Despite Senna's seemingly commanding position, Prost remained confident of winning as he had seen that his McLaren was handling better than his teammate's. He reasoned that later in the race his only problem would be getting past. Maurício Gugelmin was black flagged for adding brake fluid in contravention of the regulations. The gap between the two McLarens varied as they worked their way through traffic, but on lap 29, Prost closed the gap when Senna suffered a misfire. The problem disappeared momentarily, with Senna doing his fastest lap of the race, but then returned, worse than before. Nigel Mansell would soon retire for the 4th time out of 5 races by lap 32 with the result of an alternator failure. On lap 34, with Prost only one second back, Senna waved his teammate past as they went down the back straight and then pitted at the end of the lap. The Honda engine's electronic fuel injection system was acting up and after two pit stops to change the black box, battery and plugs, and with successive fastest laps in between, Senna retired on lap 44 with electrical problems (notably his 1st retirement of the season). Since joining McLaren at the start of it was his first ever retirement because of a Honda engine failure and the first failure of their V10 engine under race conditions. It was also only the second time in 21 races with McLaren that a Honda engine had failed, the first being when Prost's V6 turbo had blown up halfway through the 1988 Italian Grand Prix, the only race of the 1988 season that McLaren did not win. Alex Caffi, who had started in sixth in his Pirelli shod Dallara-Ford, was up to second with Senna's retirement. A stop for new tires, after being passed by Berger (whose palms were still raw and sore from his Imola crash only six weeks before), dropped him back two more spots to fifth. As he tried to re-lap his teammate de Cesaris, however, de Cesaris turned in, forcing Caffi into the wall and out of the race. After the race de Cesaris said that he simply did not see Caffi and did not even know about putting him into the wall until after the race. De Cesaris continued on to an 8th-place finish. Berger's Ferrari suffered alternator failure (meaning no power to the revolutionary semi-automatic transmission) 9 laps after