In 1982 the non-turbocharged teams running the Cosworth DFV engine faced a growing power disadvantage against the turbocharged constructors, principally Renault and Ferrari. The minimum weight regulations in force that year required cars to pass scrutineering before and after the race, but oil and cooling liquids were permitted to be refilled before the final post-race weigh-in. FOCA teams exploited this by building their cars lighter than the legal minimum and adding water tanks. The water was dumped onto the circuit as soon as the car left the pitlane at the start of the race, reducing the car's running weight and therefore improving performance. At the end of the race, the tanks were refilled so the car would pass the post-race weight check. Keke Rosberg later described the water tank as "the one that gave us at least a theoretical chance to compete with the turbos."
The practice came under formal scrutiny after the second round of the 1982 season, the Brazilian Grand Prix at Jacarepaguá. Nelson Piquet, driving for Brabham, finished first, and Keke Rosberg, driving for Williams, finished second. Following the race, Ferrari and Renault filed protests against the first- and second-place finishers, citing the use of water tanks as illegal. The case was referred to FISA in Paris, where it remained unresolved until after the next race.
In the week before the San Marino Grand Prix, the FIA International Court of Appeal ruled in favor of Ferrari and Renault, disqualifying both Piquet and Rosberg from the Brazilian Grand Prix results. Alain Prost, who had finished third, was elevated to the race victory. The ruling also changed the scrutineering procedure: cars would henceforth be weighed after the race in the condition in which they finished, with no refilling permitted. This eliminated the water-tank loophole for the remainder of the season. All other finishers' results from the Brazilian Grand Prix were upheld, including John Watson, who inherited second place, even though his car had also used the water tanks; only the outright finishers who had been the subject of the protest were penalised.
The disqualifications provoked a strong reaction from the FOCA teams, who requested a postponement of the San Marino Grand Prix to allow time to consider the impact of the revised scrutineering rules. The race organisers refused to delay the event. As a result, the majority of FOCA teams boycotted the race, which proceeded with just 14 cars — only the turbocharged Renaults, Ferraris, and a small number of other entrants. The San Marino Grand Prix was won by Didier Pironi of Ferrari.
The water-tank episode is regarded as one of the more brazen grey-area exploits of the FOCA–FISA regulatory war that characterised early-1980s Formula One. The 1982 season itself saw 11 different race winners in 16 rounds, with the power contest between turbocharged and normally aspirated machinery shaping much of the on-track and political drama. The FIA's response — mandating post-race scrutineering in race-finish condition — closed the specific loophole and contributed to the broader tightening of technical regulations that would follow in subsequent seasons.