MILWAUKEE – Days after Valley Cartage, a Wisconsin trucking firm, switched to biodiesel fuel, its trucks started breaking down on the road, according to the Milwaukee Journal-Sentinel. The vehicles’ engines began to sputter as fuel filters became plugged with an unknown, slimy substance. The problem worsened in cold weather. Minnesota was the first state to require truckers to use a two percent biodiesel blend, but the law is currently on hold while operational problems with the fuel are investigated.

Valley Cartage and dozens of other trucking firms say biodiesel has cost them thousands of dollars in engine breakdowns and time lost on the road. Valley Cartage alone changed about 50 fuel filters in a month.

The cause of the engine problems is unknown, and state officials and soybean growers still believe biodiesel is a clean, reliable fuel. Currently, researchers are testing fuel samples and examining clogged fuel filters in an effort to find the core problem.