The debate rages on about washing or not washing biodiesel, and proponents on each side of the argument swear that they are right. Further splitting the argument are the many methods of washing: should it be a water wash or a chemical wash?
We have found water washing biodiesel to be a time-consuming process, unnecessary for our purposes. We run our homebrewed fuel in older mid-eighties Mercedes-Benz diesels that have indirect-injection engines without sensors or computer controls—and they are quite tolerant of unwashed fuel. However, if the homemade biodiesel will be used in a modern computer controlled diesel engine, we recommended considering and investigating the procedure. Washing the fuel pulls out excess unreacted lye and methanol that can cause false readings from or even damage sensitive and delicate fuel sensors on computer-controlled engines.
Another benefit of washing biodiesel is improving its cold weather stability. The washing procedure pulls out minute suspended molecules of soap that may have formed during processing of the fuel. In warm climates, these particles remain small and dispersed in the fuel and they pass harmlessly through the vehicle’s fuel system. However, in cooler weather, these particles begin to thicken and cling together. As the temperature drops toward freezing, they actually begin to coagulate and cause the fuel to thicken into a soupy slurry that plugs the fuel filter and won’t flow through the fuel system. Journeytoforever.org offers a series of articles that discuss the pros and cons, as well as several methods of biodiesel washing.