Why is my pressure washer pulsing?
Most pulsing comes from a clogged nozzle, too little water, a blocked inlet screen or air trapped in the pump. Turn it off, release the pressure, clear the nozzle with the cleaning tool, clean the screen, use a short hose on a fully open tap, and purge the air for 30 seconds. If it still pulses, the pump needs a shop.
Free to ask. Local shops send quotes, and you pay only when you book one.
By Kasra Taheri, Founder, LoclisPublished
Researched and drafted with AI, then checked claim by claim against the sources listed below before it was published. Prices and rules change; the dates beside each source say when it was last checked.
Step by step
Read when to stop and call a pro, further down this page, before you start.
Step 1: Shut down and release pressure
Turn off the engine and the water, squeeze the trigger to relieve the pressure, set the trigger lock, and disconnect the spark plug wire on a gas machine.[1]
Step 2: Clear the nozzle
Remove the nozzle, clear it with the cleaning tool supplied, then rinse it with a garden hose for at least 30 seconds.[1]
Step 3: Clean the inlet screen
Take the filter out of the garden hose coupler, wash both sides under running water and refit it with the convex side facing out.[1]
Step 4: Fix the supply
Use a short, straight garden hose on a fully open tap with clean, cold water, at a minimum of 5 GPM and 20 PSI.[1]
Step 5: Tighten the connections
Check every hose, gun and pump fitting for leaks and tighten any that are loose.[1]
Step 6: Purge the air
With the water on and the gun aimed safely, squeeze and hold the trigger for at least 30 seconds before starting the engine.[1]
Step 7: Refresh old fuel
On a gas machine that has sat, drain stale gasoline and refill with fresh regular unleaded with no more than 10% ethanol.[1]
The short list of causes
A pulsing pressure washer is almost always starved of clean water or blocked at the nozzle. Work through the causes in the order below; each takes a minute or two and none needs tools beyond what came in the box.
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.
Clear the nozzle first
This is the first thing to check, and the only cause Simpson lists for a pulsing pump. 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] Its troubleshooting table lists "Pump or nozzle pulsates" with one cause: an obstructed nozzle.[1]
Take the nozzle out of the quick-connect on the wand and push the cleaning tool that came with the machine through the orifice. Then rinse it by running a garden hose into the nozzle outlet for at least 30 seconds.[1] A straightened paper clip is not a good substitute; a bent or enlarged orifice changes the spray and the pressure.
Check the water supply
A pump that is not getting enough water surges. 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 opened tap, a kinked garden hose, a thin hose that is very long, or a second hose splitter sharing the line with a sprinkler can all fall short.
Move the machine to a different outdoor faucet, open the tap fully, and use a short, straight garden hose. If the pulsing stops, the supply was the problem.
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] Never run the machine without it.[1] Sand and sediment from a well or a recently flushed main collect here quickly.
Purge trapped air
Air in the pump makes the spray sputter and surge. Simpson's fix is 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 and all the air is purged, before starting the engine.[1] Running a pump without water will severely damage the seals and other internal components.[1]
Loose hose connections let air in too. Check each fitting and tighten any that are loose. Hose, spray gun and pump connections should be cleaned and lubricated with a thin film of lithium grease regularly to prevent O-ring damage and leaks.[1]
If it is a gas machine that surges
Spray that pulses in step with the engine note, rising and falling, points to the engine rather than the water. Old fuel is a common reason. Penn State Extension says engines that are rarely or seasonally used are especially prone to difficulties with ethanol-blended gasoline.[2] Simpson says not to use gasoline that is older than 30 days, and not gasoline that contains more than 10% ethyl alcohol.[1] Drain the old fuel and refill with fresh if it has sat. A carburetor that stays gummed up after that is a shop job.
Stop and call a pro if
- The pulsing continues after the nozzle, the supply, the screen and the air purge. Worn pump valves or seals are the likely cause, and that is repair work for a service centre.
- 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]
- A high-pressure hose or fitting leaks. Simpson warns never to touch a leak in a high-pressure hose or fitting, because the pressure can cause fluid injection injuries, severe lacerations, amputations or death, and says never to attempt to repair a high-pressure hose.[1]
- The engine surges even on fresh fuel. Engine service is best left to an authorized service center, which is the manufacturer's own recommendation.[1]
Getting help in Virginia
If the cause turns out to be inside the pump, read how much pressure washer repair costs in Virginia and whether a pressure washer is worth repairing before you pay for it. If pressure is low rather than uneven, see how to fix a pressure washer that will not build pressure. 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.
Common questions
- Is it the pump or the nozzle that is pulsing?
- Simpson's troubleshooting table lists a single cause for a pump or nozzle that pulsates: an obstructed nozzle. Clear it with the cleaning tool that came with the machine and rinse it with a garden hose for at least 30 seconds before looking at anything else.
- Can a garden hose cause pulsing?
- Yes. The pump needs an uninterrupted supply of clean, cold water at a minimum of 5 GPM and 20 PSI. A kinked, thin or very long garden hose, or a tap that is only partly open, can starve it. Try a short, straight hose on a different faucet.
- Why does it pulse right after I start it?
- Trapped air is the usual reason. With the water on and the gun aimed in a safe direction, squeeze and hold the trigger for at least 30 seconds, or until the water runs steady, before you start the engine.
- Is it safe to keep using a pulsing pressure washer?
- Not for long. Simpson says an uncleaned nozzle can let excessive pressure develop, possibly damaging the pump or accessories, and running a pump without water severely damages the seals. Stop, find the cause, then restart.
- Could old gas make it pulse?
- On a gas machine, it can make the engine surge, which you hear and see in the spray. Penn State Extension notes that seasonally used engines are especially prone to trouble with ethanol-blended gasoline. Drain the stale fuel and refill with fresh.
Sources
- [1] Pressure Washer Use and Care Manual (part 7110656NOCA, rev. B, February 2026), Simpson Cleaning (FNA Group). Checked October 28, 2026.
- [2] Fuel Ethanol: Hero or Villain?, Penn State Extension. Checked October 28, 2026.
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