Jay Leno told TMZ how he lit his face on fire.  He was fixing a clogged fuel line on a “Steam Car” that was made by the White Motor Company in 1907.

He accidentally sprayed gasoline on his face and hands . . . and at the same time, a spark triggered an explosion that lit him up.

Jay says a friend quickly jumped on him and smothered the flames, but he still suffered third-degree burns on his face and hands, and might need skin grafts.  He’s still in the hospital, and he might have to stay there into next week.