1998 GMC Envoy Owner's Manual - Page 250

Fuel You can be injured and yourvehicle could be damaged if you try to do service work on a vehicle without knowing enough about it. 0 Be sure you have sufficient knowledge, experience, the properreplacement parts and tools before you attempt anyvehicle maintenance task. 0 Be sure to use the proper nuts,bolts and other fasteners."English" and "metric" fasteners canbe easily confused. If you use the wrong fasteners, parts can later break or fall off. You could be hurt. Use regular unleaded gasoline rated at 87 octane or higher. At a minimum, it should meet specifications ASTM D48 14 in the United States and CGSB 3.5-M93 in Canada. Improved gasoline specifications have been developed by the American Automobile Manufacturers Association (AAMA) for better vehicle performance and engine protection. Gasolines meeting the AAMA specification could provide improved driveability and emission control system protection compared to other gasolines. Be SUI-e posted octane is at least 87. If the octane is the less than 87, you may get a heavy knocking noise when you drive. If it's bad enough, it can damage your engine. Adding Equipment to the Outside of Your Vehicle Things you might add to the outside of your vehicle can affect the airflow around it. This may cause wind noise and affect windshield washer performance. Check with your dealer before adding equipment to the outside of your vehicle. If you're using fuel rated at 87 octane orhigher and you hear heavy knocking, your engine needs service. But don't worry if you hear a little pinging noise when you're accelerating or driving LIPa hill. That's normal, and you don't have to buy a higher octane fuel to get rid of pinging. It's the heavy, constant knock that means you have a problem. 6-3