406 TD Black smoke acceleration, standing fine?

Hi. I've got a peugeot 406 TD, 1997, 145k on clock. standing, it blows out fresh air from exhaust. on low revs on acceleration, or generally accelerating when turbo kicks in, belches black smoke. i've changed air filter, fuel filter, oil filter. flushed the oil on oil filter change, changed oil twice and put injector fuel additive in fuel tank. Still no joy. Don't know if electronically related, but rev counter sometimes kicks in on start up and stays on until i turn key off again. when revs working, black smoke doesn't seem as bad?! might be coincidence. would really appreciate advice on anybody that has had exact same problem or mechanic please?! cheers! Chris