What is the firing order of spark plugs for a 6- cylinder 1998 F150?

The firing order for a 1998 Ford F-150 6-cylinder (likely a 4.6L V8, as a 6-cylinder inline isn't common in that year, and the 4.2L inline-6 was phased out earlier) is 1-5-4-2-6-3-5.

However, it's crucial to double-check this with your specific engine's specifications. The firing order can vary slightly depending on the exact engine code. Your owner's manual or a sticker on the engine itself will provide the definitive answer. Using an incorrect firing order can cause serious engine damage.