Karcher KB 499

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

My account pin on 0711825797

Getting to much air to carb probably.

The "manifold" as talked about above is also called the "Control head".
For a picture of the very small cracks in the control head see here: Karcher Control Head cracks

The video linked to below talks about the cracks in the control head at 7:00 through to 7:08. They guy incorrectly calls it the cylinder head in the video. How to FIX Karcher pressure washer

If Karcher made the control head from an aluminium casting, or used a metal insert near the point of failure, their product range would be a LOT more reliable.

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

what you describe indicates a problem hose or block nozzle jet
if it doesn't play up with a normal nozzle then it is not the hose or jet but that particular wand and nozzle you are using
do you have a sufficient inlet water supply for the pump

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

Is there a leak from the bottom ?
If so Problem, the leak is from the seam of the Manifold housing. There is a small crack in the seam. A design weakness.. A New Manifold Complete Replacement approx. $61.03.
If not you may replace the Pump Manifold Housing from Karcher, or locate a local Karcher Service Center.
If under 1 year old, and you can find a receipt,Karcher will replace the entire Pressure Washer under warranty.

You did winterize the machine? Leaving water in a powerwasher will freeze and damage the pump. It's a common happening, because people don't think of it or are not told by the maker.

I have a Karcher K 3.86 M electric PW with a similar problem: water leaks from the pump near the outlet and the pressure surges on and off continuously at intervals of 1-2 seconds. The leak appears to be coming from a very thin crack in the plastic pump head in what I think is the "unloader" valve portion. I read elsewhere that if you leave water in these PW's and it freezes it will crack, causing leaks. I don't know how you can empty all the water out so I think we must store these PW's in areas that won't freeze in winter.