Worcester 24i pump problems

[mellmo]

Hi guys
I have a worcester 24i RSF combi boiler, which was in the house when i moved in around two years ago. the systrem worked fine until we fired up the central heating side of it about a month ago. Now, the hot water side of it is fine, but when i start the central heating, it starts up and the radiators start to get warm, but then after about five mins you can hear the water in the boiler start to boil and it cuts out. I have just spent the last two hours tring to find whats up, and it seems to be somthing wrong with the pump. With a volt meater connected to the out put of the PCB going to the pump, and a screw driver resting on the pump and my ear on the screw driver i start the central heating, all is ok, volt meater reads 240-245V and their is a noise coming from the pump, all seems ok, then after about five mins the pump stops, the water in the systerm starts to boil and the boiler shuts down. Does anyone have any ideas, or is it just a stait forward pump replacement.
Thanks in advance Melly

 

[malkie]

Right undo the vent on the pump in the middle,then turn it on and see if its running if its not has it got 240 there whe nyou try your heating ,is there any valves turned off under boiler or is pressure ok and is system all vented ans auto air vent open

 

[mellmo]

Their is a constant 240v going to the pump, all the time that the heating is on, but the pump just stops after about five, i've removed the cap in the middle of the pump and placed a flathead screw driver in the stot on the pump shaft to see if it turns, it turns without any reel force requierd, ti just don't work after about five minuets of use

 

[malkie]

when theres 240 to it is the centre spinning fasat against your screwdriver or do you turn it yourself