David and Eileen Wetzel converted a 1986 Volkswagon Golf to run on vegetable oil as fuel a couple of years ago, and have been driving around with it for a while now. The Illinois Department of Revenue is investigating them for criminal charges, mostly for not paying tax on fuel that the car doesn't even use, retroactive to the point at which he re-worked the car's engine. The couple (who are in their late 70's) had to post a $2500us bond (no mean feat when living on a fixed income, as many retired people do), and have to pay $244us …