Karcher K3.91 compressor run for a few seconds every 10-15 second

When water is running through system, the motor has to run to keep the pressure built up. So, if the water is leaking out, the motor think it needs to keep running to keep the pressure built up. The leak needs to be fixed to the motor will not run constantly.

Your pressure washer is either not fitted with a run/stop device in which case like mine you have to switch off within a few seconds of closing the trigger. Check your model features?
The humming noise is the motor stalled out against the pump because there is nowhere for the water to go. The shutoff is caused by the safety overload switching off after overheat.
Or, it has a pressure operated run/stop which is not working. Because water and electricity don't mix, I would suggest getting more specific help. You can download Karcher manuals /spares lists online from Karcher specialists but remember electricity is the silent killer.

You need to expel all of the air that has ran through the supply hose and into the pump. Do this without the machine running. Squeeze the trigger and let the water run through the machine for 30 seconds or until all the air has bled through. You can the turn the machine on.
Cheers Ziggy.

Hi and welcome to FixYa, I am Kelly
The seals do have a tendancy to harden / dry out or... corrosion prevents the unloader bypass valve from moving / operating properly. I recommend you put a little oil (2 oz maybe...) through the unit on purpose via the inlet water connection. Then connect your water hose and force the oil though the unit with the air in the water supply hose while it purges. DO NOT turn the unit on until your sure the water supply hose has purged out all air. Then turn on the unit making sure that you cycle the wand trigger rapidly for a few minutes by cycling the trigger until the high pressure hose is not so rigid then let the high presure hose charge fully again. Cycle trigger wait, cycle trigger wait. 1 second bursts until you get full pressure. Then use normally. This usually wakes up pressure washers that have been resting :)

Thanks for choosing FixYa,
Kelly

The motor is holding pressure in the line.
Check the manual.
I don't think it is supposed to cycle that much.
If it's not designed to do that, then there is a seal or a bearing that is leaking.
It's letting water pressure escape the hose by letting water seep back into the pump.
The motor is pushing more pressure into the line to keep up with the internal leak.
Two times a second seems like a lot, and the noise is probably annoying.
If the unit is new, take it back.
If it's doing the job, run the equipment until it dies, or else take it for repairs.

We had a similar problem with a paint sprayer.
Paint sprayers also hold pressure on a line, except the fluid is paint instead of water.
When the machine sensed a loss of pressure, it would cycle once to restore pressure.
After a while it was cycling constantly, just like your machine.
Working indoors with the constant noise was a nuisance.
I had the machine rebuilt, and used it for another year before buying a heavier machine.
I don't know if there is a business that rebuilds your equipment, or how hard parts are to find.

It sounds like there may be a pressure leak somewhere in the system. The machine cuts off when there is no flow of water past it. If it is leaking then there is intermittent flow. Which would make the machine run then stop continuously.

are you making the machine **** for water, if so you have got a air leak on the suction hose.
try force feeding with water by connecting your hose to the mains water hose and see if you can see a water leak.
could be starved of water also,chack the filter

The machine has a pressure leak. Check for a cracked controll head or either a leaking outlet elbow. The machine will short cycle if the holding pressure can not be maintained

To determine if your problem is due to an easily replaceable part, follow these steps:

1. Connect the water supply hose, the pressure hose, and the wand WITHOUT any nozzle tip.

2. Turn on the water supply, and press the trigger until water is smoothly running out of the nozzle
after any air is all purged out. Release the nozzle trigger.

3. Start the engine, and let it warm up for about 30 seconds.

4. With the engine running smoothly, press the nozzle trigger. The water should flow out of the nozzle (without the tip installed) at a faster rate, while the engine noticeably loads down as it used to do before your "no pressure" problem. If it does not load down, turn it OFF, and disconnect everything, and contact a Karcher repair facility.

5. If the engine does load down, release the trigger, and install one of the nozzle tips (not the black one).

6. Press the trigger, and the engine should load down, and you should have normal pressure from the wand. After you release the trigger and press it again, you will probably again have no pressure, and the engine will not load down. Removing the nozzle tip, pressing & releasing the trigger, and re-installing the tip should give you normal pressure again, but releasing the trigger will cause no pressure until you again remove the tip, press the trigger, and re-install the tip.

This is caused by a bad UNLOADER VALVE, which is located under a black cap with a metal button on it located on the top front of the pump. I don't know if you can purchase the replaceable parts from a Karcher repair shop, but when a neighbor & I took his washer to a repair place, it looked pretty simple to do this repair, and he was charged about $60 to replace a $5 (maybe) part, and it took about 5 minutes.

To determine if your problem is due to an easily replaceable part, follow these steps:

1. Connect the water supply hose, the pressure hose, and the wand WITHOUT any nozzle tip.

2. Turn on the water supply, and press the trigger until water is smoothly running out of the nozzle
after any air is all purged out. Release the nozzle trigger.

3. Start the engine, and let it warm up for about 30 seconds.

4. With the engine running smoothly, press the nozzle trigger. The water should flow out of the nozzle (without the tip installed) at a faster rate, while the engine noticeably loads down as it used to do before your "no pressure" problem. If it does not load down, turn it OFF, and disconnect everything, and contact a Karcher repair facility.

5. If the engine does load down, release the trigger, and install one of the nozzle tips (not the black one).

6. Press the trigger, and the engine should load down, and you should have normal pressure from the wand. After you release the trigger and press it again, you will probably again have no pressure, and the engine will not load down. Removing the nozzle tip, pressing & releasing the trigger, and re-installing the tip should give you normal pressure again, but releasing the trigger will cause no pressure until you again remove the tip, press the trigger, and re-install the tip.

This is caused by a bad UNLOADER VALVE, which is located under a black cap with a metal button on it located on the top front of the pump. I don't know if you can purchase the replaceable parts from a Karcher repair shop, but when a neighbor & I took his washer to a repair place, it looked pretty simple to do this repair, and he was charged about $60 to replace about a $5 (maybe) part, and it took about 5 minutes.