I keep coming across stories of people with Karcher pressure washers experiencing problems with leaks and a pulsing from the machine when the trigger is off. People have even filmed their machines doing this and posted the videos to YouTube, I’ve included two below.
The issue seems to be caused by hairline cracks forming in the cylinder head of the plastic pump that Karcher use in their cheaper model.
I’ve seen suggestions this can be down to allowing the pressure washer to freeze during the winter with water still in the pump. Alternative causes seem to be down to restricted water flow into the machine from not having enough inlet hose unrolled or from using Hozelock Aquastop connectors to connect the inlet hose to the pressure washer.
Replacing the faulty cylinder head seems to cure the problem. If you feel confident about doing this yourself spares are widely available, one web site listing them is Buyspares.