depends what van really, if its a vauxhall 1.7td then its almost certainly the fuel pump (useless korean part), if its common rail then you would get a trouble light on the dash if it was an injector or injector pump so it sounds like its probably the turbo, it may drop bit into your engine, but also another thing they do is that when the seal goes the turbo starts pumping your oil into the engine (which it will happily run on) until you run out of engine oil and the bottom end goes.
Also on a citroen/peugeot I have known the Cat to collapse, 90% blocking the exhaust and giving a similar experience.
good luck mate!