General Motors Corp., in an action that affects 1,100 jobs and raises questions about the acceptability of plastic-bodied vehicles, yesterday announced plans to scrap its two-seat, subcompact Pontiac Fiero sports coupe by Sept. 1.

The announcement comes despite several major improvements made recently in the Fiero to boost its attractiveness to buyers in an increasingly tight market for small, two-seat cars.

"We didn't want to do this," one Pontiac Division official said of the corporation's decision to discontinue the car, which was introduced in September 1983. "But we didn't have the votes" in GM's executive committee to keep it going, "and we didn't have the sales, either."

Fiero sales have fallen steadily since 1984, when the company sold 101,720 of the plastic-bodied cars produced in Pontiac, Mich.

Fiero sales totaled 90,691 units in 1985, 71,283 in 1986 and 47,156 in 1987, and are expected to drop to 30,000 units this year.

The Fiero was GM's first plastic-bodied vehicle aimed at a relatively large audience -- young buyers looking for two-seaters in the $10,000 to $15,000 category.

The substantially more expensive Chevrolet Corvette is aimed at a much smaller group of potential buyers.

However, GM officials said yesterday that they will not abandon plastic-panel technology because of disappointing sales. Pontiac is working on a plastic-bodied van, which it plans to introduce in 1990.

The Fiero plant has the capacity to build 115,000 cars annually using two shifts operating full time.

But about 1,200 Fiero workers already were on indefinite layoff because of declining sales, and 1,100 more probably will be laid off when the plant closes late this summer, GM officials said.

"The marketplace has changed, become more crowded, with a forecast for at least a 'no-growth' position in the sporty, two-seater segment," said J. Michael Losh, Pontiac Division manager and a GM vice president.