The Ford Bronco's inline-six engine firing order depends on the specific engine used. Ford used several different inline-six engines in Broncos over the years. There's no single answer.
To find the correct firing order, you must identify the specific engine (e.g., its displacement – 170 cubic inches, 200 cubic inches, 240 cubic inches, etc.) and then consult a repair manual or a reliable online resource specific to that engine.
