▁used ▁to ▁operate ▁dies el ▁vehicles ▁using ▁straight ▁veget able ▁oil . ▁ ▁Cont am ination ▁by ▁water ▁B iod ies el ▁may ▁contain ▁small ▁but ▁problem atic ▁quantities ▁of ▁water . ▁Although ▁it ▁is ▁only ▁slightly ▁mis cible ▁with ▁water ▁it ▁is ▁h yg ro scop ic . ▁One ▁of ▁the ▁reasons ▁b iod ies el ▁can ▁abs orb ▁water ▁is ▁the ▁pers istence ▁of ▁mon o ▁and ▁dig ly cer ides ▁left ▁over ▁from ▁an ▁incomplete ▁reaction . ▁These ▁mole cules ▁can ▁act ▁as ▁an ▁em uls ifier , ▁allowing ▁water ▁to ▁mix ▁with ▁the ▁b iod ies el . ▁In ▁addition , ▁there ▁may ▁be ▁water ▁that ▁is ▁resid ual ▁to ▁processing ▁or ▁resulting ▁from ▁storage ▁tank ▁cond ens ation . ▁The ▁presence ▁of ▁water ▁is ▁a ▁problem ▁because : ▁ ▁Water ▁reduces ▁the ▁heat ▁of ▁fuel ▁comb ust ion , ▁causing ▁smoke , ▁harder ▁starting , ▁and ▁reduced ▁ ▁power . ▁ ▁Water ▁causes ▁cor ros ion ▁of ▁fuel ▁system ▁components ▁( p umps , ▁fuel ▁lines , ▁etc .) ▁ ▁Micro bes ▁in ▁water ▁cause ▁the ▁paper - element ▁filters ▁in ▁the ▁system ▁to ▁rot ▁and ▁fail , ▁causing ▁failure ▁of ▁the ▁fuel ▁p ump ▁due ▁to ▁ing estion ▁of ▁large ▁particles . ▁ ▁Water ▁free zes ▁to ▁form ▁ice ▁cry st als ▁that ▁provide ▁sites ▁for ▁nucle ation , ▁acceler ating ▁g elling ▁of ▁the ▁fuel . ▁ ▁Water ▁causes ▁p itting ▁in ▁p ist ons . ▁ ▁Pre viously , ▁the ▁amount ▁of ▁water ▁cont amin