$75 for a thermostat........ but yes it's not that the engine thinks it's running cooler, it actually is, the Nismo is like 149*. The cooler the engine the more fuel it will dump because when the engine is actually cold, intake, head ect. the fuel will condense on the cold surfaces, once the engine is hot more of the fuel being injected is atomised and vaporised by the heat. Since the ecu thinks engine never reaches operating temp feul enrichment doesn't taper off, even as the intake track becomew heat soaked, so yes it'll probably run rich.