Thermostat Problem?

[cleverdicky]

Have you had any work done on you cooling system, new hose rad etc?
Because it could also be air in the system. In which case it needs bleeding.

Remember car cooling water temps would be far hotter than what we see on the gage.
Its only because it stays under pressure that it stays well below boiling point.
If that pressure is reduced by small leak or air in system expanding, temps will fluctuate.

 

[survey]

Have you had any work done on you cooling system, new hose rad etc?
Because it could also be air in the system. In which case it needs bleeding.

Remember car cooling water temps would be far hotter than what we see on the gage.
Its only because it stays under pressure that it stays well below boiling point.
If that pressure is reduced by small leak or air in system expanding, temps will fluctuate.

No, no work on the car at all! It is going absolutely fine and the temperature does sit at a steady 92 on the gauge, just as the previous poster said with his C220. My attention was drawn to this only because of the initial fluctuation on the way up 80 then 75 then 80 etc until it rises to 90-92 and stays there.

 

[whitenemesis]

The boiling point is elevated by increasing the pressure in the system. The temperature is what creates the pressure. So water at 90c under 2bar pressure is still 90c at atmospheric. Water at 110c at 2bar will not boil but at atmospheric pressure will boil but it is still only 110c

 

[cleverdicky]

???
Well it wouldnt hurt in the least to ensure any air trapped was bled.
Did it just start doing this or been a while?

Could also even be a faulty sensor or gage.
Might be a waste of time, but get hold of an obd reader and see if the temps match.

 

[television]

Not all thermostats open smoothly, never any air in these systems

 

[Smacsor]

Hi Simon - thanks for your timely response. Just to recap you are saying that your car sits at 92 degrees (that's exactly what my C220 did and what my present car does).
Are you also saying that the going to 80, back to 75 etc etc and then up to 92 is what is does NOW after the thermostat has been changed or was this how it was performing BEFORE it was changed?

Yes, my car will sit at 92 all day once it gets there.

It didn't start to fluctuate immediately after the stat was changed, its only done it for the last couple of weeks since the really hot weather (which may be coincidental). I'm not too worried about it as it gets to 92 after 10miles/20mins or so but I'll get it checked when it goes in for a service in about three months time. Would be intrigued to hear what the outcome is with yours though?

Simon

 

[hotrodder]

My money's on the thermostat being on it's last legs- getting sticky/slow to react etc. I wouldn't worry about the 82° versus 92° thing... as long as it's more than 80 and not constantly nudging/over 100 it'll be fine

FWIW the pressure cap is to stop the coolant boiling after switching off a hot engine when coolant temps in the cylinder head rise due to lack of flow/heat sink effect. Same reason some cars have an additional leccy pump in the system to keep coolant circulating if the engine is shut off and coolant temps are sky high.

Neat ethylene glycol boils at ~ 197°C, the usual 50/50 mix has a boiling point of ~107 @ atmospheric pressure. It's diluted 50/50 with water because antifreeze has a poor heat capacity, around 1/2 - 2/3 that of water
The thermostat is a (necessary) restriction in the system which raises the coolant pressure in the engine a chunk above that provided by the pressure cap when the waterpump is running. Without a thermostat (broken or otherwise) restricting flow coolant pressure in the cylinder head drops and despite the fact the temp gauge will usually show a cool running engine the coolant around the combustion chambers can boil which creates steam pockets and quickly leads to localised overheating- once a steam pocket has formed heat transfer in that spot effectively stops

 

[television]

FWIW the pressure cap is to stop the coolant boiling after switching off a hot engine when coolant temps in the cylinder head rise due to lack of flow/heat sink effect. Same reason some cars have an additional leccy pump in the system to keep coolant circulating if the engine is shut off and coolant temps are sky high.

I do not see anything wrong with the running temp of 90ºc sure as you say, when you turn off the thermal runaway takes the heat to the outer surfaces and the water jackets are in the outer section.

I do know as I said before that with 80ᵒ c the car heater will be none too hot in the winter.

As I could not find the temp range for the thermostat for this car it could be marked on the box, and a phone call to ECP might give an answer

 

[survey]

Yes, my car will sit at 92 all day once it gets there.

It didn't start to fluctuate immediately after the stat was changed, its only done it for the last couple of weeks since the really hot weather (which may be coincidental). I'm not too worried about it as it gets to 92 after 10miles/20mins or so but I'll get it checked when it goes in for a service in about three months time. Would be intrigued to hear what the outcome is with yours though?

Simon

Simon - I have come to the conclusion that my car is behaving normally. Mine exhibits exactly the same characteristics as yours with your new thermostat. Maybe the warmer weather accounts for the hunting around the 80 degree mark before getting up to 92 and staying there. I am not convinced that my dealer is correct with his assertion that 82 is correct and anything more than that merits a replacement thermostat!

It will be interesting to learn of the running temperature of ther owners' cars. In any event I will post what happens at my dealer's workshop next week.

 

[brandwooddixon]

Mine runs at around 85-90 degrees. Sometimes it will push up towards 95-100 when stopped in traffic, but then the cooling fan cuts in to reduce the temperature.

I would think that the thermostat starts to open at 82 centigrade and is fully open by around 88.

 

[television]

Mine runs at around 85-90 degrees. Sometimes it will push up towards 95-100 when stopped in traffic, but then the cooling fan cuts in to reduce the temperature.

I would think that the thermostat starts to open at 82 centigrade and is fully open by around 88.

It starts to open at 87c on most 113 engines, this one being a newer type of diesel I was not sure of anything definite, in theory it should be in the 90's as that is the best point for max efficiency and it will be interesting to hear what the outcome is

 

[survey]

It starts to open at 87c on most 113 engines, this one being a newer type of diesel I was not sure of anything definite, in theory it should be in the 90's as that is the best point for max efficiency and it will be interesting to hear what the outcome is

I totally agree with your comment.

I have telephoned another MB dealer and they support what seems to be the consensus that 90 is about right and the 'hunting' between 75-80 initially could be due to the warmer weather when all fans are going full belt.

I have just emailed MB Customer Services for a definitive answer and will report the outcome!

 

[television]

I have done another search and I see that your cooling circuit is a 2 stage one with a heater in the system

Not one word anywhere on the temp on the 651 lump

I have printed the circuit off if you would like to read it I can send it

 

[st4]

OM646 engine and OM642 engines I've had have been on 90c and an OM651 I borrowed sat at 90c. The dealer is wrong

 

[survey]

I have done another search and I see that your cooling circuit is a 2 stage one with a heater in the system

Not one word anywhere on the temp on the 651 lump

I have printed the circuit off if you would like to read it I can send it

Thanks Malcolm for your efforts. I don't think I need the circuit diagram, thanks.

 

[survey]

OM646 engine and OM642 engines I've had have been on 90c and an OM651 I borrowed sat at 90c. The dealer is wrong

Yes, I am now convinced of this also. It will be interesting to get MB Technical's response.

 

[television]

Thanks Malcolm for your efforts. I don't think I need the circuit diagram, thanks.

OK it explains how the cooling works more than any thing else and very different to any other engine that I have seen

 

[brandwooddixon]

Could the fluctuation actually be due to extra cooling as the fan cuts in to cool the airconditioning heat exchanger and a slow reacting thermostat? It's usually situated in front of the engine radiator and so would mean that both are reaping the benefit of the cooling fan's labours.

 

[survey]

Could the fluctuation actually be due to extra cooling as the fan cuts in to cool the airconditioning heat exchanger and a slow reacting thermostat? It's usually situated in front of the engine radiator and so would mean that both are reaping the benefit of the cooling fan's labours.

Could well be. It will be interesting to hear what MB Technical say. One of their dealers says it is fine, the other wants to diagnose and probably replace (am I being cynical that this is easy money for the garage as MB pay under warranty)?

 

[Smacsor]

Could the fluctuation actually be due to extra cooling as the fan cuts in to cool the airconditioning heat exchanger and a slow reacting thermostat? It's usually situated in front of the engine radiator and so would mean that both are reaping the benefit of the cooling fan's labours.

Interesting you say this. I thought there couldn't be a connection but I have noticed that if I turn off the A/C completely the car goes quickly goes straight to 92 without the hunting around 75-80?

I did a 90 mile round trip today and it took 25 mins and 20 miles (primarily motorway) before it settled at 92 after the usual hunting around 75-80 but then it stuck there all day without moving a micron.

Simon