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Pressure Washer Unloader Valve Symptoms

If you have used pressure washers for a while, you may have experienced a faulty unloader valve (also known as a bypass valve) without knowing it.  This is because the symptoms that arise from a bad unloader valve can mimic other problems, like no water pressure when the trigger is pressed, or the engine stalling for apparently no reason.  Often these problems are incorrectly diagnosed and lead to unnecessary repairs, when the problem is a stuck unloader valve.  Today we will discuss what an unloader valve is, what it does, and what happens when an unloader valve fails.

What Is an Unloader Valve?

An unloader valve is used to divert water from one place to another and is usually controlled by changes in water pressure.  In a pressure washer, this means the system maintains available water pressure any time the pressure washer is running, even if it is not being used.  This is in contrast to an “on demand” style of pump, which essentially sits idle until the trigger is activated.  Unloader valves are very important in the pump industry, as they function as a gatekeeper to prevent dangerous water pressures from building.  Since they are usually controlled by the inlet and outlet water pressure, these valves automatically open and close, keeping the water pressure constant, but immediately available.

What Does an Unloader Valve Do In a Pressure Washer?

An unloader valve essentially tells the pressure washer when to allow more fresh water to enter the pump.  Unloader valves are usually used with positive displacement pumps, which provide constant water flow to either the pump itself, or the wand.  For example, when the pressure washer is running, but the trigger on the wand is not activated, the unloader valve simply returns the water to the pump to be recirculated.  However, when the trigger is depressed, the unloader valve senses the change in pressure, allows water to exit the wand, and allows new water to enter the system.  

How Do I Tell If My Unloader Valve is Bad?

Often, one of two things will happen when an unloader valve fails:  

No Pressure From the Wand

In most occurrences, the first symptom of an unloader valve going bad is no pressure from the wand when the trigger is pressed.  This happens when the unloader valve gets stuck open and does not allow water to enter the hose and trigger.  In this instance, the unloader valve cannot sense the difference in pressure, so it simply allows the water to recirculate back into the pump.  As such, the water pressure cannot be discharged, yet the water supply into the pump remains constant and can cause the engine to overheat due to a lack of fresh, cool water to cool down the pump.

Too Much Pressure

When an unloader valve gets stuck closed, the result is often a stalling of the engine and leaking of the spray tip.  This occurs because the unloader valve cannot release the pressure as it should, even when the trigger is depressed.  Because the pressure being built up by the pressure washer cannot be released via the valve, often the result is a leaking spray tip.  In most situations,  the engine will then stall when it reaches the maximum pressure it can provide.  

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