I have replaced the MAP sensor close to the throttle body, the pressure sensor on top of the intake tube, the diverter valve, and the purge line as well on my 2012 C250 1.8l turbo.
I used to have codes p0299 and p0238 at different times and by replacing the parts I was able to clear the codes. However, the car still revve bad when getting my foot off the gas pedal. 120k miles. there are no codes on my MBII obd other than the p0340

If I'm driving automatic, D, The engine revve at above 50miles per hour and when switching to manual 6, the engine sounds better and less noisy and when switching to 5th gear at about 50 miles per hours would sound the quietest but when switching to 6 gear from 5 would get louder and back to D would be the loudest

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???
Are you saying that the engine is turning higher RPMs in D than in 6th and then higher in 6th than in 5th? That makes no sense. Or it just that it’s noisiest in D and quietest in 5th?

I’m not trying to be sarcastic or to insult you, but you have been working on this auction car for about a year now. Maybe drive it over to CarMax and see what they will give you for it.

 

The car drives good but after the over and lower boost codes I had and replacing the parts I mentioned, the engine is still noisy on D and I have to shift it to 6th and maybe 5th to be quiet. The RPM does not get high when the engine is loud at that speed. I shift to 6th which is less noisy and 5th is the best.
it just that it’s noisiest in D and quietest in 5th?
I know I am trying to resuscitate the car but I got rid of all the codes it had except the p0340 which I know does not have anything to do with the issue now.....
Basically I spent time and money on the car and going to go further until this last issue is solved. I do not get it why the OBD MBII does not give me a code anymore for the over boost which I think is the issue but not sure. I do not know when the engine gets loud means over boost. The car is drivable but annoying when engine gets loud.
The transmission is very smooth shifting. there is no noise when shifting but I wonder if the fluid might have to do something with it. Does it?
One last detail about the issue< I am driving 6thgear at 70mph and then shift to D, the noise that come out from the engine as if I was driving standard car and shifting from 5th to 2ed gear at high speed except the jerking and the speed maintained

 

The symptoms don’t make sense to me. It should be quietest in D7. At 50 MPH the engine is only turning about 1500 RPM. Lack of codes is puzzling. Have you considered bringing it to an independent Mercedes shop for diagnosis only?

 

Should I have not gotten codes had something wrong with car other than the camshaft and especially by using an OK OBD scanner? Don't you think the transmission oil has anything to do with it? When air is going through the pipes into the intercooler and then to the intake through the throttle and throttle closes, does the air get trapped in the pipes and intercooler and thus the loud engine with slight vibration but again there is no codes.
I am going to look for a shop and will post the result

 

I don’t see how it could be the transmission. Anything there should set a DTC and maybe turn on the CEL.

I think it has something to do with either the air intake or the turbo output. But you have replaced the bypass control valve and the MAP sensor. If something is resonating because it’s restricted or loose, maybe no DTC.

Looking for a shop is a good idea because someone experienced with these cars may have seen or heard it before.

 

I am still looking for a shop that can check it for me,
I cruised at 70 mph and drove it on D for a few minutes and the RPM was at 2200, then I maintained the speed and shifted to 6 gear and the RPM was at 2500 , then shifted to 5th gear and the RPM was at 2800. the loudest engine was when was On D.