Almost every "my pressure washer lost pressure" and "how do I turn the pressure down" question we get at the shop ends up at the unloader valve. The unloader is the brass valve on or near the pump with an adjusting screw or knob on top, and that screw is the pressure adjustment on your machine. Here is how to set it correctly, and why turning it is usually the wrong fix.
The short answer: adjust pressure with the nozzle, not the unloader
The unloader's job is to set a ceiling and to dump water back to the inlet when you let go of the trigger. It is not a throttle. Once the unloader is set to the pump's rated pressure with the correct nozzle, leave it alone. If you want less pressure for a delicate surface, put a larger nozzle on the gun. Turning the unloader down chokes flow, and flow is what cleans; you can add 40 percent to your cleaning time and still not protect the surface as well as a bigger tip would.
Five rules before you touch the adjustment screw
- Know your pump's maximum rating. GPM and PSI are on the pump tag or the spec sheet. Everything below is measured against that.
- Never exceed it. Running a 3,500 PSI pump at 4,000 PSI because the unloader will allow it is how pumps and unloaders die early.
- Do not use a nozzle so small that the pump can exceed its rating. The nozzle orifice is what lets pressure build; too small an orifice lets it build past the rating.
- Once you find the right nozzle, stop adjusting. Pull the adjusting knob off if you have to. The setting should not change job to job.
- Lower pressure with a bigger nozzle, not the unloader. Keep the #4.5 that gives you rated pressure for flatwork and carry a #10 or #12 for siding and soft surfaces.
Step by step: setting the unloader with the right nozzle
Say your pump is rated 5.6 GPM at 3,500 PSI. A nozzle chart will suggest a size, usually around a #5.0 or #5.5 for that flow. Treat the chart as a starting point; manufacturer ratings are optimistic.
- Remove any downstream injector from the line so it is not restricting flow while you set pressure.
- Connect the hose and gun, install the chart's nozzle, and put a pressure gauge on the pump outlet. Without a gauge you are guessing.
- Start the engine and hold the trigger open.
- Turn the adjusting screw in slowly, watching the gauge, until you reach the rated pressure: 3,500 PSI in this example.
- If the gauge will go past 3,500 with more adjustment, the nozzle is too small. Go up a size and repeat until rated pressure is the ceiling.
- Snug the jam nut. You are done. Write the nozzle size on the machine.
Where the pressure adjustment screw is, and which kind of unloader you have
The adjustment is on top of the unloader body: a knob, or a bolt with a jam nut. There are two families of unloader and they adjust differently.
Flow-actuated unloaders (Pulsar, K7, most trapped-pressure units)
Adjust with the machine running and the trigger closed. Loosen the jam nut and turn the adjusting bolt all the way into the body. On a flow-actuated unloader, turning the bolt out raises pressure. Go one full turn at a time, then open the gun, wait a few seconds for the reading to settle, and check the gauge. This is the type we run on every build, block-mounted on the skid rather than hung off the pump, because pump vibration is what shakes unloaders out of adjustment and eventually kills them.
Pressure-actuated and compensating unloaders (knob or nut on a spring)
Start with the knob turned all the way out so there is almost no spring tension, or with the adjusting nut backed off after loosening the jam nut. Turn the knob or nut clockwise a little at a time until the gauge reads rated pressure with the trigger open. Never compress the spring fully; a bottomed-out spring causes a pressure spike every time you close the gun, which is hard on hoses, guns and the pump.
Two tools you need
- A pressure gauge rated at least 1.5 times the pump's maximum. For a 3,500 PSI pump that means a 5,250 PSI gauge or better.
- An engine tachometer. A gas pressure washer needs roughly 3,600 RPM at the engine to make rated flow. If the engine is running slow, you lose horsepower and the pump cannot hit its numbers no matter what the unloader is set to.
If the pressure is low and the unloader is set right
Check the nozzle for wear (a worn orifice is bigger, so pressure drops), check the inlet filter and buffer tank supply, check for a leaking or stuck injector, and confirm engine RPM. Nine times out of ten it is a worn nozzle or a starved inlet, not the unloader. If the unloader itself is chattering, weeping, or will not hold a setting, it is time to replace it: we stock pressure washer unloaders including the K7 series and the Pulsar flow-sensitive unloader we build with, plus nozzles in every size.
Still stuck? Call the shop at 503-403-6185, Monday through Thursday, 8am to 4pm Pacific. Real builders answer, and troubleshooting is free.
