I had a similar problem on my 420SEL, check your secondary air pump (smog pump) It should be located on the passenger side in front of the engine, under the alternator (That's where is is in my car. There are two hoses that run from it. One goes to the air filter housing and the other into the lower engine block. Check to see that the hoses are not broken off. I fixed mine and the problem went away. This is pretty easy to check, you should be able to see any problems clearly. If that is not it, ahve it checked out, it could also be as mentioned above.