Be careful when starting up your cab that no one is standing next to the exhaust pipe... They will be splattered with black oily soot. This usually clears once the engine is running. Exhaust gunge can be a problem when trying to get your cab through it's MOT. Apparently you can add engine cleaning solutions such as 'Millers Diesel Power Plus' or 'Red X' to help clean up your act. Here's a message I found on a forum somewhere about this problem..
"Put some injector cleaner in the tank and drive the thing for 100 miles to get it nice and hot. Then replace the air filter and fuel filter. Make sure before you take it in for test that you have given it a good long run, plus ask yourself when was the cambelt was last changed as they have to rev it to maximum for the smoke test..."
A chap by the name of "The Dodger" who is running a black cab in Australia gives this bit of useful advice though..
"There are some additives 'Spitfire' for one. This goes into the diesel input at prescribed dosage. However, I had this problem with my FX4S and on good authority I was advised thus... Get the engine really warm, park the cab away from walls and fences etc. turn on the garden hose to medium flow and as the engine runs, push the hose up the exhaust pipe as far as the baffles and let it flow until it starts to squirt out. Give the throttle several bursts every 30seconds or so. The stuff that comes out is gobsmacking, so stand clear. When it's just clear water, take out the water house and keep the engine running until no water comes out. Do it again the day before the MOT test is booked. My cab failed the strict EPA in Australia the first time at $1000.00 a test!! Then someone told me about the above and next time it passed!!!!"