The league's most prolific side put three past Jeju without reply to take their tally to 18 this season and build on their midweek victory over Gyeongnam to assume top spot.
Lee Dong-Gook's penalty ensured Jeonbuk Motors took maximum points against Ulsan, and the Steelers took full advantage with a dominant display.
Kim Jae-Sung took his tally in the league to three with a brace, while Kim Dae-Sung continued his prolific form with his fifth of the season 12 minutes from time to make it eight games unbeaten in all competitions.
Ulsan's defeat was compounded by the fact Suwon Bluewings and Jeonnam Dragons both picked up victories to finish the weekend above them.
The former moved second with a 3-0 win over rock-bottom Incheon United - Kim Eun-Seon, Jong Tae-Se and Santos Junior on target - while Jeonnam are a place further back on goal difference.
After a whirlwind opening 23 minutes that saw Ahn Yong-Woo and Lee Jong-Ho score either side of a Yang Dong-Hyun equaliser, the hosts came out on top to go level on points with Suwon above them and Jeonbuk below them.
Ulsan will likely have their work cut out next week when they host Suwon, while Pohang have an ideal opportunity to continue their fine form when they travel to strugglers Seoul.
Sangju Sangmu built on their first league victory of the season in midweek to take a point off Seongnam, and Seoul and Gyeongnam also failed to register in their meeting.
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