Slang for a diesel engine manufactured back in the day by GM Diesel (later known as Detroit Diesel, later known as MTU). They were "screamin'" because they were loud, and "Jimmy" being casual slang for "GM." (Also known as "Jimmy Diesels.")
My job in 'Nam was to make sure them Screamin' Jimmys kept screamin' on the patrol boat, otherwise we'd have been sitting ducks for the vietcong.
by Rico Detroit March 2, 2008