James F. Isabell, 23, of 3432 Walnut St, Port Huron, died early Sunday in Mercy Hospital of injuries suffered when his car left Gratiot Blvd. and struck a tree.  He was born Jan. 21, 1946 in Port Huron and was graduated in 1964 by Port Huron High School.  A US Navy veteran with service in Vietnam, he was honorably discharged Jan. 10. He was a parishioner of St. Mary's Catholic Church.  He is survived by his parents, Mr. and Mrs. Robert C. Isabell; a sister, Miss Janet M. Isabell; a grandmother, Mrs. Gertrude Isabell, all of Port Huron; two uncles, Jack S. Isabell, North Miami, FL, and Robert E. Lohstorfer, Anaheim, CA; three aunts, Mrs. Harley Rowbotham, Warren, MI; Mrs. Paul R. Jones, Port Huron, and Mrs. Richard Forshar, Grand Rapids.