1996 Ford Bronco Owner's Manual - Page 281

Octane recommendations Your vehicle is designed to use regular unleaded fuel with an octane rating of 87. In most cases, it is not necessary to use a fuel with an octane rating higher than 87. At service stations, the octane rating is displayed on a label on the pumps. Typical octane rating label In some parts of the country, "regular" grade fuels are sold with octane ratings of 86 or even less, especially in high altitude areas. We recommend that you do not use these fuels. Always use a fuel with an octane rating of 87, even if it is sold as a "midgrade" or "premium." Do not be concerned if your vehicle sometimes knocks lightly when you drive up a hill or when you accelerate. However, if a fuel with the recommended octane rating knocks heavily under all driving conditions or knocks lightly driving at cruising speed on level roads, see your dealer or a qualified service technician. Persistent, heavy knocking can damage the engine. Fuel quality If you are experiencing starting, rough idle or hesitation problems when the engine is cold, it may be caused by fuel with low volatility. Try a different brand of fuel. If the condition persists, see your dealer or a qualified service technician. 284