It's a good trend that automotive manufacturers are now moving towards diesel engine powered cars and sedans (average of about 40+ miles per gallon / 17 km per liter). One of them would be VolksWagen and you can truly tell by their pretty aggressive advertising campaign. I even do notice more VW's (in general) running on the roads these days.
The reason why I brought this topic up is that it is part knowledge sharing should anyone be interested in going B100 (100% biodiesel) on diesel automotive engines (and in no part saying anything bad about VW TDI's, they are good products).
It would be great and also especially for the Big 3 (General Motors, Ford and Chrysler) to keep those SUV's and Minivans, but have them run on diesel which is more fuel efficient and more power / weight ratio as demanded by the vehicle capacity.
VW introduced the post ignition injection technology in their TDI engines in order to burn away the soot in the collection area, and hence producing a cleaner exhaust for regular petrol diesel type fuel combustion. For regular petrol diesel (dino-diesel) or at max up to B5, the post ignition injection is ok and doesn't drip too much biodiesel into the bottom engine oil chamber.
For higher levels of biodiesel, what happens is that biodiesel is not as highly combustible as compared to dino-diesel, and the post ignition injection spray results in the wetting of the combustion chamber, and this is not needed especially if you are using B100 (no / very very very little soot), which results in the biodiesel escaping / dripping down from the cylinder walls into the engine oil below, and hence fouling up the cooling + lubricating system. This does lead to possible engine failure.
So if you are shopping for specifically VW TDI's, look out for these used year models which DOES NOT have the post injection technology in place:
The PD version were available in New Beetles (2004 - 2006), Jettas (2004 -2006), Jetta Wagons (2004 - 2006), Golfs (2004 -2005), B5 Passat (2004 and 2005), and the 2005 V10 Toureg. The PD found in the New Beetles, Jettas, and Golfs are rated at 100hp and 177ft.lbs torque from a 1.9l engine. The B5 Passat with it's 2.0l TDI pumped out 134 hp and 247 ft.lbs of torque. The king of power is the 5.0l 310HP V10 Touareg that puts out 553 ft.lbs of torque.
The above is quoted straight from this web site:
Until they or someone figures out a way to disable the post injection electronic controls or substitute the post injection squirt with something else / blank air injection, your best bet would be to buy a young and in good condition slightly older models of the TDI's.