I have a Troybuilt 2500psi pressure washer. when I hook up and turn on the water, the water flows through the pump and into the pressure hose and out when I pull the trigger on the wand. When I start the engine the water immediately stops flowing through the pressure hose and what appears to be some kind of bypass outlet. I remove the pressure hose but had the same symptoms. water flows until I start the engine.

Sounds like a pump failure. Does yours freeze in winter? Need to winterize like cabin plumbing.
Repair/replacement of pump could be close to new unit.

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Disconnect hoses.
Look for kink in water hose.
Check incoming water connection for filter washer
Replace filter washer
If you don't have a filter washer, put one in.
Hook up water, and turn on
Water should come out other side of pump.
Turn on machine.
Do you get pressured water out of pump.
If no pressure, then pump is suspect
Hook up washer hose without gun
Does pressure come out of washer hose
Finally, the gun trigger may be bad.

We had to buy guns now and then.
Search for pressure washer guns
Following link shows guns we used:
http://www.ultimatewasher.com/pressure-washer-trigger-guns.htm

Hydraulic oil?
Is this a pressure washer?
Pump should be half full of SAE20 or SAE30 non-detergent motor oil.
Engine should have 10W30 motor oil.

If you have a pressure washer ... and not water is coming out end of gun:
Take all hoses off machine.
Look at incoming water hose connection.
The should be a filter washer.
Clean or replace filter washer.
Hook up water and turn on water.
Water should come out other side of pump.
Turn on power washer and see if water coming out of pump is pressurized.
Now hook up pressure hose without gun.
Turn on pressure washer and see if water comes out of hose and is pressurized.
Finally, the gun may be bad.
We used to replace guns when they went bad.

that is sure signal of insufficient water supply!

try easy solution first:
make sure that your mains is fully open, that tap on whihc hose is hook up is fully open and hose is of "decent" diameter to allow sufficient flow.

if above solution do not work then you need to make another, more difficult arrangement:
prepare 20litres pressure tank with inlet connected to mains and outlet by short hose to power washer, fill it with water from mains, keep it under mains pressure and use karcher power washer normaly. this will solve water supply problem. twenty litres will last about 2min, because it is re-filled, under standard pressure therefore it will last about 3min...

third possibility is to stop washing...

if you find my advice helpfull, please do not hesitate to left the comment and vote for me, thanx!

first sheck the inlet screen where the garden hose attaches,if there is water flow then see if it has a manual pressure regulator, this will be a knurreled knob somewhere on the intake manifold of the pump, ifyou have one, turn it clockwise and see if there is a difference. If still no luck, disconnect the high pressure hose from the pump, start it and see if warer flows at that point, Your gun may be clogged or your hose may be delaminating on the inside and need to be replaced. If all this fails I suggest you bring it to an expert. These pumps can require special tools and expertise to repair internally

When I use my machine I hook up the water to the pump, hook up the high pressure line and gun, then turn on the water full and squeeze the triger to make sure all air is out before starting the pressure washer(mine is gasoline powered). Then I install the nozel on the spray gun. If you are having surgeing water pressure Check the hose washer where the water hose hooks to the machine, and be sure the supply hose is not restricted and valve is on full.
If this was helpful please leave Feedback. Thanks, SP

Is there a check valve on the hose inlet that is stuck, and won't let the water in???????

Remove spray tip and clean with a paper clip, Check pump oil resevior and add 10w 30 pump oil if needed, also look for turn knob ,usually black with -+[plus /minus symbols ] and turn towards plus for extra pressure. May need new pistol/wand assembly and make sure hose is clear by seeing if water will run through hose freely and generously.

Thanks cnbode and please vote me a Fix Ya!!!

JJ505

Are you priming the water lines before you switch on the power?
i.e. with power OFF, hook up hose and turn hose on (doesn't need to have much hose pressure), then take your spray gun or attachment and pull the trigger and allow water to flow through the system (at hose pressure obviously). You might need to gravity feed it if its not flowing through...

But you will generally only get pressure once all the air is removed from the lines and the pump is pushing water only... Once you have water flow and lines are primed, switch on pump and see if that solves it.