The residents of Cottonwood Street, after years of waiting, hoped to have sewer service this summer in their small, mostly black neighborhood in McLean. But their elation has turned sour, with residents charging that Fairfax County is abandoning its promise and leaving the neighborhood with the foul odor of a backed-up septic field.

"Can you imagine, here in 'rich, exclusive McLean,' we're still having our septic tanks backed up?" asked Ernest Love. "The stench is all over the neighborhood. It smells like someone has died."

The source of Love's discomfort is the property next to his, where the back yard septic field has been overflowing regularly for a month.

Yet Love and his neighbors say they are angriest with Fairfax County officials, who they said have neglected the neighborhood because of racial prejudice.

Fairfax officials denied the allegations yesterday, saying that residents were given no promises about when sewer construction would be finished and that the project will be completed, with only routine delays, by December 1987.

"I'm unaware of any commitment that was made," by the county, said Supervisor Nancy K. Falck (R-Dranesville), who is the neighborhood representative on the County Board of Supervisors. "It's difficult for the residents to realize that their project is alive and well and moving . . . . The wheels of government do not always move with great rapidity."

Quing N. Yng-Wong, the owner of the property next to Love's, said he agreed that the neighborhood is being neglected, and, in response to the county's delays, was taking his time fixing his broken disposal system.

"Tell them I'll fix it in a week or so," said Yng-Wong, who owns the property on Cottonwood Street but does not live there. His children live at the address with their mother.

"This neighborhood has been sort of a little ghetto in the midst of McLean," said Yng-Wong, a celebrated acupuncturist known by his professional name of Dr. Wu. "This has been a stepchild to whole area."

Fairfax officials said that most houses in the county have sewers that were built and paid for by developers during construction, but that the older dwellings along Cottonwood Street have septic systems.

Falck said yesterday that she pushed the county board to approve sewer service for the neighborhood of about 15 houses last year after hearing for the first time that residents were eager to be hooked into the county's disposal system.

Cottonwood residents, most of whom paid an installation fee of $1,350, said they believed the project would be completed by next month.

One neighborhood resident who made plans based on that hope was Franklin C. Hall, owner of a newly built $113,000 house on Cottonwood Street. The house remains empty because of the lack of sewer service.

Hall's father, Chris C. Hall, said yesterday that his son refuses to pay the $5,000 or more to install a septic system when sewer service should be on the way. "They've got us trapped," he said.

Love, a former White House mailroom worker, attributed the delays to fears that if the Cottonwood sewer line is completed, residents would subdivide their property and more blacks would move into the neighborhood.

"You know what it is. They just don't want any more blacks," said Love, who added that developers had offered him several hundred thousand dollars for his property, which is surrounded by affluent, mostly white subdivisions.

"This has nothing to do with the color of anyone's skin," countered Falck, who said the project had actually received more favorable treatment than some others in the county.

"I can understand why they want it right away," Falck said. "I wish things were going faster, but the neighborhood will get the sewer."

In addition to the odors emanating from Yng-Wong's septic field, residents are concerned about the impact on health.

Dennis A. Hill, a sanitation official with the Fairfax County Health Department, said the department instructed Yng-Wong to clean up his septic system last year after investigating complaints, and had received another complaint yesterday.

"If we have raw sewage on the ground, there is a potential health hazard," including diarrhea and other types of gastrointestinal illness, said Hill. "It's much more than just a rotten smell."