Terrencehuang

This car has an estimated mileage of 78,000km, c200 2018. Bought 2021 and drove for slightly over 3 years

The car has been sitting in the workshop for 10 days. My mechanic and I had been scratching our heads over this issue.

Currently, I met with this issue.
In May 2024 around 66,000 km mileage, the car is met with top up coolant message.
I sent it for servicing with my mechanic and saw engine oil in coolant tank.
So we serviced the car and changed the following parts; Oil Cooler, Thermostat, Water Pump and the connecting rubber pipes.

In 1st week July, The top-up coolant message came up, we changed the transmission coolant reservoir as well.
In 4th week July The top-up coolant message came up again, I went to check with my usual mechanic and discovered a large amount of engine oil in the coolant tank.
The engine also lost around 1L of Engine Oil.

The symptoms of the car engine are without misfiring and no vibrations. So we rule out the cracked head gasket fault. The engine has not been overheated in any of the scenarios as I am an engineer myself and a gentle driver (Light Footer).

Seeking your information and guidance on this issue as I am a MB car lover.

My experienced team of mechanics at my usual workshop, they also mentioned that the symptoms are rare and had not encountered such an issue.

mxl556

Unless the oil from the oil cooler somewhat getting into the engine coolant, I would be checking the engine compression and/or do a leakdown test. I would also pressurize the cooling system to see if there are any internal leaks. I would also consider another mechanic as low coolant message should have been diagnosed and parts should not be replaced for no reason. It sounds like the oil cooler, thermostat, water pump and hoses were replaced for no apparent reason.
Unfortunately I would not rule out the head gasket or a cracked/warped cylinder head.

good luck.

chassis

Did the transmission reservoir change cause a problem? Double and triple check the work.

MB engines are known for piston cracking and failed cylinder heads. As mentioned above, check compression and read codes.

Terrencehuang

Mine being 2018 c200 w205 m264 1.5L eq boost and this m264 1.5L Eq boost engine is still used in the various versions in 2019 w205 c180, 2021 w206 c180 and c200 as well
https://en.wikipedia.org/wiki/Merced...60/M264_engine
This is a relatively new engine. For an engine block failure, it will mean the Mercedes-manufactured engine is on the downslope.
M264 E15 DEH LA

• 2019–present W213 E 180 (without EQ Boost for Pakistan, Singapore, Egypt and Vietnam)
• 2020–present C257 CLS 260 (Chinese market)
• 2019–present W205 C 180 (without EQ Boost)
• 2021–present W206 C 180 (with EQ Boost)
• 2021–present W206 C 200 (with EQ Boost)

If the engine block is a problem, then MB wouldn't have used it in the current make for w206.

Primary diagnostics from the mechanic team would be hairline crack in engine block that had caused the engine oil to leak into the coolant flow.
I have not replaced anything yet. i am suggesting various methods to look into this problem to my mechanic as well in the recent encounter, the car has been in workshop for 17 days.


because ....
linking up the cases

1st coolant leak encounter is leaking coolant pipe causing lose of coolant replaced coolant pipes. this means engine over pressurized causing higher chance of wear and tear on coolant pipes ?
2nd oil in coolant issue, changed oil cooler and oil filter housing and parts
3rd oil in coolant issue, changed tranmission cooler and flushed engine

4th encounter oil in coolant tank and lose 1 litre of engine oil means ….. ?

last check from mechanic
no contamination in the transmission coolant.
no coolant in the engine oil

I suspect that a faulty twin-scroll turbo system and its sensor would have driven more air into the manifold causing excessive pressure into the engine to damage the oil cooler or transmission cooler again to have lost 1L of engine oil into the coolant reservoir.
I suspect over-pressurized build-up is caused by the twin-scroll turbo and its sensor or faulty pressure release values and its sensors.
Please correct me if I missed anything.

W205C43PFL

Hmmm. Do you often drive in the city or on the highway?

The W206 uses M254 instead.

​​​​​

W205C43PFL

Sorry I should have mentioned North American W206 uses M254 I am not sure about Europe or Asia. My apologies.

Terrencehuang

​my usual drive highways 60 percent. City 40 percent car parked in shade during summer or rain. Humidity all year round would be 80% thereabout

https://en.m.wikipedia.org/wiki/Mercedes-Benz_C-Class_(W206)
Not sure. But engine type for w206 listed is m264 for w206 c180 c200. Also Mercedes listed the engines and engine parts still in production.
M254 engines should be for higher specs. c300 w206 4matic should be the ones using m254.

Please correct me if i am wrong