General Motors Corp. believes it is "on the threshold" of ending automobile air pollution, key corporate officials said at a news conference they called here today on the GM proving grounds.

Some 2,300 1978-model Pontiac Sunbirds sold in California will have a variation of what GM calls its "Phase II" air pollution control system. That will cut emissions significantly but at the same time keep fuel economy equal to non-California Sunbirds while adding about $175 to the car's cost, they said.

The Phase II catalytic converter is a "three-way" device that adds rhodium to the platinum eliminate pollutants. It requires very precise control of the air and fuel mixture, and speeds the reaction of the oxide nitrogen to harmless gases in the same fashion the platinum in earlier converters makes the hydrocarbon and carbon monoxide harmless.

To achieve that control, a spark-plug-like sensor sends constant signals to the carbureator about the amount of oxygen in the exhaust gas. The fuel mixture going into the engine is adjusted 600 times every minute.

The sensor has to be changed every 15,000 miles. To encourages the driver to do it, a flag rises and hides the mileage on the odometer of the 1978 California Sunbird.

Volvo and SAAB first put three-way systems on 1977 California cars; the Environmental Protection Agency says Volvo, SAAB, GM, Ford and Mercedes have certified such systems for 1978 models.

Fred W. Bowditch, GM's top air pollution expert, said that, with the successful development of the new systems for 1978 and '80 California cars and all 1981 cars, "We will have essentially achieved GM's long standing commitment to take the auto out to the picture as a significant contributor to air pollution."

Putting the Sunbird converter on all new American cars today would consume the entire world output of rhodium, Bowditch said.

According to Earl Pierce, AC Spark Plug enginnering director, GM has a target of reducing to one-tenth the rhodium per converter, or .002 Trey ounces per vehicle.

He said GM still is trying to develop "base metal" converters that don't need platinum, palldium or rhodium, but doesn't expect that in less than three years.

GM said it burns 88,000 gallons of gasoline a month testing its cars for EPA certification, an item that provoked immediate fire from the agency.

Eric O. Stork, head of automobile testing at the EPA, said GM's gasoline consumption "could first be a lot less because they do a lot of contingency work.

"Second, they sell more than 75 per cent of the cars with air conditioning at about $500 each, and more than 40 per cent with vinyl roofs costing about $120 each," he said.

About three-way catalysts, he said "the principal is not new but the development of the hardware has made rapid strides in recent years with benefits not only in emission control but also in fuel economy and driveability."

None of the aerospace electronics involved would have appeared on cars "if it were not for the Congress and EPA," Stork said.