Fuel additivation and diagnostics system for a vehicle with internal combustion engine comprising: ▪ a device for dispensing at least one additive, into a fuel circulation circuit of the engine of the vehicle which comprises: - a tank containing the additive, - a chamber communicating with the fuel circulation circuit and into which is inserted the tank containing the additive, at least one mobile and sealed wall (32, 33, 34) between said chamber and said tank ensuring, on the one hand, a tight separation, and, on the other hand, maintaining an identical pressure between the additive in the tank and the fuel in the chamber, - additive injection means linked to the tank and to the fuel circulation circuit and making it possible to dispense the additive into the fuel circulation circuit, said means comprising a dispensing channel (16, 16a, 16b) linking the tank and the fuel circulation circuit, - a fuel input orifice, a fuel output orifice, an additive dispensing orifice and a means (21, 25) for generating a pressure difference between the fuel input orifice or output orifice and the additive dispensing orifice; ▪ a fuel analysis device for measuring the additive content in the fuel; ▪ means for sending information as a function of the results of the analysis of the fuel obtained by said analysis device. ▪ a device for dispensing at least one additive, into a fuel circulation circuit of the engine of the vehicle which comprises: - a tank containing the additive, - a chamber communicating with the fuel circulation circuit and into which is inserted the tank containing the additive, at least one mobile and sealed wall (32, 33, 34) between said chamber and said tank ensuring, on the one hand, a tight separation, and, on the other hand, maintaining an identical pressure between the additive in the tank and the fuel in the chamber, - additive injection means linked to the tank and to the fuel circulation circuit and making it possible to dispense the additive into the fuel circulation circuit, said means comprising a dispensing channel (16, 16a, 16b) linking the tank and the fuel circulation circuit, - a fuel input orifice, a fuel output orifice, an additive dispensing orifice and a means (21, 25) for generating a pressure difference between the fuel input orifice or output orifice and the additive dispensing orifice; - a tank containing the additive, - a chamber communicating with the fuel circulation circuit and into which is inserted the tank containing the additive, at least one mobile and sealed wall (32, 33, 34) between said chamber and said tank ensuring, on the one hand, a tight separation, and, on the other hand, maintaining an identical pressure between the additive in the tank and the fuel in the chamber, - additive injection means linked to the tank and to the fuel circulation circuit and making it possible to dispense the additive into the fuel circulation circuit, said means comprising a dispensing channel (16, 16a, 16b) linking the tank and the fuel circulation circuit, - a fuel input orifice, a fuel output orifice, an additive dispensing orifice and a means (21, 25) for generating a pressure difference between the fuel input orifice or output orifice and the additive dispensing orifice; ▪ a fuel analysis device for measuring the additive content in the fuel; ▪ means for sending information as a function of the results of the analysis of the fuel obtained by said analysis device.