Nelson Core was driving up a mountain road near Morgantown, W. Va., when his 1981 Chevrolet Malibu lost power.

"I couldn't accelerate beyond 20 miles per hour," he said. "I realized that if I made it over that mountain, I wouldn't make it over another. So, I turned around . . ."

Core thus became one of thousands of victims of "catalytic converter plugging," a defect that could affect as many as 753,000 of General Motors Corp. 1981 and early 1982 models, and could cost the company up to $150 million.

Most of the cars involved are Chevrolets with 3.8- , 4.4- , 5- and 5.7-liter engines. Some Pontiac Firebirds and GMC Caballeros with 5- and 5.7-liter engines also are affected.

GM has written a controversial prescription for the disorder, and has raised the ire of environmental and consumer-interest groups in the process. The company wants the Environmental Protection Agency to waive its latest carbon monoxide (CO) emission control standards, and wants to treat the patients--the customers and their cars--on a complaint-only basis.

GM does not plan to make house calls by way of recall notices.

"We don't think the condition is a safety hazard. If we did, we'd recall all of the affected models immediately," said GM products spokesman Stanley D. Hall.

The EPA plans to grant, "in the near future," according to one agency official, GM's request for the CO emission standards waiver on repairs of its 1981 models. (The waiver was requested because GM's solution is to replace the faulty converter with another type that isn't as effective in controlling carbon monoxide emissions.) Such a waiver was granted for GM's 1982 passenger vehicle lines. But the dispensation came too late for some of this year's cars.

"We thought we had avoided the problem for 1982, but it showed up in some cars" produced earlier in the year, Hall said.

The problem is with the dual-bed catalytic converter, located between the engine and the tail-pipe assembly. Catalytic converters change poisonous compounds in engine exhaust fumes into relatively harmless matter, such as water vapor, carbon dioxide, and nitrogen, before the fumes move from the exhaust system into the atmosphere.

GM put the dual-bed unit in its 1981 passenger models to meet more stringent EPA requirements limiting CO emissions to 3.4 grams per mile (gpm). The previous CO standard was 7 gpm.

The dual-bed converter succeeds in reducing CO pollutants. But with the engines in question, the unit tends to self-destruct. GM engineers disagree on why; some say it is caused by excessive heat, others say it is related to vibration. But they agree on how the failure occurs:

Ceramic pellets in the upper bed break up and fall through louvered openings into a narrow air chamber in the middle of the converter. The broken pellets plug up the lower louvers, significantly retarding the flow of exhaust gas.

The blockage increases exhaust system back-pressure, interrupting engine efficiency and, as a result, causing a substantial loss of engine power. Under these conditions, the stricken car can only attain a top speed of about 30 mph. In severe cases, the car can't start.

GM officials say converter plugging symptoms occur soon after startup of the affected vehicles. They say there is no evidence that the condition occurs at high cruising speeds, bringing about sudden and potentially dangerous deceleration. Nor has there been a known instance where the condition brought a car to a sudden stop, GM officials say.

Core's experience seems to verify parts of those findings. He said his car was moving at about 55 mph, but gradually decelerated as he tried to climb the mountain. His car had about 8,000 miles of use at the time of its converter plugging, compared with the average 17,700 miles after which the other affected converters failed.

"This was my first GM car and the first time I had taken it out of the Washington area," said Core, a Treasury Department employe. He said he rented another car for the return trip home, leaving his car in Morgantown for repairs.

The car rental fee amounted to nearly $100. Core said GM "does not seem to want to pay for that." But the company did assume the $200 cost for replacing the dual-bed converter with an earlier designed, single-bed unit. In fact, as of last week, GM had made and paid for about 50,000 such replacements.

GM has been willing to accept the replacement costs, because it says the costs of repairing the dual-beds would have been greater.

"The combined labor cost of removing the converter from the vehicle . . . and refilling the pellets and resealing is substantial," the company said last April in a report to the EPA, submitted in support of its CO standards-waiver application.

"Until we are confident that the problem can be permanently solved with different pellets, this approach is not practical. Hence, at the present time, it is more appropriate to replace the converter rather than attempt to repair it," the company said. But the solution caused another problem.

Like the double-bed converters, the single-bed converters contain "three-way catalysts"--pellets that convert CO pollutants, hydrocarbons (HC) and nitrogen oxides (NOx) into acceptable gases.

The single-bed units meet government standards for HC and NOx conversions, and they also deliver more fuel efficiency than their dual-bed counterparts. But the single-beds fail to meet the EPA's latest standards for control of CO emissions. That is why, according to a GM letter to the EPA dated April 2, 1982, "the requested waivers" on CO standards "are of vital concern to General Motors."

The waivers and converter failures also are of vital concern to the Washington-based Center for Auto Safety, a consumer advocacy group, and to officials of the California Air Resources Board (CARB). Both the center and CARB have been pressuring GM to notify its customers about the potential defect. And both have demanded that EPA reevaluate its plans to grant what amounts to a retroactive standards waiver on the affected 1981 models.

"This is a recall situation rather than a waiver situation," said the center's executive director, Clarence M. Ditlow III. "We don't see any need for a waiver . . . Consumers want their cars to work. They want to know why their cars are stalling on them. You do that through recalls. Even if the EPA goes ahead and grants a waiver, GM has no intention of instituting a recall," Ditlow said.

Only two of the affected engine groups, the 5- and 5.7-liter machines, are in operation in California. But officials there say they are concerned that many car owners with the converter problems may wind up paying repair costs that should be paid by GM.

"People can easily go to a repair shop that has no connections with GM. These people could wind up paying full repair costs; or, worse, they could 'cheat' in terms of environmental protection and remove the converters completely from their cars," said CARB spokesman Bill Sessa.

"We've been discussing this problem with GM since December, but we just haven't been able to change their minds about notification," Sessa said.

EPA officials say they have no current basis for an emissions recall in the dual-bed converter case, because when the converters break down, the cars stop running, and when the cars stop running, there is no pollution.

"There is no evidence at this point, even with the plugging, that emissions standards have been exceeded" on the cars in question, said Charles N. Freed, director of EPA's manufacturers' operations division. "Before the catalytic converter gets to a point where the emissions standards can be exceeded, the cars manifest such poor driveability that the customers bring them in for repairs," Freed said.

Officials at the National Highway Traffic Safety Administration, which has jurisdiction over recalls for safety violations, gave a similar response. "We don't get into emissions," said a NHTSA spokesman, who added that most of the recalls handled by his agency are voluntary.

GM argues that its complaint-response approach to the problem is valid because of what it calls the lack of a safety issue and because the emissions systems in question are covered by a five-year, 50,000-mile warranty.

"There are no current plans to contact owners," the company said in its report to the EPA. "The emission warranty booklet, which has already been provided to all owners, clearly states that the catalytic converter is warranted for five years or 50,000 miles," the company said in response to the EPA question: "What notification of available remedies will owners receive?"

GM says it will handle complaints on vehicles no longer covered by the warranty on a "case-by-case" basis.

Core said he would have preferred a different answer. "I think a general notice of some kind would have helped me," he said. "Had I known about the defects, I would have taken my car to a dealer before I took it to West Virginia."