I have a 3.91M pressure washer and it does not have pressure. Water runs through it but no pressure.\Its a 1700 PSI

SOURCE: Karcher pressure washer won't start

Dear ralph_birtwi,
Most pressure washers when their gun is released, the machine stops automaticaly. This is done ny means of a control valve. When the gun is closed...pressure is built and the control valve moves and switches a micro switch off. In your case the micro switch is still closed in off position. Now this happens if a machine is left and stored with pressure in it., This control valve could get jammed and leaving the micro switch in off position. It could also stay in off position by sticking, due to incorrect chemical.
So the control valve or even called spill valve, must be renewed.
Sometimes a little lubrication with the right type of silicon grease will get the control valve working again. This job is sometimes delicate and better done by a qualified technician.
Regards,
fulltech.

SOURCE: I was using the pressure washer and it stopped

This problem, where the unit continually switches on and off when the handgun is closed, is quite common. Unfortunately it has at least six different possible causes, only two of which are easily checked. 
The first thing you can do is look for leaks.  Even the slightest leak will cause this pulsing because the unit gets up to pressure, switches off, loses pressure, switches back on again, comes back up to pressure, switches off... and so on.  This repeated switching is not good for the motor and should not be allowed to continue for longer than necessary to observe what's happening.
So... 1. Does the handgun shut off completely, or does it drip? If it drips, it may simply be because a seal has failed. The handgun can be dismantled, checked, and any defective parts replaced; be sure to lubricate O-rings with silicone grease when reassembling. As a last resort, the whole gun may need to be replaced.
2. Are there any visible leaks at the high-pressure hose screw connections?  If so, most likely an O-ring needs replacing - again lubricate the new one well with silicone grease.
3. If there are no visible leaks, either from the gun or from the connections, all other possible causes are internal within the cylinder head or control head of the pump. New parts will be needed and this normally calls for a professional repair, although it's not beyond the scope of a truly competent amateur.
I hope this helps. Any further questions, just ask! Please remember to rate this response.

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: I have a 3000 psi karcher pressure washer with

sounds like you need a seal and bladder rebuild < if you liver in a cold environmet for winters please dont forget to winterize your washres every year before winter

SOURCE: My Karcher pressure washer pulsates

your pressure bypass valve is leaking water pressure by and not building full pressure