I've heard that on the fuel filter (of most diesels), there is a screw which you open up at the service interval to drain away excess moisture which has been trapped by the filter. Is this true? What will happen if you don't drain it?
I think that water in the diesel affects the compression cause it compresses differently to fuel.
Bad diesel can have moisture in it and I know that some diesel engines have a drain for this (Mazda 6). The TDDI doesn't though. Not sure how it copes with the moisture but obviously they have some way of getting rid of it.
Not much help I know... :L