Faulty Karcher pressure washer - 'pulsing when off' (2023 Update)

[chris French]

My Dad has a Karcher pressure washer - K550M, it's few years old now, it's recently developed a fault.

Symptoms are that when turned on (with the water connected and turned on), the pump 'pulses' on and off - with cycle time of say about 4 seconds - instead of staying turned off until you squeeze on the trigger. It will sometimes pump water ok once the trigger is pulled, sometimes it cycles on off. - I've fiddle about using a different jet etc. Though that shouldn't make any difference anyway to the main problem

I'm guessing it's something to do with the pressure/flow sensor (or whatever it has) on the pump. Anyone know if there is anything I can fiddle with before I have look at it.

[The3rd Earl Of Derby]

Try the washer without the gun attached and see if it does it.

[Lobster]

Mine went through a spate of cycling like that, but only when you pulled the trigger - it was down to a small part-blockage somewhere and completely cured by cleaning. Not sure if that might help put you on the right track?

David

[The Medway Handyman]

Two possible causes, if on suction rather than mains tap pressure it could be air getting sucked in. Check hose, clips etc.

Sounds like the by pass valve may have gone out of kilter. When it was new, what happened when the trigger was shut. Did the electric motor continue to run or did it switch off?

[The Wanderer]

I've had this two or three times with my rather ancient Kew pressure washer[1]. Usually a small bit of grit or similar inside the pressure lance, which after bit of fiddling and cursing about trying to rinse the lance out from the nozzle end seems to clear itself. I did once get problems with using a stop connector onto the hose coupling on the washer. The pump was actually sucking water out of the hose and intermittently closing the stop connector. I changed the connector for an ordinary one and the problem was immediately solved.

[1] Bought, at a guess, about 25-30 years ago at a 'Special Show Price' at the Ideal Home exhibition. When I pick it up now, I find myself wondering how on earth I managed to lug it back on the train. :-)

[chris French]

Nope on mains pressure. My washer (also Karcher) works fine there.

The motor turns off when the trigger is shut- or rather did.

Thanks everyone for their comments, In reply to points others have raised.

Note- this problem is occurring without actually using the lance or anything. Just turn on water, turn on washer, instead of it just sitting there quietly until you squeeze the trigger it does this pump on/off cycling thing all the time.

It's not blockage in lance, I've had that problem before which is only obvious when you try to use the washer.

It's not low water flow - I have that problem at home sometimes (crap flow from my garden tap) - but that again is only evident when using the washer, sometimes it will stop for couple of seconds when using the lance.

Yes, I've swopped the lance/trigger unit for one from my own washer, same problem.

I'm certain it's inside the unit itself, I was just hoping someone might be able to say - ah yes, it's x, or give me some idea what to look for before I delve inside

[The Medway Handyman]

Its a non return valve on the shut off system.

When the machine is in use operating at say 100 bar and the trigger is shut, the rise in pressure opens a valve and allows the water to flow to a pressure switch. This switch will then 'break' to turn the motor off. There will be a non return valve which is designed to keep the switch under pressure so the motor remains off.

When you pull the trigger again the pressure on the switch drops so the pressure switch will 'make' and turn the motor back on.

Whats happening is that the non return valve is leaking slightly, so the pressure on the switch drops a little, allowing the switch to 'make'. The motor starts and the pressure rises 'breaking' the switch so it stops. It will continue to cycle like this.

Never had a K550M apart, but there are some good parts diagrams on line with Karcher spares agents. Google for Karcher Spare Parts.

Take the pump head off (loads of little bits!) and you should find inlet & outlet valves. The non return will be somewhere between the outlet valves and the outlet of the machine. It might even be accessible from the outlet itself. It will be near an electric cable that operates the contactor when the pressure switch operates. Probably a dodgy O ring.

HTH

[Sam]

I had identical symptoms on mine (K300) when there was a slight leak on the discharge connector "O" ring. It was more of a seepage than a leak and was caused by a split "O" ring. I haven't actually tried it as yet but I'm as sure as can be that was the problem.

If there is no external leak on the high pressure side then there could be one internally (pool of water under washer when in use?). The other alternative could be that the non-return valve(s) between the pumping head and the discharge could be allowing water to pass back to the low pressure side internally i.e. it has to be this if there are no external leaks.

Best of luck.

[chris French]

Thanks, for the suggestions from you and sam, I'll report back once i've had a chance to look at it.

[Fixed phil]

Its the head leaking inside the pump.

[Bertie]

Do you think they still have it after 16 years?