Michael Johns -- Enlarged Heart Killed Him

"American Idol" singer Michael Johns died from an enlarged heart, but a "fatty liver" contributed to his death ... this according to the Coroner's report.

The 35-year-old singer died suddenly Aug. 1 after his leg became swollen and bruised ... reportedly from a sprained ankle. Family sources claimed the injury had triggered a blood clot, but his best friend claimed the singer drank himself to death.

The Orange County Coroner's report concludes Johns died of natural causes -- the result of a disease called dilated cardiomyopathy, a condition that inhibits the flow of blood to the body. The disease caused his heart to enlarge.

As for the "fatty liver" ... it can be caused by excessive alcohol intake, but there are other causes as well. Law enforcement told us ... Johns often drank to excess and did so hours before he died.