Have just bought a U.K spec e300, its the straight six 3 litre, it will tick over and run fine, then the engine raises the tickover speed itself ( and you can hear a hissing noise) when its like this it runs kike a dog and black smoke comes out the exhaust. the R.A.C recovery guy was stumped, and reckons it is running way too rich when it does this. Have only just bought the car after waiting ages please help!!!!!!

P.S had an M.O.T test two weeks ago is it possible someone has been tinkering to get it through the emission test????