As Busch led the field into the final lap, his No. 18 Toyota blew a front left tire and slowed sharply in Turn 1. Suarez, Busch's teammate at Joe Gibbs Racing, then took the lead.
But moments later Suarez's No. 19 Toyota began running out of fuel and slowed on the back straightaway of the two-mile track.
Busch then drove his hobbled car ahead of Suarez to retake the lead. But Dillon — his No. 2 Chevrolet still running at full speed — slipped past Busch on the outside, scraped the Turn 4 wall and earned the victory.
"Sometimes things just go your way," Dillon said. "We kind of stole one today."
Dillon said he "didn't think we had a car to do that" and that he "was worried about our fuel" as well.
Busch finished second, Darrell Wallace Jr. was third and Suarez finished fourth.
Busch had dominated the race — he led 133 of the 150 laps — and hoped to make history by becoming only the second driver to win four consecutive races in NASCAR's second-level series. The other is Sam Ard, who did so in 1983.
Busch, 30, also hoped to extend his record of having the most Xfinity Series wins at the two-mile Fontana track; he has six Xfinity victories at Auto Club Speedway.
Dillon also won the pole position for Sunday's NASCAR Sprint Cup Series race at the Fontana speedway.