My karcher k3.99 keeps pulsing when in use.It's just as if you turn the switch on and off.You can watch the fan stop and start ?

SOURCE: Karcher Motor Pulese On and Off

hi mark, you do have a problem with the lance.
you need to disconnect the lance and straighten a paper clip and insert down the nozzle end.
use the machine to wash the dirt in the nozzle backward down the lance.
Dirt only in nozzle end.
if still no good, purchase some nozzle cleaners and file out nozzle very slightly.

SOURCE: karcher 2.56 keeps pulsing on and off whenever high pressure nozzle is attached. I have tried different nozzles and same thing is happening. when i press the lance without any attachment it works fine

Make sure you have bled all air from the system. Also make sure you have sufficient water supply; if house water pressure is too low or if your hose is too long or doesn't allow full flow, you may not be getting enough water to the unit.