The fire has burned 3,800 acres and is 25 percent contained. Officials say Camp Williams mobile home park, which had been evacuated, should be out of danger by Wednesday morning.
"It's a lot better than it was when it started, and most of the fuel in this area has burned off, so I feel much better," said Rita Nolan, a Camp Williams resident.
More than 770 firefighters are battling the blaze, which broke out Sunday afternoon in the mountains north of Glendora. Ground troops are getting help from more than a dozen water-dropping helicopters and air tankers. Firefighters hope to have the fire under control by next Thursday.
"It's just going to take awhile because it's so steep in here," said Nathan Judy of the U.S. Forest Service.
The heat also made the fire difficult to tame. Four firefighters suffered from heat exhaustion on Monday. They are expected to be OK.
The fire is burning near Highway 39 at East Fork Road on the north end of the San Gabriel Reservoir. Park campgrounds have been off limits to visitors since then. The fire forced the evacuation of the Camp Williams mobile home park, though about 30 residents stayed behind.
"This morning I woke up, and there was no visible flame," said Kevin Brown, a resident of Camp Williams.
No structures are threatened. Crews worked through the night, but thick brush is fueling the fire. Officials said the area had not burned since 2003.
Highway 39 into the San Gabriel Canyon will be closed indefinitely as firefighters try to get a handle on the flames. The U.S. Forest Service says the road may reopen in a few days to residents only.
The cause of the fire remains under investigation. The cost of battling the Williams Fire is so far $1 million.