A warrior off-road vehicle, after running for a period of time, the coolant continues to emerge from the radiator pressure cap. Fault detection: for the fault phenomenon, first check the pumps, fans and belts, and find no abnormalities. Then the thermostat was removed for inspection, and the technical condition was normal. The cylinder pad was replaced, and the failure could not be eliminated. After careful examination, it was found that the temperature of the coolant thermometer was not high when the engine was "open", so the pressure cover of the radiator was examined carefully. It was found that the opening pressure of the pressure relief valve was much lower than the standard opening pressure (88 + 14.7) kPa, and the failure was caused by the failure of the pressure relief valve spring. Troubleshooting: replace the radiator pressure cap after trial run, failure phenomenon disappear, troubleshooting. Fault analysis: all radiator of water cooled engine is equipped with pressure cap. The internal pressure of the cooling system working normally is slightly higher than the atmospheric pressure, thereby increasing the boiling point of the coolant and enhancing the heat transfer ability of the external heat exchanger. The pressure cap has a relief valve. For the cooling system of the general vehicle, when the system pressure reaches 88, 14 thousand and 700 PA, the valve opens. The warrior's car is damaged by the pressure relief valve spring, and the cooling fluid circulating in the cooling system will flow out of the air without control, creating a similar "open - pot" illusion.