A dry fall and piles of uncollected leaves have caused three cars in Arlington to catch fire in the last week, prompting county officials to warn motorists to keep idling vehicles away from autumn foliage.

It took firemen 1 1/2 hours Wednesday night to put out a blaze on North 26th Street after Paavo St. Dennis pulled his family's 1971 Chevrolet station wagon onto a pile of leaves one to two feet high.

The next day, a 1981 Jaguar caught fire at the corner of Rock Spring and Glebe Road, causing an estimated $10,000 in damage. A third vehicle, a 1981 Chevette, also burned on Guinea Road last Sunday night.

"We've had a few more automobiles going up this year," said Arlington Deputy Fire Marshal Buck Swartz. "I attribute it to the elements. It's been an extremely dry fall and the leaves have come down earlier."

The combustions have been caused by hot exhaust systems that can take 10 minutes to cool after a car's engine is shut off, county officials say. Only one of the three cars burned in Arlington was equipped with a catalytic converter, an anti-pollutant device considered a main cause of auto fires.

Clarence Ditlow of the Center for Auto Safety said spark plug misfires are the principal reason for overheated converters, which, like regular exhaust systems, reach an average temperature of 400 degrees under normal operating conditions. Misfires can raise temperatures to 800 degrees, he said. Piles of dry leaves, grass or brush ignite at temperatures as low as 575 degrees, according to a 1975 study in Los Angeles County.

A General Motors spokesman said yesterday that warnings about parking over dry leaves or grass have been put in the owner manuals for all new cars and that a 1975 government study concluded the anti-pollutant devices were not a hazard.

Arlington is not the only local jurisdiction to report cars igniting dry leaves this fall, although other fire department spokesmen were not able to say whether the number had increased. In Prince George's County, two burning cars have been reported, both equipped with catalytic converters.

In Arlington, the auto fires have increased complaints about leaf collection this year. County officials admit that collections have fallen behind schedule, but they blame delays on the sudden accumulation of leaves earlier this month.

"We're using the same number of trucks for the same level of attack," said Dennis Johnson, operations divisions chief in the Department of Public Works. "The problem is all the leaves fell at once, on the 4th, 5th and 6th of November."

Johnson said the county, which has deployed its $300,000 leaf collection operation slighty differently this year, has cleared more than half of the streets so far.

"It's never gone this long," said St. Dennis, who said some leaf piles on North 26th Street have reached five or six feet. "The main thing is that the neighborhood didn't go up in flames."