The symptom pattern

You release the trigger, the machine stops as designed — then a few seconds later it grunts back to life for a moment, stops, and repeats indefinitely. The cycling continues whether or not you’re using the machine. It usually appears gradually: occasional restarts at first, then constant pulsing.

What’s actually happening

K-series machines use an auto-stop system: releasing the trigger closes the outlet, pressure builds against a switch, and the motor shuts off. The system assumes that trapped pressure holds. If anything lets pressure bleed away — a weeping O-ring, a worn valve seat, a nicked hose seal — the switch sees the drop and restarts the motor to restore it. The machine isn’t malfunctioning; it’s faithfully reporting a leak somewhere between the pump and the trigger.

That also means pulsing is worth fixing promptly: every restart is a hard start under load, and a machine left pulsing is wearing its motor, switch, and pump for no work in return.

Working through it

The search is a process of elimination from the outside in, cheapest first. Most leaks are external and visible: run the machine, release the trigger, and inspect every joint — gun, lance, nozzle, hose ends, detergent tank connections — for drips or fine hissing. Swapping the lance for direct gun-only operation splits the system in half and quickly tells you which side the leak is on.

Quick-connect O-rings deserve particular suspicion. They’re cheap, they’re disturbed every time the machine is packed away, and a barely visible nick is enough to bleed pressure. Kärcher sells O-ring service kits for exactly this reason.

If everything external is dry and the pulsing continues, the leak is inside the pump head — usually the non-return (unloader) valve seals letting pressure bleed backwards. The valve is accessible on most K-series machines, but this is the point where parts diagrams, patience, and a decision about repair economics enter the picture.

Verdict

Chase it from the trigger backwards and you’ll usually fix it for the price of an O-ring kit. Internal valve seals are still a viable repair for a confident owner. If the pump head itself is worn, weigh the parts cost against a replacement machine — but don’t scrap a K4 for what is, nine times out of ten, a rubber ring.