The machine runs, and water comes out. It just does not hit like it used to. Or it surges, or it holds for a second and gives up.
Almost everyone starts at the pump, because that is the expensive component and worry follows the money. That is the wrong end.
Work from the tap towards the pump, cheapest and most likely first. Most faults turn up before you get halfway down the list. None of the first five steps cost anything.
First, read the symptom
The symptom pattern tells you where to look before you touch anything.
- Pulsing or surging, pressure rising and falling rhythmically. Points at supply, air, or pump valves
- Steady but weak, no surging, just tired. Points at the nozzle, a leak, or the pressure adjuster
- Nothing at all, water dribbling out. Points at a blockage, a badly stuck unloader, or a serious internal fault
- Fine for a few seconds, then drops. Points to supply running out, or an unloader that is not resetting
- Weak and the motor sounds laboured or slow. Points at the electrical supply rather than the water side
Keep that in mind, then work the list.
1. Measure the water supply
This is the cause in a very large share of cases, so it goes first. Measure it properly, rather than glancing at the tap and hoping.
Put a bucket of known volume under the tap you use, open it wide, and time the fill. Ten litres in forty seconds is fifteen litres a minute.
Compare that against your machine's inlet requirement, which your supply must exceed rather than match.
Then check the delivery. A narrow hose, a long run or a coil left on the reel will strangle the flow. So will a quick connector that necks the bore down.
We have gone through this in detail in getting the water supply right. It remains the single most useful page we have written.
2. Check the inlet filter
Pull the inlet strainer and look at it, because grit, rust flakes and scale collect there. Even a partly blocked filter produces the same symptoms as a poor tap.
Rinse it, refit it, and replace it if it looks damaged.
While you are there, fit an inline filter if you do not have one. The alternative is that debris ends up in the valves.
3. Purge the air out
Air in the system makes a pump surge and hammer, and it cannot compress its way out.
With the machine switched off, open the water wide. Squeeze the trigger until a steady, bubble-free stream runs from the lance.
Only then start the machine, which takes twenty seconds and is a good habit regardless.
4. Check the nozzle
Two separate faults here.
A worn nozzle passes more water at less pressure. The machine is mechanically sound, but it cannot build against an orifice that has eroded larger.
Look at the spray pattern: ragged, lopsided or wider than it used to be means it's worn.
A blocked nozzle gives high pressure with almost no flow, or an erratic spray. Soak it or back-flush it.
Never push wire through it, because that ruins the orifice for good.
Also confirm the nozzle size is correct for your model. A tip meant for a different machine gives you the wrong pressure, in one direction or the other.
5. Hunt for leaks
Run the machine and walk the whole water path with your eyes.
Check the hose along its length, particularly where it has been run over or kinked. Check every coupling and quick release, and check the trigger gun.
The gun is a wearing item, and it often weeps internally long before it drips where you can see it.
A small leak under pressure is a large loss of pressure. Worn O-rings cost pennies, and they are the fix in a surprising number of cases.
6. Check the pressure adjuster and unloader
Sounds obvious, and it catches people out regularly. Someone may have wound the pressure adjuster down, or knocked it in transit.
Beyond that, the unloader valve diverts water when you release the trigger, and restores pressure when you pull it. When it sticks, the machine either will not build at all, or builds and then dumps.
The tell: pressure that is fine for a moment, then collapses. Or a machine that behaves one way after a short trigger release, and another way after a long one.
There is a repair kit for it. On a machine that has done real hours, this is a routine replacement rather than a disaster.
7. Suspect the pump valves
Supply, filter, nozzle, leaks and unloader all clean, and it still pulses? The inlet and outlet valves in the pump head are the next candidate.
They wear, they pit, and grit holds them off their seats. A leaking valve means the pump cannot complete a full compression stroke, which shows up as surging and lost pressure.
Valve kits are available, and on most machines this is a workshop job. It is still a repair rather than a write-off. That is the advantage of a machine designed to come apart.
8. Look at the oil
Take a look at the sight glass before you do anything else drastic.
Milky or creamy oil means water is getting past the seals into the crankcase. That is a seal kit job, and it needs doing promptly.
Contaminated oil stops protecting the bearings, so a seal failure left alone becomes a bearing failure.
Weeping from the joint between the pump head and the crankcase points the same way. We stock the water seal kits, and this is a standard service item. Leave it, and it turns into a much larger bill.
9. Check the electricity
If the motor sounds strained, slow to start, or trips out, stop looking at the water.
Long or thin extension leads are the usual culprit, dropping voltage at the machine and starving the motor. So is a socket shared with other heavy loads. Plug it directly into a proper supply, with the shortest lead you can, and see whether the symptom moves.
Does it start and trip at once? Stop, and get it looked at rather than resetting it repeatedly.
When to stop and book it in
You have supply, filter, air, nozzle, leaks and unloader covered, and it still will not build. Or the oil is milky, or it is tripping electrics.
You are into the pump now, and the workshop is the cheaper route on a machine you earn with.
Our express service is £85 including VAT while you wait for a cold water machine. We always credit the £39 inspection fee against the work if a repair follows.
Frequently asked questions
Why is my pressure washer pulsing?
Pulsing almost always means the pump cannot draw a full charge of water on each stroke. Check the flow rate at the tap with a bucket and timer.
Uncoil the hose, clean the inlet filter, and purge the air before starting. If the supply measures up, worn pump valves are the next candidate.
Why does my pressure washer lose pressure after a few seconds?
Usually the unloader valve failing to reset. Or a supply that cannot sustain the flow once the first water in the hose has gone.
Check the supply first, then look at the unloader, which is a serviceable part with an available repair kit.
What does milky oil in a pressure washer mean?
Water has passed the seals into the crankcase, and it needs a water seal kit fitted promptly.
Contaminated oil stops lubricating, and a seal failure left running takes the bearings or the crankshaft with it. That is a far bigger job.
Can a bad extension lead affect a pressure washer?
Yes, and by a long way, because a long or thin lead drops voltage at the motor. That shows up as slow starting, a laboured sound, weak performance and sometimes tripped breakers.
Always run from the shortest, heaviest lead you can. Plug it into a supply that is not shared with other heavy loads.
Is it worth repairing a pressure washer that will not build pressure?
On a serviceable machine, almost always. Nozzles, O-rings, unloaders, valve kits and seal kits are all available individually.
The expensive castings and the motor survive, and the machine returns to full performance for a fraction of replacement cost.
The bottom line
Start at the tap and work towards the pump. Measure the flow, clean the filter, purge the air, check the nozzle, hunt the leaks, then look at the unloader.
Most faults turn up before you reach the pump. The ones that do not are usually still a parts job rather than a new machine.
Kränzle UK stocks the seal kits, valve kits, unloaders and nozzles that fix every fault on this list. We run the workshop for the ones you would rather not. Describe the symptom to us, and we will tell you which part you need.