Karcher Model3.91 Pulsing and low water pressure Can I fix it?Thanks

Isn't there a bleed off valve on the unit. Typically they get stuck and pulsing is the result.

My account pin on 0711825797

1 water inlet filter clean ?? not limed or ??
2 nozzle clean.
3 pressure control valve properly screwed
Sometimes water hoses are provided with a slot at the hose connection with an internal water seal, which is the wrong way to the water tap instead of to the machine

sounds like your unit has a bypass problem either needs adjustment or replaced

Just google " stripping down Karcher KB4040" A YOU TUBE CLIP will show you hiw to fix it and what tools you need

it sounds like you have a air leak make sure all your fitting are tight if you have a filter make sure the filter isnt clogged if that doesnt fix it it sounds like your pumps ceramics might have been damaged in the pump its self

I do not have specific experience of your machine.
However, the high pressure pump will be controlled by a pressure switch on the high pressure outlet to the pipe and jet.
Pulsing (turning off) suggests that the outlet (high pressure) pipe is blocked or partially blocked. Try checking the nozzle and (with the unit turned off) clearing it with a fine piece of wire.
Hope this works
Nic Williamson

Could be the input water pressure is not sufficient. There is a pressure valve inside the machine that will cut off the motor if input pressure is too low or non existent.

Try a different source for the water input.

there is probably dirt or a little stone in the nozzle of the gun. its building up a high pressure and goes idle. clean nozzle and try again

sound like water seals my have bl owen or vales are damaged check your oil too,make sure you have no water there. if your un-loader fails can create a high pressure point and blow seals. good luck, and let me know