Pressure Washer Pump Teardown (ProUser PP150 5.5hp 2200psi)

My ‘cross season has gradually gotten more serious, with two bikes and a pressure washer; both vital at last Sunday’s National Trophy. I’ve always wanted a pressure washer anyway (partly for washing bikes but also because I like mechanical things!) and this £100 GumTree special couldn’t be turned down. It’s been good to me so far but it’s clear the previous owner didn’t look after it. I noticed one of the pump mounting bolts had vibrated loose and after some suspicious drips, then loosing a bit of oil, I decided to take the thing apart to learn about it and give it a service.

I’d assumed that I’d find a turbine pump but the design is something I’ve never come across and is really neat. Like a human heart, it works by changing the size of chambers to force the incompressible liquid through one-way valves; using pistons rather than contracting muscle.

Attached to the drive-shaft is a cam in the x-axis, creating an eccentric motion that knocks into, and depresses each piston. Each piston sits above a chamber in the pump casing with one-way valves leading into, and back out.

Pump Diagram

Opening it up, here’s a diagram to figure out how it worked. This is the casting on the right in the video, with the flow other-side overlayed in blue pen. From the inlet, the water is sucked through a one-way valve as the piston pulls back and into the piston chamber. With three pistons pulling at different phases there are three one-way valves to maximise each engine revolution, each chamber filling at a different stroke and creating a more consistent pressure. On the downstroke, the water is forced through another one-way valve positioned vertically at the top here and then along to the outlet. The vertical valves at positioned in series so all the chambers lead to the same final outlet pipe.

Included in the design are various pressure releases to avoid build up, which would damage the valves and maybe even casting. Part of the reason I took it apart in the first place was because it was dripping at the bottom; I assumed the seal had perished. In fact, the chambers all have holes at the top of the stroke which drip down to the base of the pump. The idea being that on the suck stroke, any extra water
will drip away to avoid a water lock. Second to this, the little plastic thing at the top is a sprung pressure valve that releases water from the top manifold if the hose is full; you’re not using the lance. This also leads to the same drip channel. So dripping is normal.

The pump wasn’t too soiled but a blast of carb cleaner and a scrub cleaned up any grime that had built up. The oil certainly needed changing. It was gray with lots of crap in and contaminated with water. It’s meant to be changed after break in and I’ve a feeling the previous owner didn’t bother.

I got some 30wt 4-stroke equipment oil from Halfords and filled to just above the rotating cam (what other pump models seem to suggest). There is a drain hole on the pump but I’m not sure of the exact quantity to put in. Bolted the thing back together with all four bolts so it’s nicely seated now.

12 replies on “Pressure Washer Pump Teardown (ProUser PP150 5.5hp 2200psi)”

Hi, I have the same pump as you and Iam replacing the end plate on the pump as the connection that the lance hose connects too is snapped off! Can you please tell me where you put the oil into the pump and what quantity? Maybe by sending me a picture? The guy who had it before me left it outside all the time and it was in a bad way when i got it (for Free), the oil that came out is similar to yours..a milky creamy colour which as you said should of been changed a long time ago..I dont want to put it back together and it be wrong..I look forward to hearing from you..


David Brooks

Hi. The easiest way to fill the oil to the correct level is with the end plate off and the cam exposed. You need to pump on pointing up so if it’s still on the crankshaft you’ll have to tip the washer 90deg. Fill the oil to just above the cam, as this video shows: (@ 5:37)

Hope that helps. There is a oil fill hole (brass allen key) at the top but I don’t know the exact quantity to put in should it would be guess work – above is the best method.

David, I have the same pump driven by a 5.5 HP Chinese motor. The motor runs ok with no load but as soon as I hook up the water, the motor stalls and stops due to the built up in pressure. I’m unsure if the Piston Rings are worn in the motor or the pump’s faulty. The motor’s valves are good and compression reads 640kpa (92psi); I don’t know if that’s good or not. Your explanation on the pump was excellent. I’ve stripped and cleaned the pump but the fault is the same. I noticed that when the motor is about to cut out as the pressure builds that no water is coming out of the bottom outlet, which I thought relieved any pressure build-up when stopped using the wand. Also, I stripped the top relief valve, which was very difficult to remove without damaging. I’m a little unsure of how many ‘o’ rings were supposed to be there, either one or two. Any advice would be appreciated.

Hi, the guy you are after is called John, i got my washer working again with Johns help, daft question but do you have enough oil in the engine? They have an auto cut out when the oil is low..

David, Thanks for getting back to me. Yes, I’ve changed the oil and it’s full. The motor runs fine with no load, it’s only when I connect the house that the motor stops as it can’t handle the pressure build up. Could you please tell me if, when you have the hose connected and your not spraying water, is there water coming out of the small overflow hole under the pump? As John explains, this hole is for that reason when the wand is not in use, any pressure build up is released through this hole. Also, did you pull apart the pressure relief valve on the top? I damaged the ‘o’ ring trying to get this out and was wondering if there were supposed to be one or two ‘o’ rings in this valve. The pump seems a simple design and it’s got me stumped why it’s cutting out. I was trying to avoid unnecessarily replacing the piston rings as I’m not sure if the 92psi is within specs.Thanks again for you help.
P.S I was original asking John but I’m glad you responded.

Hi David,

Thanks for the comments. It sounds very much like the pump is not releasing the pressure build up and stalling the engine. You could check this by holding the lance trigger to let the water out whilst starting the engine. I don’t know the exact compression rating for the engine but it sound about right (I assume there is a good level of resistance pulling the pull start?)

When you stripped the pump down, the brass (I think) piston sleeves have small cut away sections, which should point down toward the drain outlet (matching sections in the casting). If they don’t it won’t drain, a couple were twisted on mine causing it to stall every now and again. You can see by shinning a torch up the drain hole at the bottom of the pump. I’d suggest this could be your problem.

There should be a little water spraying out of the pressure relief valve too, so perhaps that is not firing correctly after being stripped. This video shows detail of another pump’s one:

Let me know if this helps.

John, I did check the brass bushes and all the cut outs do face an outlet. The motor does get difficult to pull start once pressure builds in the pump. There is no water coming out of the bottom hole at any time. I’m unsure why or how much water should be discharging from this hole with the hose connected and the wand not in use. It seems that there is no pressure relief in the pump. Is there anything else I may of overlooked? Don

In my experience the hole at the bottom where the brass bushes output only drips a little during use (if at all). They’re there to stop the chambers taking in too much water as I understand it. The top pressure relief should spray a little when the lance isn’t being used, so it sounds like that is the problem. You could try connecting a water source with no pressure such as a water butt to see if the pump is working correctly.

John, I have all the brass bushes with the cut outs facing the same cut outs in the casting and all the plastic one way valves work. Therefore, If the problem is in the top relief valve, could you please tell me what to look for. I’ve stripped the valve and the spring loaded metal valve is working. Could you also tell me if the following configuration is correct: From the bottom of the relief valve and spring there is a small ‘o’ about 10mm dia, above the spring there is a nylon washer/seal about 15mm dia (unsure if another ‘o’ ring should be above or below this nylon seal.) I think there should be as there is a gap of about 4mm where the nylon seal/washer is positioned. Then at the top there is a nylon block with another ‘o’ ring. This nylon block also has a very small ‘o’ ring about 5mm up inside the block.
If I could send you a picture some how it would be easier but I’m unsure how I could do this. Thanks again for your help as I have no one else to turn to and I don’t want to dump the equipment if its only something simple, like a worn seal.

Good evening

I have a ProPower/Premium Pressure Washer (both names on frame) 5.5 hp 2500psi.I have tried unsuccessfully to buy seals (water in oil) or a similar pump.
Any help will be greatly appreciated.



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