A diesel exhaust will always look 'dirtier' than a petrol. If you have it on tickover and floor the accelerator (whilst in neutral!), does it sound as if one cylinder is missing? If so, could be you have got a dodgy injector.(This would also account for the smoke). You should be able to get the van tested (on a Crypton or similar) at a decent garage, or look in Yellow Pages for a diesel engine specialist. Injectors for my truck are about �30 each exchange, so hopefully for a van they would be about �10 each, but don't hold me to it!!