Almost one out of every five 1973-1978 automobiles on the road today has had its emission control equipment teampered with, according to a study released today by the Environmental Protection Agency.

The study also noted that another 48 percent of all cars tested had their systems altered, but is was not clear whether the tampering was deliberated.

Nearly 2,000 cars were tested in six states, including Virginia, last summer by EPA personnel. Most of the inspections came while autos were going through their usual state safety inspection.

The EPA study found that emissions of various fumes were considerably higher on the autos that had their systems tampered with. "By the time a vehicle reaches 50,000 miles," The EPA said, "the average hydrocarbon and carbon monoxide emissions will be nearly four times that of a vehicle with no tampering."

Specifically tampered with were the Exhaust Gas Recirculation systems (EGR), catalytic converters, gasoline tank inlets designed to prevent the motorist from using leaded gasoline and the vacuum spark retard.

EPA assistant administrator for enforcement, Marvin B. Durning, said the study revealed a "disturbing failure rate."

"Consumers are spending millions of dollars for pollution control benefits they are not receiving," he said. "In addition, urban smog conditions are being made worse by cars that should be helping to solve the problem."

The rate of tampering, according to the report, increases as cars get older. While 7 percent of the 1978 cars checked had been purposely tampered with, the 1973 models checked showed a 32 percent rate.

The EPA tests also revealed that more than 5 percent of cars that are supposed to take only unleaded gas had leaded gas in their tanks, confirmation of a situation that has concerned EPA for sometime.

"Somewhat surpisingly,c the report added, 75 percent of those cars involved in "fuel switching" did not have the restrainers on their gas tank filler tubes removed, indicating that the owners used funnels or that many leaded gas pumps have improper nozzles that fit into vehicles earmarked for unleaded guel.