Just before the exit point of the exhausts, designed to give enhanced sound. Variable opening from 2500 rpm upwards I think, fully open at 6000.
Usual fault is a motor failure, £600 at main dealer, needs programming with DAS/Star. If you turn off the engine but leave the ignition switched on you'll hear the motor cycling trying to reset.
The flap motors do seem to be a weak point on the car. Mine are ok at the moment but I did have one replaced under warranty a couple of years back. Somewhere on the forum there is a thread about exhaust flap simulators that plug in instead of the OEM flap motor. They trick the car into thinking that the flaps are fully operational so no codes are thrown. They seem like an alternative for anyone who wants the flaps to be permanently open or replace the rear section of the exhaust with something custom. Haven’t tried them myself.
I had one replaced on warranty last year.
Symptoms were - Quiter than normal, intermittent power at high revs, there were no fault codes.
Visible inspection showed that the drivers side flap was always shut, and the passenger side flap was cycling between open and closed.
When they do this, the flap that is shut is causing the problem, and the other cycling flap is attempting to resynchronise position, and this fails due to the stuck flap.
MB replaced the drivers side motor head (Actuator) and all was fine, they also confirmed that at first inspection they separated the flap and actuator to prove that the flap was not stuck.
The part took over a week to arrive from Germany, there was no UK stock, as it wasn't a "Fix first visit" part.
They also commented that the actuators are problematic on other vehicles, but I can't remember the exact model, may have been a GLC ?
My confusion may be the result of my use of the US version of the English language.
As I understand ignition switches; In order to stop the engine you must turn the ignition switch to the Acc or Off position. That in and of itself does not leave the ignition switch turned to the on/run position.
Is the meaning of the instructions to turn the engine off by turning the ignition switch to the Acc position and leave the switch in that position?
“I do not understand how you can turn the engine off and leave the ignition switched on at the same time.”
Start your engine, let it run and then turn it off to the first detante, position 1.
The engine has stopped but the ignition (ie electrics) are still on and active.
You’ll hear the flap constantly recycycling.
That is what I thought but . . . I learned a long time ago not to assume anything and I wanted to clearly understand the procedure in case I had the problem at some point in the future.
The exhaust flaps have several functions that are important to the later model R172SLK 55 V8 engines. These engines are built with direct injection and the ability to shut down four of the 8 cylinders to improve fuel consumption efficiency. When the engine goes into eco-mode , four-cylinder, it changes the back pressure level and can cause a problem with maximizing fuel efficiency. In four-cylinder mode the flaps are closed to modulate back pressure to the optimal level . In eight cylinder mode the flaps open to improve overall engine performance by decreasing back pressure. I’m not sure about how the flaps operate in between the two extremes but it sounds like they are variable depending on multiple data points such as MAF measurements and throttle position. I don’t think a workaround is a good idea unless you want to drive in V8 mode all the time.