Pressure Washer World (fix it!)

Any pressure washer experts on here? I have a mcculloch pressure washer that was inherited when we bought our current house, and it’s broken. At risk of seeming stingy I’d definitely rather try to fix it than tip it.

I have had it working in the past, but was using it for about 20 minutes thinking it was just a bit crap, then all of a sudden the pressure kicked in. I assume this is a bleeding/airlock issue but despite watched plenty of videos online regarding running water through them for a bit I’ve had no joy.

• If I attach the water source without the pressure hose attached the water shoots out, and if I turn the unit on the pressure increases- I think at this point it’s working correctly.
• If I then attach the pressure hose (no pressure shows on the dial and it doesn’t work pressurise) and remove it again, the water that was previously shooting out has changed to a dribble. It’s like by attaching the pressure hose it causes a problem somewhere.
• To get any pressure back I have to turn off the water source, and turn it back on.

I hope that makes sense, can post a video if it helps.

First things first, check any filters are clear.

There are no obvious filters… The feed hose appears to connect directly to the pump, which would require disassembly to access.

Is it possible to unscrew the mains water inlet fitting? That’s where you might find a filter.

Pressure washers break, cheap ones like Mculloch sooner than others but non of them seem to last that long. If you really want to fix it just strip it down but it probably won’t go back together easily and spare parts will probably non existent for that brand.

The real answer is to buy a  ‘Kranzle’ (with an umlaut over the ‘a’ ).

But that defo won’t tick the ‘cheapskate’ box!

Pressure washers use triple piston reciprocating pumps*. Each of the 3 pistons has an inlet and outlet check valve in order for it to function so 6 in total. If any of these ‘stick’ or get foreign objects stuck under them then the pump will ‘miss’ on that piston and pulse.

Providing the simple things have been checked (filter is clear, pressure switch starts the motor on demand) then removing and cleaning the check valves is the next course of events. If they cannot easily be removed then a sharp crack with a toffee hammer in their vicinity when the pump is running often fixes them.

*I use these type of pumps in the chemical industry for high pressure metering of low viscosity and cryogenic fluids. Im a hoot at dinner parties when I get on this subject.

Sounds more like the unloader valve to me. Strip it down, regrease it all, change the oil in the. pump and rebuild.

Thanks for the tips so far- here’s one possible candidate. The pistons are all moving freely.

Just run it with the pistons exposed (dunno if that’s a bad idea or not 😆) and they seem to be working fine

I’m not a pressurewasherologist, but my thinking is that, given they switch off when you let go of the trigger, there must be a switch to detect flow and/or pressure. It sounds to me like that’s where your problem lies – no outlet hose is seen as trigger “open”, but the hose itself is causing enough pressure / little flow that it’s seen as “closed”, so the pump’s not on.

Or your hose / nozzle is clogged, can you poke anything in the nozzle?

Good thinking, but I think this one is supposed to run all the time. It certainly *does* run all the time, regardless of whether the hose is attached.

RustyNissanPrairie

Full Member

Pressure washers use triple piston reciprocating pumps*. Each of the 3 pistons has an inlet and outlet check valve in order for it to function so 6 in total. If any of these ‘stick’ or get foreign objects stuck under them then the pump will ‘miss’ on that piston and pulse.

Providing the simple things have been checked (filter is clear, pressure switch starts the motor on demand) then removing and cleaning the check valves is the next course of events. If they cannot easily be removed then a sharp crack with a toffee hammer in their vicinity when the pump is running often fixes them.

*I use these type of pumps in the chemical industry for high pressure metering of low viscosity and cryogenic fluids. Im a hoot at dinner parties when I get on this subject.

I’ve tried tapping them with a hammer.. there doesn’t seem to be a way to tell if they’re clogged though. They look like they should be removable with a socket but I can’t get them to turn.

If the pistons are reciprocating / motor is running, and there is definitely water getting into the pump head then it can only be some/all the check valves are not seating.

Also check if there is a pressure relief valve on the outlet of the pump – ie something that stops it overpressuring if the outlet blocks. Make sure this isn’t passing.

RustyNissanPrairieFull Member
If the pistons are reciprocating / motor is running, and there is definitely water getting into the pump head then it can only be some/all the check valves are not seating.

Also check if there is a pressure relief valve on the outlet of the pump – ie something that stops it overpressuring if the outlet blocks. Make sure this isn’t passing.

Really appreciate the help, thanks!

Managed to remove all of the check valves. Cleaned them and they were all operating smoothly. I’ll have a look for pressure relief.

I use these type of pumps in the chemical industry for high pressure metering of low viscosity and cryogenic fluids. Im a hoot at dinner parties when I get on this subject.

*Adds RNP to the dream dinner party guest list. Do I lose Stephen Fry or Sir David Attenborough? 🤔*