Try putting Veg. oil or Biodiesel through a Delphi pump-equipped diesel and you'll learn a VERY EXPENSIVE LESSON
You caneasilymake Biodiesel from waste cooking -oil...a lot of older diesels will,indeed, run well on pure "chip-fat"... otherwise , filter, titrate and convert properly.. If you get your titrationwrong, you can get a residue of NITRO-GLYCERINE instead of ordinary Glycerine. -not recommended for a longand healthy life.....YOU STILL CAN'T USE IT IN A LOT OF MODERN DIESELS especially Common-Rail.
UK pump-diesel is now mandated by law to have , IIRC, 5% bio-content.
Research the problems with the Renault engine....fitted to :- Mazda6, Volvo V40/ S40, Renault Megane /Espace..Mini diesel...........Don't take my word, it's all out there onthe motor forums....yes, I know I'm a Saab dastard as well.
