All 1967-74 Models; 1975 and Later Models Without Catalytic Converter
Chevrolet and GMC trucks are designed to operate on regular grades of fuel (1967-71) commonly sold in the U.S. and Canada. In 1972-74 (and 1975 later models without catalytic converter), unleaded or low-lead fuels of approximately 91 octane (Research Octane) or higher are recommended. General Motors recommends the use of low-leaded or unleaded fuels (0-0.5 grams per gallon) to reduce particulate and hydrocarbon pollutants.
Use of a fuel which is too low in anti-knock quality will result in spark knock. Since many factors affect operating efficiency, such as altitude, terrain and air temperature, knocking many result even though you are using the recommended fuel. If persistent knocking occurs, it may be necessary to switch to a slightly higher grade of gasoline to correct the problem. In the case of late model engines, switching to a premium fuel would be an unnecessary expense. In these engines, a slightly higher grade of gasoline (regular) should be used only when persistent knocking occurs. Continuous or excessive knocking may result in engine damage.
Your engine's fuel requirement can change time, mainly due to carbon buildup, which changes the compression ratio. If you engine pings, knocks, or runs on, switch to a higher grade of fuel and check the ignition timing as soon as possible. If you must use unleaded fuel, sometimes a change of brands will cure the problem. If is is necessary to retard the timing from specifications, don't change it more than about 4°. Retarded timing will reduce power output and fuel mileage, and it will increase engine temperature.1975 and Later Models With Catalytic Converter
Chevrolet and GMC trucks with Gross Vehicle Weight Ratings (GVWR) which place them in the heavy duty emissions class do not require a catalytic converter. However, almost all 1975 and later light duty emissions trucks have a catalytic converter. The light duty classification applies to all trucks with a GVWR under 6,000 lbs. (2724 kg) through 1978, except for 1978 trucks sold in California. 1978 California models and all 1979 models with GVWR's under 8,500 lbs. (3859 kg) fall into the light duty category. In 1980 and later, the light duty classification applies to all trucks with GVWR's under 8,600 lbs. (3904 kg).
The catalytic converter is a muffler shaped device installed in the exhaust system. It contains platinum and palladium coated pellets which, through catalytic action, oxidize hydrocarbon and carbon monoxide gases into hydrogen, oxygen, and carbon dioxide.
The design of the converter requires the exclusive use of unleaded fuel. Leaded fuel renders the converter inoperative, raising exhaust emissions to legal levels. In addition, the lead in the gasoline coats the pellets in the converter, blocking the flow of exhaust gases. This raises exhaust back pressure and severely reduces engine performance. In extreme cases, the exhaust system becomes so clocked that the engine will not run.
Converter equipped trucks are delivered with the label "Unleaded Fuel Only'' placed next to the fuel gauge on the instrument panel and next to the gas tank filler opening. In general, any unleaded fuel is suitable for use in these trucks as long as the gas has an octane rating or 87 or more. Octane ratings are posted on the gas pumps. However, in some cases, knocking may occur even though the recommended fuel is being used. The only practical solution for this is to switch to a slightly higher grade of unleaded fuel, or to switch brands of unleaded gasoline.
Diesel engine pick-ups require the use of diesel fuel. Two grades of diesel fuel are manufactured, #1 and #2, although #2 grade is generally the only grade available. Better fuel economy results from the use of #2 grade fuel. In some northern parts of the U.S., and in most parts of Canada, #1 grade fuel is available in winter, or a winterized blend of #2 grade is supplied in winter months. If #1 grade is available, it should be used whenever temperatures fall below +20°F (7°C). Winterized #2 grade may also be used at these temperatures. However, unwinterized #2 grade should not be used below +20°F (7°C). Cold temperatures cause unwinterized #2 grade to thicken (it actually gels), blocking the fuel lines and preventing the engine from running.
Do not use home heating oil or gasoline in the diesel pick-up. Do not attempt to thin unwinterized #2 diesel fuel with gasoline. Gasoline line or home heating oil will damage the engine and void the manufacturer's warranty.