USE OF UNLEADED GAS LEADED GAS VEHICLES

ALL MODELS

Requests have been received from dealers for information that may be used to advise customers of the effect of using unleaded gasoline in trucks designed to use leaded gasoline.

The need for leaded gasoline in the U.S. is decreasing as older vehicles designed for leaded gasoline are replaced with certain newer ones requiring unleaded gasoline. This does not include vehicles with heavy duty emissions over 8,600 GVW. Furthermore, the U.S. Environmental Protection Agency (EPA) is phasing down the concentration of lead in leaded gasoline starting in July, 1985. These two facts taken together could result in limited supplies of leaded gasoline being available for vehicles designed for such a fuel.

The lead phasedown is based on health considerations and a desire to eliminate fuel-switching the practice of using leaded gasoline in vehicles equipped with catalytic converters. Because lead destroys the emission control properties of catalysts, leaded gasoline should not be used in engines equipped with these devices. GM supports this EPA effort to reduce lead emissions and to reduce fuel-switching.

All GM gasoline-fueled engines, 1971-78 trucks less than 6,000 lbs. GVW, and starting in 1979 trucks less than 8,500 lbs. GVW were designed to used unleaded gasoline and are unaffected by lead reduction efforts.

For all trucks with GM gasoline-fueled engines manufactured prior to the 1979 model year and over 6,000 lbs. GVW, and trucks over 8,500 lbs. GVW manufactured beginning with the 1979 model year, and depending on operating conditions, engine knock and/or exhaust valve seat durability conditions may arise when such engines are operated on gasoline without lead.

The octane quality of leaded regular gasoline is generally higher compared to unleaded regular gasoline. Thus, switching from leaded regular to unleaded regular in vehicles designed for leaded regular may cause some engines to knock. Occasional light knock is of no concern. However, persistent, heavy knock can cause engine damage and should be avoided. Two alternatives are available to avoid knock. The first is to use unleaded premium gasoline (or a mixture of unleaded regular and unleaded premium): The other is to retard the engine's basic spark timing.

The lead in gasoline creates a "cushion" between the valve and valve seat to minimize wear. Valve seat wear is aggravated by operating at high engine speeds and loads, particularly for long periods of time. Engines designed to use unleaded gasoline have hardened valve seats to compensate for the removal of lead.

Engines designed to use leaded gasoline can avoid excessive wear if operation is limited to reasonable speeds and loads. However, if excessive valve seat wear does occur, certain model engines have cylinder heads that may be rebuilt and upgraded with hardened valve seat inserts to avoid recurrence of the problem.

For those engines designed for leaded gasoline, GMC recommends that they continue to be fueled with leaded gasoline as long as such gasoline is available.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.