My beloved Sprinter motorhome is based on a 1998 410 with the 2.9 5 cylinder diesel engine. She has 280k miles on the clock and still runs well and always starts on first touch of the key. At tick over there's not much to be seen from the exhaust but on acceleration there is a billowing black cloud of soot. I have changed the air filter and put a bottle of Cataclean in the tank. Can anyone relate to this problem and its cause.? All advice will be appreciated.