Why Does My Pressure Washer Pulse at Low Pressure? Water Supply, Air, Nozzle and Unloader, in Order

What pulsing means

A pressure washer pump delivers a steady stream only when it is fed more water than it pumps out. When the supply falls short, the pump pulls in air and bubbles, the pressure rises and collapses in a rhythm you can feel at the wand, and the motor note surges with it. The same symptom appears when air is trapped in the pump, when a blockage at the nozzle makes the unloader valve cycle, or when the unloader itself sticks. The order below runs from the most common and easiest cause to the least, and most pulsing is cured at step one or two.

Step one: the water supply

Every homeowner pressure washer needs a minimum flow, printed on the rating plate in gallons per minute, and it needs it from a hose that can deliver it. Check the tap: it should fill a 5-gallon bucket in about two minutes or less. Check the hose: 5/8-inch diameter, under 50 feet, no kinks, no quick-connect fittings with narrow bores, and not a lightweight expanding hose, which collapses under suction. Check that the hose is not run through a Y splitter with another hose open. Then check the inlet screen where the hose connects to the pump; it is a small mesh filter and it clogs with grit and scale. Rinse it. A starved pump pulses, cavitates and wears out its seals, so this step matters for the life of the machine, not just the symptom.

Step two: air in the pump

A pump that has run dry, a hose connected after the motor started, or a loose inlet fitting lets air into the pump, and the air compresses and expands with each stroke to cause pulsing. Purge it: connect the water, leave the motor off, remove the nozzle from the wand or hold the trigger open, and let water run through until it flows steady with no spitting. Tighten the inlet connection. Start the motor only with water flowing. On a gas unit also check the inlet hose washer is in place; a missing washer draws air.

Step three: the nozzle, wand and gun filter

A nozzle orifice partly blocked by grit raises pressure behind it until the unloader valve opens, then pressure falls, the unloader closes and the cycle repeats as a pulse. Remove the nozzle and clean the orifice with the pin tool supplied or a straightened paper clip, then back-flush it with water. Check the small screen inside the gun inlet and the wand for debris. Try a different nozzle; if the pulsing stops, the nozzle was the fault. Also confirm the right nozzle is fitted: a soap nozzle at full pressure or a turbo nozzle on a low-flow machine can behave oddly.

Step four: the unloader valve

The unloader diverts water back to the pump inlet when the trigger is released, and it must close cleanly when the trigger is pulled. A valve stuck partway by scale or a worn spring or seat lets pressure bleed off, and the pump surges as the valve chatters. Symptoms are pulsing at the wand with full flow, pressure that climbs and then drops after a few seconds, or a machine that will not reach rated pressure. On many pumps the unloader can be removed, cleaned and its O-rings replaced; on others it is a bolt-in assembly. Do not adjust the unloader pressure screw to chase the symptom; that risks over-pressure.

Step five: pump valves and seals

Inside the pump, inlet and outlet check valves open and close with each piston stroke. A valve with a chip of debris under it, a cracked valve spring or a worn seat lets water flow backward, and the output pulses. Worn piston packings let air in. These are the causes when steps one to four are clean, and they are the usual reason an older machine pulses. Pump valve kits and seal kits are sold for most axial and triplex pumps; on cheap axial pumps a replacement pump often costs less than the kit and the labor.

Step six: temperature and detergent

Hot inlet water, above about 100 F, makes the pump cavitate and pulse; feed cold water. A detergent injector with its pickup tube open to air draws air into the stream on units that inject on the inlet side; close the detergent valve or cap the tube. On electric units, a machine that stops and starts at the motor rather than pulsing at the wand is doing its normal total-stop cycling, and a hum that cycles with a weak stream is usually low supply flow again.

Preventing it

Run the washer on a full 5/8-inch hose from a strong tap, rinse the inlet screen each season, purge air before every start, flush the nozzles after use, run pump protector fluid through the pump before storage and never let the pump run more than a minute with the trigger released. A washer treated that way pulses only when something is clogged, and the order above finds it quickly.