my citreon dispatch,smokes when cruising along. PLEASE HELP!!!!!?

PLEASE PLEASE HELP!!!!! My dispatch van smokes when cruising along at same speed , i have had two garages look at it and they say it doesnt seem to be the pump!! It has had a service with all filters etc changed , timeing checked twice , injecters just been re-tiped and set , hpi reading fine (blowing daisies) so one said. Tried blocking off ERV valve no success.Its a lovely van very clean , and drives nice , but i cant afford to do much else can anyone HELP??????????????????? its 1.9 Diesel non turbo Treg.