Fuel - Use of Unleaded Gasoline In Older Vehicles

Number 85-6(Gasoline)-34 Date 6/85

Subject: USE OF UNLEADED GASOLINE IN GENERAL MOTORS VEHICLES DESIGNED FOR LEADED GASOLINE
The need for leaded gasoline in the U. S. is decreasing as older vehicles designed for leaded gasoline are replaced with newer ones requiring unleaded gasoline. 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 in (a) passenger cars starting in 1971, (b) 1971-78 trucks less than 6,000 lbs GVW, and (c) trucks less than 8,500 lbs GVW starting in 1979 were designed to use unleaded gasoline and are unaffected by lead reduction efforts.

For all GM gasoline-fueled engines in (a) pre-1971 passenger cars and trucks, (b) 1971-78 trucks over 6,000 lbs GVW, and (c) trucks over 8,500 lbs GVW starting in 1979, concerns exist about engine knock and exhaust valve seat durability 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, cylinder heads may be rebuilt with hardened valve seat inserts to avoid recurrence of the problem.

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