I thought i'll make an simple troubleshooting guide if you're dealing with vacuum leaks/idle issues instead of scrolling through endless threads.

If you're experiencing a high/hunting idle in park/neutral and also increased fuel consumption, the chances are very high that you have a vacuum leak.

I suggest to start with the following to pinpoint if you might have a vacuum leak. When the engine is running on high idle, take off the oil filler cap. If there is no vaccum leak, the idle should change ( because you're now creating a massive vacuum leak ) and the engine might stutter and the revs should go down momentarily. If there is no change to the idle when removing the oil cap, you for sure have a vaccum leak, 100%.

The best and safest way to troubleshoot this is with a spraycan filled with water. Only use carb cleaner when the engine is cold! I have heard that there is a chance that it might catch on fire with an warm engine.
When you spray water on a leak the engine should shutter and the revs go down.

One failure point with the m112k engines and vaccum leaks is the breather hose ( part nr 1120100382 ) that connects beneath the throttle body ( make sure this one is completely plugged in to the throttle body, especially if you have removed the TB for cleaning etc.
This same hose connects to both valve covers and might get brittle over time. The problem is that it's impossible to remove and replace without taking off the TB. Also remember to replace the TB gasket ( part nr 1121411380 ) it might be fine to reuse, but better to just replace it with a new one.

When removing the TB, have a look a the IAT sensor that is plugged in to the metallic y-pipe further down. This y-pipe connects to both intake manifolds. There have been issues where the IAT sensor has popped off and caused idle issues. See this thread for pictures and more information ( https://mbworld.org/forums/c32-amg-c...low-input.html )

Another hose that some members have had problems with is the metallic hose that connects to the supercharger, just under the TB and left intake manifold. See this thread for a better explanation and a picture ( https://mbworld.org/forums/c32-amg-c...idle-more.html )
Also check the hose that comes from the plastic Y-pipe that is connected to the TB. This hose runs on the right side to the valve cover when standing in front of the engine.

I have read threads where members have more problems than only an idle problem. Error codes indication P0120, ESP error, no throttle response, hunting idle etc. If you already have checked all vacuum lines and haven't found anything, the problem might lie in the throttle body harness ( Part nr 1131500028 ) When reading old threads this problem seems to occur when disconnecting the harness from the TB too often and it causes the pins to come loose on the female connector causing all sorts of problems. Also make sure that the female connector is thoroughly pressed in. Before connecting this to the TB again, you might use some electric cleaner on both sides.

If you have removed the TB for cleaning and moved around the butterfly when doing so, you might have to reset the idle for the TB. This is very easy. Just turn the key to position 2 where all the lights come on to the dash and let it sit for a minute or so. You will hear the TB adjusting. When it's done, go ahead and start the car.

Maybe we could make this thread a sticky, and if others with their expertise would like to add more information, this would be great!

 

Thanks for this information. I'm getting ready to go down this rabbit hole.