Raw water pump not priming at idle RPM

[nigel1]

In the last week, I noticed that the Jabsco water pump">raw water pump is not priming when the engine is started and left at idle, it needs the throttle opened to about 1800 rpm before the pump primes.

Shortly prior to this, I carried out an overhaul on the cooling system of the Volvo Pemta TMD 22 engine. Cleaned out the heat exchanger, the mixing elbow, renewed most of the hose's, new freshwater/coolant pump, new stat, replaced the raw water pump impeller, and dismantled, cleaned and re-assembled the vetus anti siphon loop.
When the work was completed, I started the engine to check for leaks, and the pump primed OK at idle rpm.

Boat was not used for a couple of months, until last week when I moved the boat from England to Scotland. On starting the engine, I noticed no water from the exhaust, so gave a quick increase on the revs and the pump then picked up suction.

Then did some checking.

With engine stopped, the water in the suction and discharge hoses drains down (as it should do). On the suction side, the water level in the hose ends up about 6 inches below the suction port of the pump, which I assume is at hull water level.

On starting the engine at idle, I do not see any water coming into the inlet filter, or any rise in the water level in the suction hose.

Increasing the RPM causes a sudden rush of water into the filter, and the pump primes. After this, dropping RPM back to idle, the pump continues to work OK.
With the pump going OK, I dont see any air in the suction hose or discharge hose up to the anti siphon loop. After the anti siphon loop, I do see air in the discharge hose.

Another thing I tried. Started engine, left it at idle, and briefly closed the inlet valve. This caused the water in the suction hose to rise up to the pump suction port.

I have checked all the hose clips, they are tight, no signs of leaks.
Removed cover plate from the pump, impeller looks good, put in new sealing O ring on the pump cover.

Any one have any idea's what could be the problem. Seems odd that this problem has started so suddenly.

Thanks