It could be that the valves were not adjusted properly last time - e.g. using the cold clearance values on a hot engine or vice versa.

Also, the valve clearances are held with a double-nut setup. The top nut (a cap nut) is adjusted to spec, and the bottom nut is tightened against it to lock it into place. If the nuts weren't tight enough against each other, they could un-adjust relatively quickly (i.e. over 1500 miles).

I just hate to see diesel owners (and their cars) get burned by mechanics who don't know about these engines. I speak from (almost) experience, having dealt with a guy who told me I needed a new timing chain when all I needed were clean injectors, and also told me I needed a new rear differential when all I needed was rear swar bar linkages. It sounds like your mechanic has been around these engines before, in which case it's just fine to trust his judgment on a bad or burnt valve.

Also, injectors can be rebuilt if you find a shop willing to do it. I had all four of mine done for around $100 by a local shop about two years ago.

- Nathan
'83 240D "Steiner"
'00 New Beetle TDI