According to the 'King of the Hill' star's autopsy report, obtained by TMZ, Johnny was found dead in his bathtub back in August -- and the water was running when he was found ... but the drain was open and he was not actually submerged.

The M.E. says there were no obvious signs of trauma ... adding that a toxicology test showed a "presumptive positive test for cannabinoids," with the report noting drugs were found at the scene -- although it's unclear what sort of substance, precisely, was discovered.

With all that said, Johnny's cause of death couldn't be determined -- this due to the "extensive decompositional changes" his body was going through when discovered. Another interesting nugget -- the 5'7" voice actor weighed just 100 pounds at the time of discovery.

TMZ broke the story, Texas officers were called to his home in August for a welfare check, which is when they discovered his body -- he was pronounced dead at the scene, and no foul play was suspected at the time.

Johnny lent his voice to the animated show for 258 episodes of the long-running series, famously playing Dale Gribble ... only missing one episode in the show's entire run. He was also cited as returning for a supposed revival of the show, but it's unclear if he ever recorded for it before his death.

Fans mourned the loss of Johnny soon after he was pronounced dead -- and Mike Judge, co-creator of the series and voice of the show's lead, Hank Hill, told us he was "deeply saddened" by his passing.

He was 64.