snky1987

Hi,

A word of introduction:

I am an owner of 2008 W204 C200 CDI 5gear automatic Mercedes.
Car has the mileage of 90k miles (about 140k kilometers).

The issue appears when I run the car on cold engine (does not need to be cold outside) and the car is ran for the first time on that day (I do not have an issue on the 2nd start after 8h standby when I work).



How does the car behave just after the start:

When I run the engine my rpm goes slightly up and then stabilise which is more or less normal. Sometimes it flows a little bit in rpm (+- 50rpm) but then eventually get stable.



Issue description:

Then, when I start to drive and my gearbox change to 2+ gear, and I accelerate slightly (not too hard because engine is still cold) then RPM can suddenly drop from 1500/1800 (varies) to as low as 1000/1500 RPM and I hear the noise similar the the noise you have with old water pipes in the old house.

When I push the acceleration it goes up again and I have to work with acceleration aggressively to prevent this from happening. When the engine reach aprox. 80celsius then I do not have this problem for the rest of the day.



So summarising:

1) How does it look like ?
It drops the RPM when accelerating with a little power and sounds like an old water pipes in old house after RPM drop.

2) When does it happen ?
It happens when the engine is ran after the full night rest, on cold engine and only when accelerating slightly or maintaining low speed/rpm on cold engine (usually below 2000RPM).

3) Was car checked by mechanic ?
Yes, It was checked under the computer. No errors except of airbag and lock errors.



Additional information (may not be relevant):

I usually drive the motorways, 30miles each way, and the motorway os not too busy so I drive 2300RPM and speed around 50-60MPH all the way. I do not do too many 'city cycles' so I do not expect this to be DPF issue.
However, once a month I smell something like 'old eggs' or 'burnt oil' in the cabin so I always thought that this is DPF burning. The car also took about 300ml of oil after 9 months of driving last year.

From the other hand, DPF (I think so) failure would appear in the computer logs, but it was more or less clear....

Car does not make this weird resonance noice all the time on low rpm, only when it drops suddenly during acceleration on cold engine. Apart of that engine works perfectly fine.
Maybe it accelerates with a bit lag (it takes 1-2 seconds before you feel the acceleration when you start from full stop) but I always blamed automatic gearbox for this. I never drove any automatic car before so I always thought that this lag is the price for comfort you have in traffic.

Carsy

You may have more response from the DIESEL section of this forum.

I would be looking at sensors that are active whilst the engine is cold .

Good luck.

snky1987

This is something mechanic also recommended to start from "Temperature sensor" it costs almost nothing, I will replace it tomorrow and keep this post up to date. Thanks