Last Updated on September 25, 2026 by Umar Farooq
A pressure washer won’t prime when the air sitting inside the pump has no way out of the machine — and the thing most likely to be blocking its exit is the spray tip still clipped onto the end of the lance, which is exactly why the manufacturers tell you to take it off before you start. The motor runs, the pump sounds busy, and what reaches the wand is a dribble, a cough, or nothing at all.
Almost nobody types the word prime and stops there. Going through what people actually post, the same fault turns up as not priming, priming trouble, won’t pump water, won’t pull water, won’t draw from the tank, and — from anyone who has owned the machine longer than a season — it primed fine last year.
What Priming Means on a Machine With No Prime Button
There is no prime button on a pressure washer. There is no prime lever and no prime setting. What there is instead is a sequence, printed in the chapter of the manual titled Setup, which is the chapter you read once, in the driveway, with the cardboard still on the lawn.
Priming is simply replacing the air in the pump with water. A plunger arriving at the top of its stroke against a pocket of air squeezes the pocket smaller and hands it back; against water, which does not compress, it has something to push. Until the chamber is full of liquid, the pump is an expensive air compressor with a bad attitude.
Graco’s operator manual for its gas machines says when this matters and gives a finish line rather than a duration: it is “essential to prime the pump on initial start-up and each time the water supply is disconnected from the unit after initial use”, and the “unit is primed when water flow is uninterrupted by air.” Not a count of seconds. A quality of stream.
That definition also draws the line between this page and its neighbours, because the three get blended together everywhere else:
- Nothing ever arrives, however long you hold the trigger. A failure to prime. This page.
- Water arrives with bursts of air mixed into it. The pump primed, and something is admitting air into the stream. That is pressure washer spitting air, which carries the full two-stage purge.
- Nothing at the wand while the machine keeps running. Same symptom, different urgency: switch it off first, because a dry pump is being damaged while you read. The shutdown sequence lives on pressure washer no water coming out.
Two things people go looking for separately and will not find here. A stuck check valve is a symptom of an unfiltered inlet rather than a topic of its own, so it lives in the last section below. And a machine that primes, builds, then collapses after a few seconds has primed successfully — that is a supply or unloader question.
Pressure Washer Won’t Prime? Take the Nozzle Off First
The mechanism is almost insultingly simple. To prime, the pump has to push the air in front of it out of the machine, and the only exit is the end of the lance. If a spray tip is fitted, that exit is an orifice sized to turn 2.5 gallons a minute into something that hurts — and you are asking a pump with no suction yet to evacuate a column of air through it.
Graco puts this in a boxed NOTICE, which tells you how often it gets skipped: “Make sure the nozzle is not connected to the unit while priming the pump.” Its stated reason is not even the one above — priming, it warns, “allows mineral deposits to be released from the system which could obstruct or damage the nozzle assembly.” So the tip is both the thing stopping the air leaving and the thing the departing grit is about to ruin. Two problems, one clip.
The second half of the sequence is when you start the engine. Simpson’s manual numbers it: step 10 is “remove all air from the pump and high pressure hose by depressing trigger until a steady stream of water is present”, and step 11 is start the engine. Graco’s troubleshooting table catches the same mistake from the other end: against a machine that will not build, the listed cause is “gun not triggered” and the fix is to trigger the gun *while* starting.
So, on a cold machine:
1. Machine off. Tap fully open.
2. Nozzle off the lance entirely. Put it in your pocket, not on the ground.
3. Squeeze and hold the trigger. Water should arrive at plain tap pressure, looking far too gentle to be correct. It is correct.
4. Keep holding until the stream is continuous and silent. Uninterrupted by air is the test, not a count.
5. Start the machine with the trigger still held.
6. Shut down, fit the nozzle, go to work.
The longer version — disconnecting the high-pressure hose first so the pump head purges on its own, before fifty feet of hose gets involved — is laid out on the spitting air page. No point printing it twice.
And here is the part I will be blunt about. A great many of the machines described online as “won’t prime” are not faulty. They are being started in the wrong order by someone who has owned the machine three years and therefore never read the setup chapter. Both manuals quoted above put the purge before the start instruction; neither puts it after. That is not a tip, it is the procedure.
Is Water Actually Reaching the Inlet?
If the purge produces nothing at all with the machine switched off, stop suspecting the pump. With the motor off there is no pump involved — that is your tap, your hose and a screen, and something in that chain is shut.
Work along it in this order:
- The tap, fully open, not cracked open. Graco specifies an incoming water pressure of 25 to 125 psi and adds the useful aside that “a typical outdoor faucet will generally supply this psi if fully turned on.” Fully. A quarter turn is not a supply.
- The hose, laid out flat. A hose still on its reel, or kinked behind a corner, is a closed valve as far as the pump is concerned.
- The inlet screen. The small mesh filter at the machine’s inlet is the first thing to clog and the last thing anyone looks at.
- Every fitting in between. Y-splitters, inline filters, quick-connects with internal check valves that have stuck shut.
Then measure rather than assume. Disconnect the hose at the machine and fill a bucket for a minute. Simpson’s requirement for its gas units is at least 5 gallons per minute at 20 psi, with a further note most owners have never seen: if the source is a well, the garden hose may be no more than 30 feet long. The full supply-and-screen sweep is on the no water coming out page.
Most Consumer Pumps Cannot Lift Water at All
This is where good intentions do the damage. Someone sets the machine next to a rain barrel, drops the inlet hose into a bucket, and concludes from the silence that the pump has failed to prime. It has not failed. It was never able.
“Self-priming,” in pump language, means a pump that can clear the air out of its own suction line and pull water up to itself. A homeowner axial wobble-plate pump is not that pump. Look at what Graco asks for at the inlet: a *minimum of 25 psi*. That is about 58 feet of water standing above the pump. Not a pump that might struggle to lift from a barrel — a pump that expects nearly sixty feet of head pushing water into it. A container on the ground offers about minus two.
If you must run from a tank, the fixes are geometric rather than clever: outlet above the pump inlet, short fat plumbing, a tank kept fuller than you think it needs. Arimitsu’s pump troubleshooting guide describes the failure that catches people out even when the geometry is right — as the level drops, “when the water level inside the supply tank reaches a point in which the head pressure does not exceed the suction from the pump, a vortex forms allowing air to siphon into the pump,” and a baffle or a turned elbow at the outlet is what stops it. The full tank arithmetic is on pressure washer cavitation.
So: if your machine will not prime from a bucket, you are not missing a technique. You are missing a booster pump, or a tank on a stand.
Leave the Inlet Check Valve Until Everything Else Is Clean
Inside the pump head, each plunger works between two one-way valves. The inlet valve opens as the plunger withdraws; it snaps shut as the plunger advances, so the water has to leave by the outlet instead. A grain of sand under that valve holds it a fraction off its seat, and the chamber simply breathes in and out of the inlet. No suction develops, nothing fills, and the machine will do this all afternoon.
Graco’s troubleshooting table gives the remedy in one line and then adds a second instruction that is the real content: clean the inlet and discharge valve assemblies — and *check the filter*. A fouled inlet valve is almost never a random event. It is the receipt for a screen that was torn, missing, or last rinsed during a different presidency.
Three reasons this suspect goes last:
1. It needs the machine opened. On a triplex pump the valves sit under hex caps and come out with basic tools. On the sealed axial pump in most homeowner machines they are pressed in and not serviceable, which turns a diagnosis into a pump purchase.
2. Its symptom is shared. Everything above produces the same nothing-at-the-wand, so the valve is only the remaining explanation once supply, screen, nozzle and inlet geometry are proven.
3. The tell is specific. Water flows freely in with the motor off, the motor runs at normal speed and sound, nothing arrives at the wand, and the pump makes no rhythmic pumping note at all. That exact combination, with a clean screen, earns you the spanners.
FAQ
Why won’t my pressure washer prime?
Because the air in the pump cannot get out, water cannot get in, or the pump is being asked to lift water it was never built to lift. Check in that order: nozzle off and trigger held with the machine off, then the tap and inlet screen, then whether the supply sits below the pump inlet. The inlet valves come last.
How do I prime a pressure washer pump?
Machine off, tap fully open, nozzle removed, trigger squeezed and held until the stream is continuous and free of air, then start the machine with the trigger still held. Fit the nozzle afterwards. Graco defines the finish line as water flow uninterrupted by air, which beats any stopwatch.
How long should it take to prime?
On a short hose with the nozzle off, seconds. On fifty feet of high-pressure hose, up to about a minute. What matters is the stream, not the clock — if it is still coughing after a minute you are not clearing trapped air, you are drawing a continuous leak, which is a different fault.
Why won’t my pressure washer draw water from a bucket?
Because it almost certainly cannot. Most consumer pumps are not self-priming and expect a flooded inlet — Graco asks for 25 psi minimum at the inlet, which is around 58 feet of head. A bucket on the ground provides negative head. Raise the supply above the pump, or feed the machine from a tap.
My pressure washer primed last year and won’t now — what changed?
Usually the screen. A season of sediment, a winter of rust flushed out of the garden hose, and the filter that passed 5 GPM last autumn passes a trickle. Pull it, hold it to the light, and look for clear mesh across the whole face rather than a clear patch in the middle.
Is a stuck unloader the reason it won’t prime?
Rarely, and it is the wrong place to start. An unloader stuck in bypass gives you flow with no pressure rather than no flow at all. If water is arriving and simply has no force behind it, you primed successfully and your problem is downstream of this page.
Prime Before the Engine, Not After It
Both of the manuals I quoted here put the purge before the start instruction, and Graco puts the nozzle-off requirement in a box of its own. That single ordering — water, trigger, nozzle off, *then* engine — is the difference between a machine that primes in twenty seconds and one that convinces its owner the pump is dead.
So before you buy a valve kit, price a replacement pump, or type a model number into a search bar at nine on a Saturday morning: take the tip off, open the tap, hold the trigger with the machine switched off, and wait for the stream to go quiet. It is free, it takes under a minute, and it is the one thing the people who built the machine actually asked of you.
