The Department of Energy was accused yesterday by Rep. Herbert E. Harris II (D-Va.) of covering up information that Exxon Corp. contributed heavily to the long lines and short supplies at area gasoline stations last summer by slashing its deliveries 34 percent in June and 20 percent in July.

Exxon, the largest supplier of gasoline in the Washington area, was further accused by the Northern Virginia congressman of taking out "false and misleading" advertisements in the city's daily newspapers at the height of the gasoline crisis. The ads said, "In total, we will supply as much gasoline as we did last July."

The energy department denied making a cover-up attempt.

Exxon said that in July its dealers nationwide got 96 percent of their July 1978 supply of gasoline, but that those in the Washington area got less because of government regulations. It said Washington area sales in June were 85 percent of those in June 1978 and July sales were 91 percent of those the year before.

Harris based his complaint on a written report DOE sent him Oct. 1 that he said failed to include figures that some of the agency's officials gave him in person in a meeting in his office Sept. 17.

"The Department of Energy has complied data showing that Exxon shorted the Washington market, but has withheld that information [in the written report] and blamed the gas shortage on panic buying and tank topping by area motorist," Harris said.

In a letter hand-delivered yesterday to the office of Energy Secretary Charles W. Duncan Jr., Harris complained that the department's audit report requested by Harris "covers up crucial data on gasoline deliveries uncovered by the Department of Energy's own auditors."

The report sent to Harris did not specifically mention Exxon's role in the gasoline shortage "because indiviual data is proprietary, protected by law from disclosure," a DOE spokesman said.

"We believe the report to be valid" in showing that deliveries by seven major oil companies in May, June and July, through the area's two major tank farms, were 12 percent below the total for the same months in 1978," the Energy official said.

The two terminals, Colonial and Plantation, are located in Harris' congressional district, and provide about 85 percent of all gasoline in the Washington area.

Exxon declined to comment directly on Harris' charges, but a spokesman for its Eastern regional office said "the actual numbers off our books" show that sales in the Washington area were down 15 percent in June from the same month in 1978, and down 9 percent in July-to-July sales.

The newspaper ads, which appeared in The Washington Post on July 12 and in The Washington Star the next day, were referring to national supplies, the Exxon spokesman said.

While conceding that the Washington area got a lower percentage "than the nation as a whole," he said nationally Exxon supplied its dealers with 96 percent of their previous year's supply during July. The Washington area got less, he said, because of DOE regulations that forced Exxon and other oil companies to make adjustments for new businesses and other factors.

In a formal statement, the company said, "Exxon has been allocating gasoline and other petroleum products since March because of a limited availability crude oil. The allocations to its customers have been made in strict accordance to Department of Energy Regulations."

Harris discounted the effect of DOE's allocation requirements, saying "it's the gasoline that comes out of the pipeline and gets delivered" that counts. He said that if the area's top retailer cuts back by 34 percent one month and 20 percent the next, "there's going to be a crisis," even though the other oil companies delivered 95 to 98 percent of their 1978 volumes.

Harris asked DOE to audit gas supplies near the start of the shortage, on June 19. He said DOE blamed part of the long delay on getting statistics on the fact that Exxon "stores some of its records in a salt mine in Texas."

When he finally got some figures, Harris said, they pointed to a conclusion that most of the shortage was because of a cutback at the plantation storage area supplied by the Plantation transmission line, where Exxon and Shell, the area's third biggest retailer, store their fuel. Gasoline delivered to Plantation in June was down 26 percent over the previous year, Harris said, while deliveries to Colonial, which supplies gas to Amoco (number two here), Citgo, Crown, Gulf and Texaco, were off only 1.6 percent from 1978.

The report delivered to Harris by Energy's office of special counsel "not only didn't include the information about Exxon's cutback, which I was assured would be included, but it doesn't even mention the name Exxon."

A spokesman for Energy said the omission of Exxon, in a list of major gasoline suppliers, was "an oversight."

Harris said he is concerned about the long-range implications of a government agency issuing a report that is "incomplete and misleading. Do we have a situation at Energy," Harris asked, "where the industry has a disproportionate effect on the information the public gets?"

The congressman cited one of the report's conclusion as "the kind of obfuscation that drives me up the wall." He was referring to a finding that "panic demand," caused by publicity of shortages in California, resulted in "tank topping" by area motorists.

"Tank topping can cause an aberration for a few days only," Harris said. "But after you've topped it off, that's it. This problem lasted two months not because people panicked, but because the largest supplier in the area cut back so sharply, yet the Department of Energy didn't even include that in its findings."