Coolant temperature not constant

[lena]

Hello everyone,
In 2020, during a mandatory recall of my B180 type W246, the air conditioning refrigerant was replaced. The old refrigerant R134a was replaced by R1234yf.
Since then I regularly get a message that the engine temperature is not right.
I then see that the engine temperature rises a bit and then drops to about 80°C and then returns to normal. The phenomenon mainly occurs when i have to slow down or stop after a ride.
I have now noticed that this message only appears when I let the air conditioning work and when the outside temperature is higher than about 20°C.
When the air conditioning is off, there is no problem with the engine temperature.
Does anyone have an explanation for this, or any tips on how to solve the problem? Could the new refrigerant be the cause?
Thanks in advance.

 

[OneForTheRoad]

The engine cooling system and the air con are 2 separate things. 1 uses water type based coolant the other a gas . You may just need a new thermostat???

 

[Benzowner]

You should not turn off the air con, according to my air con engineer, just adjust then temperature of the air.

 

[lena]

At first I also thought about the thermostat, but because the problem does not occur when the air conditioning is not in operation, I suspect that the problem is elsewhere. Before replacing the thermostat, I want to make sure that the problem is not elsewhere.
The problem always occurs momentarily when I just slowed down after a ride at constant speed, more precisely, when I set the interior temperature lower than the outside temperature.
I read that the efficiency of the refrigerant R1234yf is much lower than that of the previous refrigerant.
Since the compressed gas is also cooled with the radiator and fan, I wonder if the air conditioning can have such a big influence on the functioning of the engine cooling. Or has the air conditioning a separate sensor for starting the cooling fan?

 

[m80]

I can only relate to the 651 engine.

Mine took an age to reach temp and then was erratic at holding it. I replaced the stat with genuine and it didn't improve anything.

Those stats are not only dependant on coolant temp to operate but get a signal from the ECU also. The ECU sees greater engine load and opens the stat in preparation for an increase in engine temp.
I didn't get to understand if another sensor might be causing my issue by being over sensitive.

 

[grober]

The air-conditioning will normally have a separate heat condenser [radiator] in the same airflow as the main engine coolant radiator. As you surmised there will be temperature limits to both systems and it may be that the new refrigerant characteristics are upsetting the engine coolant monitoring system. In other words to systems are not working in harmony any more. More modern systems may have a more efficient/larger condenser for example or it may be that some of the software settings[limits] need modification?
An air-conditioning specialist may be able to help but fitment of a more efficient condenser may be physically impossible meaning it may be something you will have to live with.

 

[Petrol Pete]

Hi Lena ,.A bit of a coincidence that all was well until the re gas took place . As grober mentioned ^^^ the condenser for the air con is quite literally squashed up against the main cooling radiator on most modern cars and shares the same cooling fan(s) . The condenser has its own temperature monitor that will switch the fan on if needed no matter what the engine temperature.

If there is a fault in the aircon system causing high temp in the condenser it will start the cooling fan drawing extra air through both radiators which in turn would effect engine coolant temperature. Do you hear the cooling fan start up even though the engine temperature gauge shows that the fan is not needed ?

 

[lena]

Hi Petrol Pete. I don't know if the fan starts when the temperature gauge indicates that it shouldn't because when it occurs I'm always on the road. Next time it happens, I'll go to the side and check.
Furthermore, as grober mentioned, I will visit an air conditioner specialist. If it is indeed the new gas, the best solution would be to replace the gas again with the old type of gas 'r134a', but that may no longer be used in Europe, says Mercedes.

 

[jc651]

Hmmmmm how did they change from r134a gas to the new stuff because the connection fittings are a different size for a start and the original system is designed to run on r134a gas. Not saying it can't be done but I wouldn't of thought it's as simple as changing the gas.