Bypass line? Thermostats are there to regulate the water flow through the radiator and regulate engine top end temperatures in an effort to keep them more stable. Without them, full flow through the radiator means not only periods of fluctuating temps, but in most cases by the coolant is flowing through the radiator at a high rate, it does not dissipate all the heat it could have if it were spending a little more time in the radiator. So the cooling cycle starts with warmer coolant then it would have and ends with it being hotter then it would have. In short, its a bad idea to eliminate the thermostat from any engine. If one has failed, yes, pull it to get home but replace it asap.