I have seen this happen when the intake valve stuck open, and while trying to start it, fuel is sucked into the head over and over. You can usually smell the gas in the oil when that happens. Sometimes you can pull the spark plug, and watch the valves with a flashlight through the spark plug hole in the head to see if they are opening and closing completely as you pull the starter or crank it if electric. I was able to fix this on my 11 hp riding mower by spraying Gumout with the long spray tube into the valve seat area through the plug hole. You could also have a leak in the float valve in the fuel bowl, so it never shuts down the flow from the tank and floods.

The pump is (theoretically) always putting out the same volume, regardless of if it is in bypass, low pressure to the gun, or high pressure to the gun. The pressure comes from the size of the orifice that the flow goes through, smaller orifice, higher pressure at the same flow. When the unloader is in bypass, the flow is just looping within the unloader and pump (and heating up so don't stay in bypass very long). The resistance to flow within the unloader will cause a pressure difference between the input and output of the unloader, which holds the unloader in the bypass position. When you pull the trigger of the gun, the pressure on the pump outlet/unloader inlet drops quickly as the hose fills and the flow goes out the nozzle. The dropping of the pressure allows the unloader to shift to the flow out of bypass, to the gun. When you close the gun, the resulting pressure spike in the hose shifts the unloader to the bypass position. Since you can get the unloader to shift out of bypass in the low pressure/big nozzle position, it appears that the unloader is working, at least somewhat. It doesn't work in the high pressure/small nozzle setting, so the assumption is that there is not enough flow to drop the pressure in the unloader to get it to shift. This could be caused by too small of a high pressure nozzle or a partially plugged one. It would also make the unit run at too high an operating pressure, which is not a good thing to do. If you have the manual for the unit, it should say what nozzle number to use to get the rated pressure at the rated flow. Since the pump puts out constant flow, you should get very close to the same gpm into a bucket at low pressure and high pressure. If there is more than about 10% difference, your pump is not very good. There are also unloaders that shift based on flow, not pressure, so whenever flow is directed to the gun, they shift out of bypass. If your pump was tired, and you had a flow sensing unloader, it may not shift out of bypass, as the flow would be too low. Happy hunting, hope you find an answer.

Do you have a rubber washer in the fitting?

My Kaercher M3.97M electric pressure washer motor/pump pulses on/off when the trigger on the wand is off. What could be the problem?