From an ESPN story at the time:

Porter, 50, who caught for four major league teams and was the MVP of the 1982 World Series with the St. Louis Cardinals, was found dead Aug. 5 next to his car in a park.

Jackson County medical examiner Dr. Thomas Young said Porter had a level of cocaine in his system consistent with recreational use. He would not be more specific, and said it was unclear when Porter had taken the drug.

Porter did not die of an overdose, Young said, but of a condition called "excited delirium," which causes excessive body temperature, "behavior that is agitated, bizarre and potentially violent," and stopped Porter's heart.

Young said heat exposure and an enlarged heart, which is common among drug users or those with high cholesterol, contributed to Porter's death.