One problem: Biodiesel is a solvent that releases gunk accumulated in the fuel tank or system. Fill the tank with it, and that gunk will probably clog the truck's fuel filter. Change the filter, though, and the truck should be ready for continued use. A bigger problem is that pure biodiesel attacks natural and butyl rubbers, so manufacturers will have to eliminate those materials from fuel systems if they're going to certify their diesels for use with B100. Blends of B20 and lower don't have that problem.

Also, biodiesel, like any other vegetable oil, will go bad if left to sit long enough. You need to be sure you have a supply of fresh oil, as old oil can spoil. It also absorbs moisture, which can corrode fuel-system components.

Biodiesel is even more prone to gelling in cold weather, making its use a challenge in winter. Blended B20 fuel may plug the truck's filter at two to 10 degrees higher temperatures than regular diesel would, according to the National Biodiesel Board. Pure biodiesel gels sooner than petrodiesel, and the solutions are similar as for #2 diesel: blending with #1 diesel, using fuel heaters and parking indoors, and using a cold-flow improvement additive. The Rocky Mountain National Park already has addressed the cold-weather issue, though. Its government vehicles burn B20 even in winter, points out Loren Beard, senior manager for environmental and energy planning at Daimler-Chrysler.

The possibility of depending less on petroleum is appealing, so burning B100 would be ideal. Unfortunately, it's tough to find a suitable supply of pure biodiesel, and your truck's manufacturer isn't going to support your environmentalism. "We strongly recommend against [using B100]," says Beard. "To do it, you would have to replace a lot of elasomers and make some changes to the fuel pump. B20 is a small enough departure that the vehicle can deal with it." And, while stories are common of running on B100 without any trouble, a fact Beard acknowledges, "I wouldn't do it if it were my truck." Richard Baker, senior technical leader on combustion and emissions in powertrain research and advanced engineering at Ford Motor Co., explains what can happen on B100. "The kinds of things that'll show up are clogged fuel filters, water pickup, and corrosion in fuel pumps and lines. The fuel system is intricate, and [it] can lead to a significant, expensive overhaul."

We may have to be satisfied burning blended biodiesel, and even then probably the lower B2 and B5 mixtures for a while until better B20 guidelines have been established. Manufacturers are solidly behind use of B5, and Chrysler even shipped new Jeep Liberties from the Toledo, Ohio, plant with that fuel. "Using B100 takes an awful lot of handholding and precautions such as changing out hoses," says Steve Howell, technical director of the National Biodiesel Board. If you want to convert, he explains, "be absolutely sure the supplier is using ASTM-certified products. We know that bad biodiesel can cause problems. The best way to feel confident is to use suppliers who've gone through NBB certification."

What about B20? The trouble is that there's no standard agreed-upon specification for the fuel as there is for B5, because of the need for careful handling to make sure it isn't too old.