500 at Pocono International Raceway in the Lola and the Bosch Spark Plugs Grand Prix at Nazareth Speedway in the March. After starting 10th at Long Beach Guerrero's best start was at Nazareth, where he started 11th. At the end of the season Guerrero finished 12th in points with 40 points, despite skipping two races. Following the 1988 season Guerrero and Granatelli split for 1989. Guerrero joined Alex Morales Motorsports, who was at the time the factory (and only) team using Alfa Romeo engines. Guerrero drove the No. 21 Alfa Romeo March 89CE-Alfa Romeo Indy V8. The engine ended up being not ready for the start of the season and the team missed the first four races of the year, including the Indianapolis 500. Guerrero was even offered to drive an extra car for Truesports. Guerrero had to decline due to his contract with Alfa Romeo. The team managed to first compete at the Valvoline Detroit Grand Prix on the Streets of Detroit. In the race Guerrero was able to finish 8th. The remainder of the season saw Guerrero score only one additional point due to a 12th-place finish at the Red Roof Inns 200 at Mid-Ohio Sports Car Course. Guerrero would finish 23rd in points with 6 points. Prior to 1990 Morales Motorsports closed down and Patrick Racing took over the Alfa Romeo project with Guerrero driving the No. 20 Miller Genuine Draft March 90CA-Alfa Romeo Indy V8. Guerrero managed to qualify for the Indianapolis 500, starting 28th and finishing 23rd due to suspension damage. The team then switched to using a Lola T90/00 starting at the Marlboro 500 at Michigan International Speedway. In the race Guerrero finished in 5th place. At the end of the season Guerrero finished 16th in the points with 24 points. In 1991 Guerrero was replaced by Danny Sullivan and was left without a car to drive. At the Indianapolis 500 Guerrero drove a second car for Patrick Racing in the No. 40 Sharp's Lola T91/00-Alfa Romeo Indy V8. He qualified 28th and finished 30th after being involved in a crash with Kevin Cogan on Guerrero's 24th lap. Guerrero then drove three select races for King Racing in the No. 26 Quaker State Lola T91/00-Buick V6, scoring a best finish of 14th at the Valvoline Detroit Grand Prix on the Streets of Detroit and the Texaco/Havoline Grand Prix of Denver at the Pepsi Center.