How do I fix a pressure washer that won't build pressure?

Most pressure washers that will not build pressure have a supply or blockage problem, not a failed pump. Work through the cheap causes in order: the water supply, the inlet screen, the nozzle, the hose, and air trapped in the pump. If none of those fixes it, the pump or its seals are worn and the machine needs a repair shop.

Simpson's manual lists the same checks for low or no pressure: an insufficient water source, a clogged nozzle, a clogged water inlet screen, a high-pressure hose that is too long, and the engine choke left in the CHOKE position.^[1] The steps below follow that order.

Before you touch anything

Shut the engine off, turn off the water, squeeze the trigger to relieve the pressure in the hose and set the trigger lock.^[1] On a gas machine, also disconnect the spark plug wire and let the engine cool. Simpson's warning is blunt: "Always disconnect spark plug wire, let the engine cool, and release all water pressure before performing any maintenance or repair."^[1] On an electric machine, unplug it.

Check the water first

The pump needs more water than many people expect. Simpson says to be sure the supply can provide an uninterrupted source of clean, cold water at a minimum rate of 5 gallons per minute (GPM) and 20 pounds per square inch (PSI) of pressure, and not to use hot water.^[1] A partly closed tap, a kinked or split garden hose, or a thin hose that is too long can starve the pump.

If the machine runs but pressure is weak or surging, the quick test is to move it to a different outdoor faucet with a short, straight hose.

Clean the inlet screen

The screen sits in the garden hose coupler at the pump inlet. Simpson says to remove the filter from the garden hose coupler, use water from a running garden hose to clean both sides of it, and reinsert it into the pump inlet with the convex side facing out.^[1] Do not run the machine without it.^[1]

Clear the nozzle

Simpson says that if a nozzle becomes clogged, the pump may pulsate and the spray pattern could change, and that an uncleaned nozzle can let excessive pressure develop, possibly damaging the pump or accessories.^[1] Take the nozzle out of the quick-connect on the wand, clear it with the cleaning tool that came with the machine, then rinse it by running a garden hose into the nozzle outlet for at least 30 seconds.^[1]

Shorten the high-pressure hose and purge the air

Simpson lists a high-pressure hose that is too long as a cause of low pressure and says the hose should be less than 100 feet.^[1] Run the machine on the hose it shipped with. Check each hose connection for leaks and tighten loose ones.

If the pump has air trapped in it, pressure will not come up. Simpson says to connect the hoses, turn on the cold water, aim the spray gun in a safe direction, and squeeze and hold the trigger for at least 30 seconds or until the water runs steady, before starting the engine.^[1] Running a pump without water severely damages the seals and internal parts.^[1]

Check the settings

Two quick ones. On a gas machine, move the engine choke to the RUN position once the engine is going.^[1] And make sure the spray wand is not set to low pressure, or fitted with a wide low-pressure nozzle. Simpson notes that the white 40-degree nozzle produces a lower-pressure, wider spray.^[1] If your machine has an unloader hand control knob, turn it clockwise until it stops to return to the preset highest pressure, and do not overtighten it past that stop.^[1]

If it is a gas machine that sat a while

Old fuel is a common reason a gas pressure washer runs badly after storage. Penn State Extension says engines that are rarely or seasonally used are especially prone to difficulties with ethanol-blended gasoline.^[2] Drain the old fuel and refill with fresh, and use a fuel stabilizer if it will sit again. A carburetor that stays gummed up after that is a shop job.

Stop and call a pro if

• Water drips from the pump. Simpson's table lists a worn or torn O-ring, worn piston packing or a pump head damaged by freezing, and tells owners to contact customer service for the last two.^[1]
• The pump still pulsates after the nozzle is clean and the air is purged. Worn packing or valves are the usual next suspects, and they mean opening the pump.
• Oil is leaking from the pump. Worn oil seals are on Simpson's list of causes, and the manual tells owners to contact customer service.^[1]
• A high-pressure hose or fitting leaks. Simpson says never to touch a leak in a high-pressure hose or fitting and never to attempt to repair a high-pressure hose, because the pressure can cause fluid injection injuries.^[1]
• The machine sat through freezing weather with water in the pump. Simpson says water left in the pump during freezing weather will cause severe damage.^[1]
• The engine will not start or runs badly. That is a small-engine job, not a pressure fault.

Simpson recommends that all service and repairs be performed by an authorized service center.^[1]

Getting help in Virginia

Before you pay for a repair, read how much pressure washer repair costs in Virginia and whether a pressure washer is worth repairing. You can also browse small engine repair answers or find local small engine shops.

When you are ready, describe the problem and local shops can quote it.