MANKATO — When the temperatures turn arctic, car batteries are tested, vehicles take longer to warm up, and snowblowers need a few extra pulls to start.

If you own a vehicle with a diesel engine, bitter cold brings an added set of problems — gelled up fuel that can leave everything from pickups to big rigs dead in their tracks.

“You don’t think of it until the severe cold hits,” said Dennis Heinze, co-owner of Lake Crystal Coaches.

Across the state there were big increases in fuel problems in school-bus fleets, semi trucks and other diesel-powered vehicles. Some point to biodiesel as the culprit, but most say it has more to do with how people mix their diesel fuels.

Gary Schmit, owner of Minnesota Truck & Tractor in South Bend, saw a rash of problems.

“We had a bunch of them. When that cold snap hit, we were changing fuel filters like crazy and they were all gelled up,” Schmit said.

Diesel’s benefit is it provides more horsepower per gallon than gasoline. One of its drawbacks is gelling in cold weather.

There are two grades of diesel — No. 1 and No. 2. The No. 2 diesel is cheaper but gels more easily because of naturally occurring paraffin wax in it that crystallizes and gels fuel in cold weather.

During the winter, No. 1, a more refined diesel, is mixed with No. 2 to lessen gelling. Fuel additives also help.

But some have pointed to biodiesel, made of soybean oil, as a possible culprit. By law, much of the diesel sold in the state must contain at least 2 percent biodiesel. Some say the biodiesel raises the temperature at which the diesel begins to freeze.

John Schaffer, of North Central International in North Mankato, has been doing more repairs of semis recently, but he doesn’t blame biodiesel. “Diesel fuel just gets thick when it gets cold.”

Schmit said the problem seems to be diesel mixtures that have too much No. 2 diesel.

Some gas stations premix the two diesels during the winter, while some truckers use hoses from two pumps to blend the fuels themselves.

“The more No. 2 you have, it’s cheaper and you get more power. So there’s an incentive to use that mix,” Schmit said. “But if it goes from 30 above to 15 below, you need to put more No. 1 in. A lot of people get caught with the wrong mixture in when it gets cold.”

A spokesman for the Minnesota Department of Commerce said there is no evidence biodiesel is adding to the problem. The Minnesota Trucking Association also said there is no evidence pointing to biodiesel.

Heinze said they hadn’t had problems with their 16 large coaches, but they did have gelled filters in their smaller mini-coaches.

“Once you replace the fuel filter and use the right mixture, they’re fine,” Heinze said.

Several school bus companies in the Twin Cities had stalled buses last week during the cold snap. The Bloomington district had to close school Friday because of bus problems.

A spokeswoman at Palmer Bus Service, which provides school bus service to the Mankato district, said they have had no problems with gelled fuel.

The state has mandated the biodiesel blend increase to at least 5 percent starting this spring.