Red Bull were the ones who brought up the issue with the FIA, not Merc
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"Before the Mexican Grand Prix, though,
Red Bull wrote to the FIA asking for clarification about whether or not utilising a system that got around the fuel flow sensor would be allowed. Such queries are often done to root out what rivals are doing,
The governing body responded by stating that such a system would not be allowed. It was clear that gaming the fuel flow rate to boost it between the point that measurements were taken would be against the rules."