My Karcher 11/5 U starts and stops when not in use

Pressure washers do this due to a valve called the 'unloader'. It's purpose is to relieve pressure from the pump when not in use so it is not under continuous strain, but there is also a pressure switch linked to the motor that will fire it up if the pressure drops to low so your washer is always primed and ready for action. This means tht the unloader relieves the pressure, then the electric pressure switch pumps it again to keep the pressure high. If it is doing it a couple of times a minute I wouldn't worry, but if it is every few seconds it would indicate to me that the pump is losing pressure constantly somewhere. Main reasons would be;

It could be a weak unloader, which is basically a strongish spring and a couple of plates and seals, but would normally come as a kit or complete valve.

A leaky seal in or around the pump. If this was the case you should see water leaking from the unit somewhere and then you could pin point the worn/damaged seal and replace it. Most washers have seal kits and it is advisable to replace seals in this way as it saves time later (especially if a worn seal, others will be wearing out too).

Failed non return valve. In the pump head there are normally two non return valves per piston (one directly after the piston and one a bit deeper into the outlet side) and also a couple in the back end of the pump head. These basically allow the pressurised water to be pushed through the pump and stop it flowing back into the cylinder set. If the water can flow back as well as forwards it will not keep pressure (like blowing a balloon up but not holding it shut between breaths). Non return valves will normally be supplied as a set and will be assembled already so just push into place when the pump head is apart. You would not see a leak if thesse valves are perished as the water will just flow back toward the source instead of out of the machine anywhere. Pressure washers do this due to a valve called the 'unloader'. It's purpose is to relieve pressure from the pump when not in use so it is not under continuous strain, but there is also a pressure switch linked to the motor that will fire it up if the pressure drops to low so your washer is always primed and ready for action. This means tht the unloader relieves the pressure, then the electric pressure switch pumps it again to keep the pressure high. If it is doing it a couple of times a minute I wouldn't worry, but if it is every few seconds it would indicate to me that the pump is losing pressure constantly somewhere. Main reasons would be;
It could be a weak unloader, which is basically a strongish spring and a couple of plates and seals, but would normally come as a kit or complete valve.
A leaky seal in or around the pump. If this was the case you should see water leaking from the unit somewhere and then you could pin point the worn/damaged seal and replace it. Most washers have seal kits and it is advisable to replace seals in this way as it saves time later (especially if a worn seal, others will be wearing out too).
Failed non return valve. In the pump head there are normally two non return valves per piston (one directly after the piston and one a bit deeper into the outlet side) and also a couple in the back end of the pump head. These basically allow the pressurised water to be pushed through the pump and stop it flowing back into the cylinder set. If the water can flow back as well as forwards it will not keep pressure (like blowing a balloon up but not holding it shut between breaths). Non return valves will normally be supplied as a set and will be assembled already so just push into place when the pump head is apart. You would not see a leak if thesse valves are perished as the water will just flow back toward the source instead of out of the machine anywhere.