fao the pressure washer expert

[Gazz]

i have a karcher home pressure washer, cant recall the model number right now, it's about 5 years old, has a metal pump body i know that much,

over hte past year or so it's been doing the following.... i use the machine, all air is out of the water supply, release the handle and the motor stops as it should, if i leave it like that for more than a minute, the motor starts up again, it dosent sound loaded as when i've pulled the trigger, and as nothing goes pop, i assume there's a bypass valve opening, and the pump is just pumping via the bypass.

a quick pull and release of the trigger shuts it off, but a minute later it does it again,

if i turn the power off when it's doing it, turning it back on it's still doing it, giving it a tap with a birmingham screwdriver makes no differance either, (thinking pressure switch playing up)

any ideas what to look at? i am assuming pressure is leaking in the pump, and thus triggering it, but where to look? no water is leaking from the pump or anywhere externaly.

[dennis]

Mine was doing that a few weeks ago. It was leaking from a joint in the pressure hose. A new O ring fixed it. It was a small leak so check the hose and lance.

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[The Medway Handyman]

This will be a machine with a pressure switch.

To explain whats occurring; Say the working pressure is 100 bar. The pressure switch is basically a piston held down by a spring that trips a microswitch.

When the trigger is closed the back pressure rises to say 105 bar and causes the piston to trigger the microswitch, so the motor stops. When the gun is opened the pressure against the spring drops allowing the piston to move away from the microswitch, so the motor starts again.

A slight leak in the high pressure side is causing that. Anywhere from the pump outlet to the trigger gun.

[John Rumm]

Normally IME of this type of fault, after a pause for seconds/minutes in the off state, they run for moment and then stop again and repeat at a similar duty cycle ad infinitum. Mine did it for a bit when the o-ring on the on the motor end of the lance perished. Ebay brought an envelope of ten new ones to the door the next day and that fixed it. From what the OP is describing it sounds more like once it starts running again, it carries on and does not cycle off again until the lance is operated? Could be a similar problem - but it sounds like the pressure switch might be sticking on in addition to having a small leak on the high pressure side.

[VAG_dude]

I notice the better domestic do what you have described below, however I have seen a commercial Kranzle machine in operation and that seems to idle when the trigger is closed, I am led to believe this will wear out the motor quicker but surely on such an expensive and supposedly top end machine this is not the case?

I am looking to buy this machine soon and apparently that also idles when the trigger is closed, it's got a brass pump made by Interpump.

(the Quicky)

[The Medway Handyman]

In theory it will, but the motor is likely to have a 'run' life in the many hundreds of hours range, so it doesn't matter.

The plus side is that you don't keep on stopping & starting the motor every few minutes.

Only downside is that leaving a machine in by pass for a prolonged period could cause the pump to overheat.

A responsible owner won't do this.

(the Quicky)

The Interpump Boxjet is a legend in the pressure washer world. Didn't know they were still available. Highly reccommended!

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[Gazz]

yup, it deffo just starts up after about a minute of non use, and keeps running, with the motor sounding unloaded (as if it was loaded it'd have popped something by now i guess) i give it a kick, turn power off and on etc, but nowt will stop it except pulling the lance trigger and releasing it,

no sign of any leaks, there's no water under the machine when i'm finished using it, and no water comes out of the case when i tip it upright onto it's wheels, (had a cheapie bosch pressure washer before this one, plastic pump and that leaked, left a big puddle under it and would do the on-off-on-off thingy as pressure dropped and was built up again,

it's a karcher 520M if it helps, appart from the annoying habbit of it starting and running on it's own, it's fine,

[VAG_dude]

Thanks, I'll be going for the Quicky due to the higher flow rate, any experience with that one? Going to be used every week or so for washing a few cars.