The vehicle spewed burning carbon from its exhaust system, igniting several fires on Friday on Oak Glen Road in Cherry Valley.
Authorities are asking anyone who may have seen such a vehicle at the time to contact investigators, according to a statement from the California Department of Forestry and Fire Protection.
The blaze in Riverside County, among several wildfires across California, had consumed more than 41 square miles of dry brush and timber since it broke out on Friday evening, fire officials said.
As of Monday afternoon, it was just 5% contained and the fire along with coronavirus precautions made for added stress at an evacuation centre, said John Medina, an American Red Cross spokesman.
Volunteers used to “close contact” with evacuees have had to adjust their approach during a time of social distancing, Mr Medina said.
“I mean, that’s part of the recovery of a disaster, is that you have to show warmth and love and caring. And that’s hard when you’re standing 6ft away. So that’s the biggest challenge,” Mr Medina told KESQ-TV.
Bill Boehm helped his mother evacuate her home with her three horses and said the pandemic has not stopped people from coming together to assist each other.
“Everyone seemed to be wearing masks and such, so that type of precaution was still going on,” Mr Boehm told the news station.
The blaze began as two adjacent fires in a rural area near the city of Beaumont, about 85 miles east of central Los Angeles.
Flames raced along brushy ridge tops and came close to houses while firefighters attacked from the ground and air.
One home and two outbuildings were destroyed, California fire officials said. No injuries were reported.
In northern California, evacuations were ordered after a wildfire that sparked on Sunday afternoon spread quickly and burned near homes near the East Park Reservoir in Colusa County.
Complicating firefighters’ efforts were temperatures in the area north of Sacramento were expected to hit 36C by Monday afternoon.
On California’s central coast, a fire in San Luis Obispo County was 60% contained on Monday after burning more than two square miles of brush east of the community of Santa Margarita. Two structures were destroyed but officials said it was unclear whether they were homes.
