Sheriff's deputies received a call around 8:10 a.m. about a Toyota Sedan parked on Mt. Hamilton, about two miles south of Joseph D. Grant County Park.
Arriving deputies found the Toyota with a man in the backseat, Sung said. They also detected a strong chemical odor emanating from the car and called a hazardous materials team with the San Jose Fire Department and Cal Fire for assistance.
The man appeared to be in his early 20s, Sung said. There was no indication of foul play.
The sheriff's office is investigating the death as a suicide.
The California Highway Patrol helped with the initial road closure on Mt. Hamilton Road near Quimby Road, CHP Officer Peter Van Eckhardt said. The road was reopened around 2 p.m.