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You're not supposed to open the pressure cap while the coolant is hot... it can be dangerous, because it will cause the coolant to boil immediately and overflow.

Can you post a photo of the leak?
 
If you open the cap while it is hot then the coolant will spew out. The pressure of it being sealed and shut keeps the coolant from boiling. As soon as you relieve the pressure the fluid will boil and expand and could potentially shoot out!

If the coolant is 'disappearing' you have a leak - no question. There are several things you/your mechanic should do to try and identify the leak;

Static pressure test - pressurize the system and ensure it holds
Coolant gases test (you can get a kit that has already been helpfully linked to tell if coolant is leaking into the combustion chamber)
Check the filler cap (any coolant in oil either through headgasket issues, or oil cooler issues, will show up here as it evaporates from the oil)

I can see you have a CL500 - W216 I assume. A possible issue on these is a leaky heater solenoid valve. It sits right by the firewall just inside from the front SAM (it sits just infront of the driver) Looks like this:

1619882678892.png

Get the car up to temperature and inspect the hoses where it joins this valve for any small drips. When it starts to go it usually 'leaks' when hot and so doesn't really leave puddles of water but your coolant will slowly drop.
 
You're not supposed to open the pressure cap while the coolant is hot... it can be dangerous, because it will cause the coolant to boil immediately and overflow.

Can you post a photo of the leak?
Will try it again tomorrow and show you 😂.
I am aware of the safety risk
 
@Riva811 , you say the car has been pressure tested successfully which means there is no leak, or the leak is tiny.

The expansion tank cap has to be removed when pressure testing, so it is not tested as part of the procedure. I really think you need to spend £10 or so on replacing that first as to me, it is the most likely cause by far. If it starts leaking after disturbing (undoing) it slightly too, another clear pointer to it being the cause.

Martin.
 
The head tank are prone to splitting between the upper and lower plastic joins.
Is this where the leak originates ?
 
Went to Klasse of Fulham told Steve this and that. He said it might be the tank overflow. I said ok have a look at it. Left it there for a few hours, he said he replaced the tank. I can see they kept the car running to detect the leak. I never saw a clean/new coolant tank in a car before :D
I hope this is the end of it.
 
Went to Klasse of Fulham told Steve this and that. He said it might be the tank overflow. I said ok have a look at it. Left it there for a few hours, he said he replaced the tank. I can see they kept the car running to detect the leak. I never saw a clean/new coolant tank in a car before :D
I hope this is the end of it.

Took it to MB Brentford and Chelsea and Brentford again after complaining to head office. MB Chelsea confirmed that they pressure tested it and there is no leak...

So MB Chelsea 'pressure test' it, did they........? 🤔 🤔 🤔 🐷 💸
 
So MB Chelsea 'pressure test' it, did they........? 🤔 🤔 🤔 🐷 💸
Yeah they tested it with a cold engine, they also did it infront of me when I called roadside assistance. Klasse let it run and raised it to see where the leak is coming from. More evidence that dealerships are crap at investigating the mysterious issues because they are in a rush to get the next car in. They did suggest I take it to Brooklands though for investigation as they say they have better mechanics due to the AMG workshop. Happy i saved on £120 investigation and a lot more on the expansion tank replacement. I have to admit Steve at Klasse can diagnose car issues by ear and eye in 1 minute. impressive
 

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