2003 GMC Yukon Owner's Manual - Page 364

Some gasolines that are not reformulated for low emissions may contain an octane-enhancing additive called methylcyclopentadienyl manganese tricarbonyl (MMT); ask the attendant where you buy gasoline whether the fuel contains MMT. General Motors does not recommend the use of such gasolines. Fuels containing MMT can reduce the life of spark plugs and the performance of the emission control system may be affected. The malfunction indicator lamp may turn on. If this occurs, return to your authorized GM dealer for service. Only vehicles that have the 5.3L V8 engine (VIN Code Z) may use 85% ethanol fuel (E-85). Many service stations will not have an 85% ethanol fuel (E-85) pump available. The U. S. Department of Energy has an alternative fuels website (http://afdcmap.nrel.gov/nrel/) that can help you find E-85 fuel. Those stations that do have E-85 should have a label indicating ethanol content. Do not use the fuel if the ethanol content is greater than 85%. Your vehicle may not operate properly if the ethanol content is greater than 85%. At a minimum, E-85 should meet ASTM Specification D 5798. To insure quick starts in the wintertime, the E-85 fuel must be formulated properly for your climate according to ASTM specification D 5798. If you have trouble starting on E-85, it may be because your E-85 fuel is not properly formulated for your climate. If this happens, switching to gasoline or adding gasoline to your fuel tank may improve starting. Your vehicle is designed to accommodate a mixture of gasoline and E-85 fuel. For good starting and heater efficiency below 32°F (0°C), the fuel mix in the fuel tank should contain no more than 70% ethanol. Fuel E-85 (85% Ethanol) The 8th digit of your vehicle identification number (VIN) shows the code letter or number that identifies your engine. You will find the VIN at the top left of the instrument panel. See Vehicle Identification Number (VIN) on page 5-96. If your vehicle has the 5.3L V8 engine (VIN Code Z only), you may use either regular unleaded gasoline or ethanol fuel containing up to 85% ethanol (E-85); also see Fuel on page 5-4. In all other engines, including the 5.3L V8 (VIN Code T), use only regular unleaded gasoline. 5-6