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You may kick yourself if you go buy a new one just yet. There are many things that can cause this same gripe. Some are very simple and some are not easy to locate or fix. Being that I am only familiar with HydroTech's I have some suggestions to look at if your machine is similar to these. (Most are but some are completely different but here goes anyway) First thing to check is the nozzle. You must remove it and see if there is any dirt in it. If clean, start making your way back toward the pump. Handle pipe, trigger, hose connectors, ect. There can be enough dirt in there to cause the pump to overload but may appear that the water flow is good otherwise. If you don't find any problems there, then you have to start checking the unit and pump plumbing for dirt. You must pull it apart because as above, the flow may look right but in actuality may not be. What you should look for is a flow or pressure control switch or valve. Every unit I have seen has one of these or the other and sometime both. I'm no pressure washer expert here but I'm given to understand these can work either as a pump safety or governor. What I have found is these are much like a venturi device with a large through port and a small control or test port somewhere. If this little hole gets plugged or the line / sending unit gets looses pressure or reading, the pump will start to pulse as you said. Depending on how big this unit is will determine just how much monitoring and control circuitry is in it. The 3 HydroTech units I service are fairly large industrial types and get quite detailed in this area. A very simple problem that will also cause this is a bad check valve in the feed circuit or loss of/fluctuation water supply pressure. If you have ruled this out, try the intake valve next. If the unit has an onboard storage tank, check for breaks/leaks in the suction line/fittings from the tank to the pump. If the unit has an output feedback circuit to govern the pump, check the lines/fittings and also ensure this is clean. I'd be willing to bet you have either a leak or dirt somewhere in the machine that's causing this. If the pump was shot, it would not build pressure/flow to normal at all. Because it's getting full pressure/flow in a pulse, this tells me the pump is good but something else is screwing it up. Another thing is to see if the pump has internal bypass/feedback circuits, some do, some don't. Like I said, I'm no expert but these are some of the things I find wrong with the units I work with. Gives you a place to start looking anyway.
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