Gas Pressure Washer: Why does water spray out of my pressure washer's pressure relief valve?

The thermal relief valve opens to release hot, high-pressure water from the pump system onto the ground to prevent damage to the pump. Hot water builds up when the spray gun is unused for more than 5 minutes while the pump is running. The unloader valve recirculates water back to the pump inlet to control the pressure and temperature. If the water gets too hot, the thermal relief valve opens to relieve the pressure by dumping hot water on the ground. The thermal relief valve closes after the pressure is released and the pump cools. If the thermal relief valve fails to open and the unloader valve fails shut, high pressure and heat damages the pump seals and check valves, and the pump won't develop pressure. If the thermal relief valve fails to shut properly, water is constantly purged through the relief valve and the pump can't build up pressure. Replace the thermal relieve valve if it fails to open and close properly to relieve pressure and heat from the pump system.

The unloader valve reduces water pressure within the pressure washer pump when the pump runs but the spray wand isn't used, by recirculating water to the pump inlet. If the valve sticks in the closed position, the thermal relief valve frequently discharges hot water onto the ground because the unloader valve can't open to allow recirculation. If the pressure washer doesn't develop adequate outlet pressure, the unloader valve could be stuck open. Replace the unloader valve if it sticks in either the closed or open position.