On my old mk1 it rarely got hot enuff for the cooling fans to kick in,had to be in traffic for ages. Seem to recall it being to do with diesels (in general)not producing much waste heat which is why mk2s have the aux heater to help things along. I would guess that if its going to overheat it would have by now. Mpg has improved because its now running at operating temp ,not on cold settings(like leaving the choke in,anyone remember those??)