As a used car dealer, Larry Westfall said he thought he had heard all possible customer stories, from payment difficulties to transmission problems. But yesterday, when a man telephoned to say that his seven-foot boa constrictor might have sought refuge in a repossessed sports car, Westfall just had to check it out.

"At first I just went in for a look," and the snake's head "just popped . . . out of the heat vent, right on top of the dashboard," said Westfall, used car manager at Spencer Toyota in Silver Spring. "I guess he was dead already. It must have been the heat. So I tried to pull him out. But he was stuck. That's when we realized this must be one big snake."

Herbert Lynch, general manager of the dealership, would not disclose the name of the snake's owner. He said the man had written a $15,000 check for the red 1984 Nissan 300 ZX Turbo, but the check bounced.

Lynch said the man came to the dealership Wednesday to discuss the matter, but left with a Spencer salesman after they failed to reach an agreement. The salesman went to the man's house, repossessed the car and drove it back to the lot.

So at first it seemed business as usual yesterday morning when mechanic Leroy Perez went into the car to look for the keys and begin cleaning it for the next sale. But the call from the snake's owner followed shortly, and Westfall went out to car and discovered the dead snake.

A Montgomery County Humane Society spokesman said that heat exposure and dehydration were probably strong factors in the boa's death, adding that only extreme temperature or old age generally cause death to such an animal.

Lynch said employes first used a coat hanger to try to pull the boa out, and "I even had a 6-3, 250-pound guy out here try pulling on it. But the thing wouldn't budge."

Pat Bisel Jr., a mechanic at the dealership, offered to help Westfall. Bisel and Westfall worked for more than two hours in the midday sun, removing the dashboard and the steering wheel before finally reaching the end of the snake.

Most employes and casual observers kept their distance during the effort, some straining to watch the brave ones pull and heave on the entangled reptile.

The heroics complete, Bisel, leaning against a van and lighting up a cigarette, said he had offered to help remove the snake "because I wanted to see how long he was. The main thing was the smell. It smells real rotten." Triumphant, he held the snake up to discover "it's a lot taller than me."

Lynch said the company disposed of the snake yesterday afternoon and will now "air out the car and put it back together."

He said the repairs would cost about $400.