Other mistake: Al's and Peggy's birthdates are around 1948/1950, respectively. Supposedly, Al married Peggy after she got pregnant the day of his 4 touchdowns in high school in 1966. How could Kelly have been born in 1971?
Other mistake: Depending on the season the Bundys' address is 9764 Jeopardy Lane. Season 4 the address is 9674. But in the opening credits when Steve is shown on his porch the address is 9674. (00:00:20)
Continuity mistake: The outside view from the Bundy sliding doors/patio is different in the opening and most scenes from the shows. In fact, I can't think of any time it's the same.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.