:
I left my car to the workshop earlier this week for a replacement of the thermostat. I didn't ask the workshop for trouble shooting the fluctuating temp, just simply replacing the thermostat.

All looked good on the short 30 min drive home. It stayed at 91C.
However tonight I had the first opportunity to drive the car little bit more as I been busy all week. The temp still fluctuates, however sometimes it sits pretty at 92C on highway and don't go higher than 93C whilst idle, but time to time it goes down to 87-88C and up to 90 etc.

One thing I've noticed is that before I start the car, when it's cold, after the first push of the start button, I can see the cold water and oil temp.
On my car the temp is:
Oil = 20C (probably temp in garage)
Water = 40C (This is what got me thinking)

What water temp does your car show on cold engine? I'm thinking if there might be some sensor that is not quite right as I am sure there is not 40C in the garage or am I wrong?

 

[Steve84N]

That's a normal range for the coolant to fluctuate within.

The oil temperature is true from outset but the water temperature doesn't read below 40 so just shows 40 until it goes above and then it's accurate. Goes over 40 very fast anyway.

 

[SLSWE]

That's a normal range for the coolant to fluctuate within.

The oil temperature is true from outset but the water temperature doesn't read below 40 so just shows 40 until it goes above and then it's accurate. Goes over 40 very fast anyway.

Just to confirm
Starting water temp of 40C is normal.
And water temp fluctuating between 87-93C is also normal?

 

[Steve84N]

Yes.

It's not actually 40 on cold start up, just the sensor doesn't read below 40.

 

[SLSWE]

Thanks for confirming! I guess it’s time to enjoy that M156

 

[badgx]

Enjoy buddy

 

[BlackC55]

Faulty thermostats can give a engine warning light. I think your coolant temps are normal. This is a common question for us at the workshop. People watch their oil and coolant temps more than ever before because there is a digital read out menu option on the AMG's. Any unusual behaviour is often picked up by the engine control unit.