Irrespective if it was run the day before, it it's stood overnight in sub-zero temps it will still end up absolutely stone cold in the morning, and the glow plugs will still be activated for about the same length of time, so I'd think they're probably OK.

It could possibly be a fuel filter issue -- there may be some muck in there. Or it could be a dirty connector on something like the cam or crank position sensor ... but that would probably throw a fault code.

You could try a tank of premium fuel (Shell V-Power or Ultimate) to see if the detergent additives etc make any difference. For the sake of an extra £3 or so it can't hurt. Unfortunately, we probably won't see too many more sub-zero temperatures until next winter now ....

It's almost impossible to nail a problem like this "over the wires" but I'll give you the benefit of a recent VAG diesel problem I solved:

Firstly, you don't give a year or engine type, but I'll assume it's a PD fitted with the cannister fuel filter with the "thermostatic Tee". The function of this device is to maintain a reasonable level of hot fuel within the filter to prevent waxing in cold weather. When the fuel gets to about 50 deg C, the valve in the Tee operates and the fuel is returned to the tank - under these conditions filter heating is not necessary.

All well and good - it works OK - until a filter is fitted which is improperly made.

The filter in question was a "genuine" (they don't make filters of course) VAG unit made in China. To cut a long story short, the bore for the thermostatic Tee was incorrectly formed, allowing air ingress at this point. This allowed air locks to form over prolonged periods giving rise to poor starting. Hot or short-term starting was unaffected.

Chinese components do not meet my own standards of fitness for purpose and I won't use them. Apparently VAG find enough of them to be satisfactory.

Having been in the business of diesel systems component testing in the past, there are only two makes of filter I would ever fit to a diesel engine - Mann & Hummel or Bosch.

When changing this filter pull the outlet hose half off and spray the joint liberally with clean solvent (WD-40 will do) before parting the joint. Then transfer the hose immediately to the new filter having removed the protective cap. This is to minimise the possibilty of dirt entering the clean side of the system. Use the new "O" rings provided with a replacement filter when fitting the thermostatic Tee - the blue one goes furthest away from the filter can. Lubricate the "O" rings with engine oil before final assembly.

659.

Edited by 659FBE on 06/03/2012 at 16:40