Most motorists believe that gasoline is gasoline.

They are wrong; and their error could lead to vehicle stalling and stopping, and costly repairs.

Many new engines and the gasolines needed to fuel them do not match. It is a problem that could affect about 31.5 percent, 3.38 million, of the 10.8 million new cars expected to be sold in the United States this year.

Those are the cars equipped with port fuel-injected gasoline engines. Their numbers have risen sharply since 1985, when they accounted for less than 20 percent of the U.S. new-car market.

By 1990, about 52 percent of the new cars sold in this country will come with port fuel injectors, according to technical experts in the auto and oil industries.

Fuel injectors replace the carburetors traditionally found in automotive engines. But their purpose is the same -- to control the mixture of air and fuel entering engine combustion chambers.

Fuel injectors have been around for years, but mostly as throttle-body injectors on passenger cars. That means they were located centrally, usually atop engines in the manner of carburetors. The throttle-body systems often involved one injector supplying fuel to all cylinders.

Throttle-body injectors outperformed carburetors in mixing fuel with air. But port fuel injectors -- one injector per cylinder -- greatly improve the process.

Port fuel injectors work by spraying gasoline into the engine's intake manifold, where the fuel joins with air before flowing to the cylinders, where the mixture is burned. Better air-fuel mixing yields better combustion, greater power, better fuel efficiency and cleaner exhaust gases.

But port fuel injectors have a problem: They operate in a hotter environment than do throttle-body injectors. The extra heat can turn the wrong kind of unleaded gasoline -- one with lower detergency, less cleansing power -- into a gummy residue that can plug fuel injectors.

The upshot is sputtering engines and fuming drivers.

"We have about 60,000 members across the country; and in the last year, they've all had customers complaining about problems with port fuel injectors," said Victor Rasheed, executive director of Service Station Dealers of America, which is based in Washington.

He said many of the complaints have been bitter. "You know, it's aggravating to step on the accelerator of a $25,000 automobile, and the thing doesn't go anywhere."

The problem is not so much with the injectors, as it is with the quality of some of the unleaded gasolines flowing into engines equipped with the devices, Rasheed said.

Unleaded gasolines cost more to produce than leaded varieties because they must go through extra refining proceses to reach suitable octane levels -- ranging from 87 for regular unleadeds to 93 octane for super-unleaded grades.

Octane is the measurement of how smoothly gasoline burns in an engine -- the higher the octane, the smoother the burn.

But to help reduce the cost of producing unleaded fuels, some refiners have been cutting back on detergent additives, said Rasheed and some auto industry officials.

"There has been a significant deterioration in the quality of fuels," said Bernard I. Robertson, director of Chrysler Motors power-train engineering division. "We've met with most of the oil companies to talk about all of our concerns about fuel-quality problems, of which port-fuel-injection plugging is one."

But oil industry officials, while agreeing that certain unleaded gasolines can create injector blockages, said the auto makers helped create the problem.

"This idea that we were just making the gasoline poorer and poorer, to the point that it didn't work anymore, is just wrong," said Robert P. Larkins, vice president for marketing at Exxon Corp.

The auto makers did not anticipate the kinds of problems they have had with port fuel injection, "and neither did we," Larkins said.

Exxon's regular unleaded gasolines worked fine with the cooler operating, throttle-body injection systems. But the high-heat environment of the port fuel-injection units presented more demanding challenges to gasolines that had served well in the past, Larkins said.

"The engines changed, not the gasoline," Larkins said.

"The car companies overlooked a problem" that occurred in the pre-production testing of their port fuel-injection systems, said James B. Smith, a product engineer with Amoco Oil Co. The problem was in the testing itself, which did not include the effects of frequent short trips and stop-and-go driving on the sensitive fuel-metering devices, Smith said.

Those so-called "severe operating conditions" can create tremendous engine heat that "soaks back," or rises, through the engine block while the engine idles, or after it is turned off. That heat cooks droplets of fuel that form at the tip of injector nozzles when the engine is running slowly, or not running at all.

The cooked droplets become gummy deposits around the fuel-injector pintle, a coned pin at the tip of the nozzle that creates the gasoline spray. The dirty pintle disrupts the spray pattern and decreases the amount of fuel going into the engine combustion chamber.

At least one auto maker agrees that there was a testing oversight.

"We put out an injector design that we had no problem with in testing. But we didn't test it under the right conditions. We didn't drive it stop-and-go," said Joseph Colucci, director of General Motors Corp.'s fuels and lubricants laboratory.

The domestic auto industry, with the approval of the Environmental Protection Agency, traditionally runs nearly continuous, 50,000-mile durability tests on new fuel system components.

But auto makers discovered that deposits will not form under that kind of driving schedule, because fuel constantly is flowing through the port fuel-injector nozzle. Air rushing past the nozzle during constant driving also helps to keep the nozzle cool, Colucci said.

"Let's not worry about who's at fault in this," Colucci said about injector plugging. "Let's learn from our experience and solve the problem, and hope that this kind of thing does not occur again."

Larkins said: "We don't want to get into a finger-pointing situation. The news is not in fixing the blame. The news is that the problem is being solved."

There is ample evidence that Larkins is right.

All of the nation's major oil companies are marketing, or planning to market, high-detergent unleaded gasolines designed to unplug injectors and keep them clean. And all of the Big Three U.S. car companies -- GM, Chrysler and Ford Motor Co. -- are working on designs to make port-fuel-injection systems more tolerant of various grades of unleaded gasoline.

The push for higher detergency is viewed on both sides as an interim step -- one designed to give auto makers more time to come up with tougher injectors. But that temporary measure seems to be taking on the character of permanency, said Joseph Lastelic, a spokesman for the American Petroleum Institute, which is based in the District.

Mobil Oil Co. started the battle in September 1985 by introducing a double-detergent, super-unleaded fuel. "Our firm claim was then, and remains, that two tankfuls of our super-unleaded will clean plugged fuel injectors," said John Lord, a Mobil spokesman in Houston.

Amoco, Arco, Exxon, Shell and Texaco soon followed, although spokesmen for all of those companies said that their organizations had been producing high-quality, high-detergent unleaded grades. A spokesman for Amoco pointedly stated that the company's new high-detergent gasoline, Amoco Ultimate, had been in development for six years -- long before the injector controversy arose.

"Amoco Ultimate is not a response" to auto industry complaints about gasoline quality, said Ralph Stow, Washington spokesman for Amoco. "We've been working with the auto companies all along. We've recognized that we've got to meet the needs of today's engines, and we've done so," Stow said.

Amoco said its new gasoline "will keep a car's entire fuel-intake system clean." Arco, in West Coast advertising, makes a similar vow.

Says Larkins of Exxon: "We're not just putting more detergents into our gasoline. We're adding completely new kinds of detergents. Our XCL-12 Premium will clean injectors with one tankful. All of our unleaded gasolines will help to keep injectors clean."

This is joyful music to the people at Lubrizol Corp. in Wickliffe, Ohio, one of the nation's largest suppliers of gasoline detergent additives.

"Our sales are going up and our stock is rising," said Fred Ruhland, Lubrizol's senior account manager.

"It's a whole new ball game," said Rasheed. "Port fuel injectors are here to stay, and I guess gasoline manufacturers have to get accustomed to that. I guess we all do."