In 1983 Ensign Racing merged with Theodore Racing and as a result Guerrero began driving the No. 33 Café de Colombia Theodore N183-Ford Cosworth DFV. Guerrero only failed to qualify at the Monaco Grand Prix and became more consistent in finishing races. Despite this Guerrero's best finishes were a pair of 12th-place finishes at the Dutch Grand Prix and the European Grand Prix. At the end of the season Guerrero again failed to score points and was as a result unranked in the championship while rookie teammate Johnny Cecotto managed to get a 6th-place finish and with it 1 point towards the championship for himself and Theodore (although Cecotto failed to qualify four times to Guerrero's single time).