As a follow-up; after running my '53 plate Ford Focus TDCi 10,000 miles on 50% biodiesel (NOT veg oil), I would say it was trouble free on B50 (50/50 mix with normal diesel / DERV).
Cold starting is the main issue, summer or winter:-
A full set of good working glowplugs are essential for biodiesel or you will get copious amounts of white smoke at startup. I had one good working plug out of 4 when I first tried bio. I got away with it with DERV, but on BIO it still started... but produced theatrical volumes of white smoke like a bonfire for first 20 sec, and then fine thereafter. Glowplugs renewed, now OK!
Cold starting tips for TDCi on biodiesel:-
Turn the key to pre-heat position for 4 seconds & off again, 3 times, to guarantee 12 seconds of heat before you start. The engine may need cranking for slightly longer than with normal diesel (DERV).
DON'T touch the accelerator during cranking, or for first 20 sec after starting. White smoke is partially burnt fuel, this may be due to the much higher flash-point of bio making it harder to ignite.
Adding Wynn's Supremium diesel additive slightly improved cold starting, or diesel additives based on hydrotreated light petroleum distillate.
IMPORTANT: Change the engine oil & filter at 6,000 mile intervals when running on high bio blends. All fuels (petrol, diesel, biodiesel) tend to migrate into engine oil, but bio doesn't readily boil off so collects in engine oil diluting it and reducing lubricant properties over months. Focus Oil & filter change only costs 25 quid at high street fitters.
Then......
I decided to try upping the mix to B100 (100% bio) in summer months... the cold starting was awful... needed 3 or 4 tries kept dying out, gave off lots of white smoke if accelerator pressed to try keep it alive, but once up and running warm it ran on it fine AT FIRST. However, after about 1,000 miles running on B100 problems started to appear - the car started losing acceleration above 3,500RPM in 3rd gear, and did not improve when run back onto normal diesel. It may be the injectors clogged as changing fuel filter (20 quid) made no difference, but running high strength Wynn's Diesel Clean-Up (8 quid) thru the fuel filter seems to have helped.
So take my advice: B100 in a TDCi is a NO-NO. Stay at B50 or below.
I don't object to these costs as I view it as funding experimentation for the greater good, feeding back these observations here on the web. If you're buying a car specifically to run on B100, search the web for makes that are guaranteed by manufacturer for B100. Hope this answers your question!
TDCi
June 2008