Motor runs but no pressure build up and when I let trigger off still runs
In my experience with karcher, and other brands, you will be money ahead to replace the pump entirely. Pumps usually cost around $125. There are some times rebuild kits available, but they will often cost over half the amount of an entire pump. Add to that the unloader valve, & you will be way past the cost of a new pump.
SOURCE: the motor is pulsing on and off without the trigger being pressed
It is probably sucking air on one of the connections or around one of the fittings at the regulator. You can also turn the pressure up allittle on the regulator, that might stop it also. Make sure the clamps on the hoses going to the regulator is tite. Also it could be that the regulator is just bypassing water and if it has air in it it will pulsate.
SOURCE: Karcher K3.91 compressor run for a few seconds every 10-15 second
They have a problem. I removed both valves and lubed with lithium grease as I was told. Unit is now working for how long who knowes.
SOURCE: Electric Karcher pressure washer surging when turned on
Dear firenutt681,
One simple thing you can do, is to check for leaks. Check if hose at machine is leaking. Check gun if it is leaking. You can also check if water still comes out of lance nozzel. These leaks however so small, will cause the machine to surge.
Another reason is due to a faulty SPILL VALVE. This spill valve( often called CONTROL VALVE) will contract when the gun is closed. When it contracts under the enclosed pressure it will switch off the machine by means of a Micro Switch. Now if this Spill valve is faulty it will loose its compression under pressure and puts on the micro switch again. This can be seen once in a while or more frequent, as in your case. I sometimes do service Spill Valves, but mostly I replace them with a new one. I do not know the model number of your machine, but normally you have to open the cover and you should spot the spill valve on the head. Most probably you see a thick wire or a cap containing the micro swith. If you find the micro switch, you have found the spill valve. Sometimes it is a bit hard to pull out the spill valve. Myself, in various Karcher models I remove the spill valve by trying to feed water and put on a bit the machine. In this way the spill valve will be spitted out.
Another reason which makes your machine surge, is leakage from from the detergent inlet.
If you do not see no leakages at all....then for sure it is the SPILL VALVE.
If you cannot do this job yourself or you do not find spare parts...you can continue using the machine as it is...but before closing the gun...switch off the electricity. I only recommend this in an emergency. You do this so that you do not leave the machine surging. This surge will for sure damage the electric motor and the capacitor (assuming it is single phase).
Regards,
Fulltech.
SOURCE: karcher 550m has lost alot of its pressure,Why
Dear emmettbrickw,
Many of the models of Karcher machines, have a valve that under pressure...will compress and at the same time it will let water by-pass and circulating inside the head. As this valve compresses it also puts off a micro switch. This all happens when the gun is closed. This valve is called a SPILL VALVE or sometimes a CONTROL VALVE. Now for various reasons (example you store the machine with pressure in the system)....the seals of this Spill Valve get damaged. So you can have a problem in which the machine stays off....or a problem like yours...the machine continues running with gun closed. Your problem with gun closed...the spill Valve is not compressing and therefore not stoping the machine. At first when this happened it caused a fault also in your machine's valves. The gun was closed...pressure grew high...and this burst the intake valves for sure.
So my advice is that the spill valve must be renewed and also the intake valves. You must also check the Miro-Switch to see if it got stuck in the on position. If micro-switch is faulty, this also must be renewed. The Spill Valve is often located where you find a micro-switch holder with an electrical wire coming out. When you find this micro-switch...detach it from machine...then put on machine for a few minutes and press micro switch to see if machine stops. If machine stops...then at least Miro-Switch is OK. Always do these tests with electrical protective clothing, Shoes and Gloves!! Now if your machine does not have a micro-switch...then you have a spill valve which only by-passes water. This type of spill valve also needs checking and if need be renewed.
In a few simple words, the spill valve does all the work after one has opened or closed the gun. Sometimes by changing the spill valve only....the pressure will get back to normal. If not ...suction and delivery valves must be checked.
Regards,
Fulltech.
SOURCE: I have a Karcher K5.55M The motor pulses and only 1/2 pressure
In the video link above, at 7 minutes,.. through to 7 minutes 8 seconds,.. the guy talks about a "minute crack in the cylinder head". He actually means to say "minute crack in the control head". This minute crack is the main reason for this common problem: the pulsating and some loss in pressure on a wide range of Karcher waterblasters.
I got a replacement control head from my local Karcher dealer for NZ$50 that fixed the problem,.. for now :-( The below picture has two red ovals around the cracks in the control head that are very hard to see. If Karcher made this control head from an aluminium casting, their product range would be a LOT better. 
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