Coolant Leaking in ~1000 kms

[WildSDI]

My 2015 w246 B class B200 CDI run 1,05,000 kms has been suffering from coolant loss from the past 7000kms. Twice on highway runs I got low coolant warning and had to top up. Recently service was done 1300 kms back and coolant cap was replaced as no leak could be identified. But today I got the coolant low warning again. The coolant reservoir shows signs of spilled coolant.

There is no identifiable leak from the radiator or any of the hoses. There is no white smoke from the exhaust. I do get a bit of black smoke on hard acceleration which I assume is normal.

Should I change the tank or open up the engine looking for a leak? Or look into the hoses going behind the wheel well?

 

[AMGeed]

I had a similar problem on my previous E63.
Run the car up to temperature and check the coolant reservoir closely for leaks.
Mine had a tiny pinprick hole that wasn't evident until the system was under pressure.

You could also wait until the engine (off) has cooled and carefully feel under the water pump for wetness.
Just a couple of suggestions to try.

 

[Bellow]

At first glance it looks as the coolant is being lost at the reservoir via the cap - which points to over-pressurisation. Not sure that diesels show 'white' at the tailpipe when a head gasket has failed as there's not necessarily vacuum to pull coolant into the cylinder. A compression/leak down test would show head gasket failure. But, aren't the EGR coolers notorious for cross leaking?

No mention of coolant in oil (engine or transmission -if auto) which could be saddle cooler failure but I doubt that could cause over-pressurisation and loss via the cap.

 

[Oleks1992]

My 2015 w246 B class B200 CDI run 1,05,000 kms has been suffering from coolant loss from the past 7000kms. Twice on highway runs I got low coolant warning and had to top up. Recently service was done 1300 kms back and coolant cap was replaced as no leak could be identified. But today I got the coolant low warning again. The coolant reservoir shows signs of spilled coolant.

There is no identifiable leak from the radiator or any of the hoses. There is no white smoke from the exhaust. I do get a bit of black smoke on hard acceleration which I assume is normal.

Should I change the tank or open up the engine looking for a leak? Or look into the hoses going behind the wheel well?

Check under the reservoir for leaks as they're prone to cracking and leaking. Also waterpumps leak at the bottom of the engine

 

[WildSDI]

At first glance it looks as the coolant is being lost at the reservoir via the cap - which points to over-pressurisation. Not sure that diesels show 'white' at the tailpipe when a head gasket has failed as there's not necessarily vacuum to pull coolant into the cylinder. A compression/leak down test would show head gasket failure. But, aren't the EGR coolers notorious for cross leaking?

No mention of coolant in oil (engine or transmission -if auto) which could be saddle cooler failure but I doubt that could cause over-pressurisation and loss via the cap.

No coolant in engine oil. Did not check in transmission oil.

My mechanic had missed the O ring in the new coolant cap and the caused the catastrophic leak of yesterday. I am waiting for a new one.

No idea why the previous leak occurred. Nothing visible externally.