Mercedes reveals cause of Valtteri Bottas' Spanish GP engine failure

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Mercedes says a turbo defect it has never encountered before led to Valtteri Bottas' engine failure at the Spanish Grand Prix.

Bottas was running third in Barcelona when the engine issue occurred, forcing him to retire from the race and costing him valuable ground to Sebastian Vettel and teammate Lewis Hamilton in the title fight. Mercedes had been forced to revert back to the power unit Bottas had used at the opening four races of 2017 when it detected a water leak in his new power unit ahead of FP3.

Though Bottas was running an older engine, Mercedes has revealed a defect led to the failure, something team boss Toto Wolff believes is proof the team must remain "double diligent" in its fight with Ferrari.

"It was painful to lose 15 valuable points with Valtteri retiring from P3," Wolff said. "We've identified the root cause of the problem, which was the turbo. We haven't seen that defect before, which shows you that you need to be double diligent.

"This is a technical sport and if you stretch your limits, you'll encounter technical problems."

The failure saw Bottas drop 41 points behind championship leader Vettel, just one race after claiming his maiden career win at the Russian Grand Prix. Though Mercedes left Spain having taken back the lead of the constructors' championship, it did so by just eight points -- it would have been 23 if Bottas had finished the race in third.

Bottas had played a key role in Hamilton's eventual win in Spain, stalling Vettel at a crucial point in the Ferrari driver's charge with soft tyres and preventing him from building too large a gap to his Mercedes teammate before his pit stop.