Karcher K4.99M Started to pulse and lose pressure so changed cylinder head which appeared to have a hairline crack. Having reassembled it still pulses but something is restricting flow of water when I try to prime it without the pressure hose fitted. Am I missing something? Performance is no better than when I started

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If theres also a lack of power that would suggest a hairline crack in the pump housing. If power is ok it might be the seals inside the pump head.

The "manifold" as talked about above is also called the "Control head".
For a picture of the very small cracks in the control head see here: Karcher Control Head cracks

The video linked to below talks about the cracks in the control head at 7:00 through to 7:08. They guy incorrectly calls it the cylinder head in the video. How to FIX Karcher pressure washer

If Karcher made the control head from an aluminium casting, or used a metal insert near the point of failure, their product range would be a LOT more reliable.

This fault is within the control head, under the detergent valve is a small spring loaded valve that acts to induce the detergent, this will have failed, the shaft of the component breaks off the disk base leaving a small hole in the centre of the disk. This is enough to allow pressure past the component. The machine will pulse when operated and will not induce the detergent. I've looked everywhere to find this part as I have a karcher with the same fault.

You have a cracked cylinder head caused by leaving pressure in the machine after turning it off .You wont be able to see the crack. The only solution is to replace the cylinder head.

I do not have specific experience of your machine.
However, the high pressure pump will be controlled by a pressure switch on the high pressure outlet to the pipe and jet.
Pulsing (turning off) suggests that the outlet (high pressure) pipe is blocked or partially blocked. Try checking the nozzle and (with the unit turned off) clearing it with a fine piece of wire.
Hope this works
Nic Williamson

This sounds like it had water left in it and it froze over the winter. The motot turning on and off signals a water leak on the output side of the pump. You may have a cracked pump housing. Locate the pump and make sure nothing is leaking in that area when under pressure. looh for fine hairline cracks.

Is there water leaking from the bottom ? If so, then you have a leak in the pump. Bad seal or crack in the Pump.
If under 1 year old and have a receipt, Call Karcher for a warranty replacement.

The motor doesn't pulsate when you are spraying and pulsattes when not spraying is normal otherwise the pressure will just keep building and burn out the pump. However, if you are not using the sprayer right away then the unit should be shut off until your ready to start again.
I hope the answer is helpful, please rate if the answer has helped in any way. Good Luck and have a great weekend.

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.