With no championship points in Formula One and no prospects to drive for a competitive team, Guerrero left at the end of the 1983 season to race in the United States.

He had an auspicious beginning to his Indycar racing career, winning both CART and Indianapolis 500 rookie of the year honors in 1984.

14 Café de Colombia/Moda Caribu Ensign N180B-Ford Cosworth DFV at the season-opening South African Grand Prix.

An official press report from Nunn stated that Guerrero was "in an inappropriate physical and mental condition to drive.

At the Monaco Grand Prix Ensign's tire supplier Avon withdrew from Formula One and that left the team tireless.

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).

The high point of Guerrero's season was a 2nd-place finish at the Indianapolis 500 where he also shared the race's rookie of the year award with Michael Andretti.

Guerrero also scored a pair of 2nd-place finishes, first at the Escort Radar Warning 200 at Mid-Ohio Sports Car Course and the Miami Indy Challenge 200 at Tamiami Park.

In the race Guerrero qualified 5th and was viewed as among the favorites, along with pole sitter Mario Andretti and defending winner Bobby Rahal.

Third gear had been damaged earlier in the race by the Bettenhausen incident and the car began to get moving, only to stop just outside the pit lane.

After his near-win in Indianapolis, subsequently, Guerrero scored four pole positions at the Miller American 200 in Honor of Rex Mays at the Milwaukee Mile, the Budweiser/G.I.

Joe's 200 presented by Texaco at Portland International Raceway, Budweiser Grand Prix of Cleveland at Burke Lakefront Airport, and the Escort Radar Warning 200 at Mid-Ohio Sports Car Course.

Just days after his Mid-Ohio victory, Guerrero was testing at the Indianapolis Motor Speedway, when he crashed and was hit by a tire off of his car.

Guerrero's recovery from his near-fatal accident was documented by IndyCar Medical Director, Dr. Steve Olvey, who said in his biography, Rapid Response: My Inside-Story as a Motor-Racing Life-Saver, that he saved Guerrero's life by lowering the pressure in his brain with injections of medicines that were brand-new to the medical field and were not widely practiced yet.

However, the remainder of the season was mostly a disappointment, such as the Indianapolis 500 where Guerrero qualified 12th and did not even make it through the first lap due to being collected by Scott Brayton.

Guerrero's only top tens after Indianapolis were the Quaker State 500 at Pocono International Raceway in the Lola and the Bosch Spark Plugs Grand Prix at Nazareth Speedway in the March.

Guerrero joined Alex Morales Motorsports, who was at the time the factory (and only) team using Alfa Romeo engines.

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.

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.

Prior to 1990 Morales Motorsports closed down and Patrick Racing took over the Alfa Romeo project with Guerrero driving the No.

At the Indianapolis 500 King updated Guerrero to a Lola T92/00-Buick V6 and the number was changed to 36 (Jim Crawford began to drive the No.

During the year Guerrero and King consistently qualified in the mid-pack, with a best start of 4th at the Valvoline 200 at Phoenix International Raceway.

21 WavePhore Reynard 94i-Ford Cosworth XB, with additional sponsorship coming from Johnny Lightning (at Disney), Newscast (at Phoenix) and Pennzoil (at Indianapolis).

At the Dura Lube 200 at Phoenix International Raceway Guerrero qualified 3rd and finished 16th due to a broken cv joint.

At the Indianapolis 500 the team updated to a Reynard 95i and Guerrero qualified 6th and led for a race-high 47 laps and battled for the win with Tony Stewart, Arie Luyendyk, Davy Jones, Alessandro Zampedri and eventual winner Buddy Lazier.

Starting at the Indy 200 at Walt Disney World Speedway the series began to use a different type of spec car with Guerrero driving the No.

Starting with the Samsonite 200 at Pikes Peak International Raceway the team switched to the Oldsmobile Aurora Indy V8.

At the season-opening Indy 200 at Walt Disney World Speedway Guerrero qualified 6th and finished 26th after retiring due to a crash after 13 laps.

Guerrero attempted to compete in the NASCAR Busch Series during the 2000 season at the Carquest Auto Parts 300 at Lowe's Motor Speedway for the Hispanic Racing Team in their No.

09 Ciclon Energy Drink Chevrolet Monte Carlo at the Aaron's 312 at Atlanta Motor Speedway but again failed to qualify for the race.