Knocking & power loss in my pre-owned W203 Mercedes-Benz C200K

My mechanic who does work on German cars and has decent experience in the field is clueless as as to what the issue is.

BHPian joygoku08 recently shared this with other enthusiasts.

Hi fellow BHPians, I am new to the forum as a member, however have been a long time follower of Team BHP.

I did try to search for the issue that I am facing related to my car but was not able to find any information within the already present threads.

I own a 2007 W203 Mercedes C200 Kompressor 6 speed manual (Facelift version & the last batch of W203 in India).

I bought the car back in March 2022, and got the car completely serviced recently which included complete engine oil drain, intercooler drain, brake fluid drain, new oil filter, air filter, spark plugs, mechanical arms & bushes, brake discs and pads. The car when it runs, runs like a charm but occasionally knocks at idle. When this happens, irrespective of whether AC is on or not the RPM fluctuates from about 950-1000RPM (Ideal idling RPM) to about 600 or 700 RPM. When this happens, I can revv up the motor to stop the car from shaking/vibrating, but if I don't do so, the motor will eventually completely stall in neutral or will self recover. When this happens, I also experience immense power loss, and the car feels like the supercharger is not running.

I got the V-Belt checked and its as good as new. Can't find any issues with the belt tensioner either. No error related to the supercharger flap came up when scanned. I am also aware of timing chain issues with the m271 engine but the car has only run around 52,500KMS in 15 years of its life.

Since not many of these cars were sold in India (even if they were, I barely see any on the streets of Kolkata), I cannot find any one to guide me on what the issue is here. My mechanic who does work on German cars and has decent experience in the field is clueless as well.

We have checked the vacuum pipes. Found one damaged by a rat. That was fixed, but the intermittent knocking & powerloss remains.

An update: I got the car scanned and turns out that there is misfire in cylinder 1 and 4. I opened up the engine cover and noticed that the ignition coil on cylinder 1 is snug fit using some kind of butter paper. (Hate how the first owner kind of people treat these German beauties).

There is also an error related to the grounding on the O2 sensor. I am getting my car worked upon on Monday and will get the fuel lines checked too. Ignition wiring also will be checked.

I read on certain Mercedes forums that a coolant leak can block the O2 sensor and the coolant pipe did have a leak when I initially bought the car. There are also reports of a leak in the Pressure Control Valve located below the air filter.

Thanks everyone for your advice.

Here's what BHPian SS-Traveller had to say on the matter:

Check for water in fuel - I am presuming you use E10 petrol. Drain the fuel lines and tank, and refill with fresh fuel.

Here's what BHPian Jaggu had to say on the matter:

Get the plug wires, coils and even the new spark plugs checked once. Knocking can be a timing or spark related issue also.

Here's what BHPian Jeroen had to say on the matter:

This is not engine knock. This is a problem with just idle running. These cars, for some reason are susceptible to poor idling and it can be a real bear to trouble shoot.

Check the dedicated Mercedes forums for some ideas.

Yours appears to be intermittent, which makes it more difficult to find. Usually it is related to either a vacuum leak somewhere in the air intake or a wonky electrical connection.

Here is what I would do: when the poor idling occurs, open the hood. With the engine running start to tuck gently at all the parts in the air intake circuits. See if anything moves, listen whether the RPMs change. When the RPMs change you are tugging/pulling on something that is either leaking or not connected properly, or corroded.

Ideally, spray starting gas on all connections and joints in the air intake. If the RPMs goes up, you have found your problem.

Not quite sure, but if this engine has an idle air valve, or auxiliary air valve or whatever Mercedes might be calling it, I would be suspecting that too. When it idles poorly, just pull the plug on it. If the engine runs fine, you have identified the problem.

Good luck, let us know what you find.

Jeroen.

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