Bit of a strange one.................well got me stumped anyway. Freshly, professionally, rebuilt 6 pot engine, rad fully flushed out and flows as it should, block chemically cleaned. Alloy pump manifold with new water pump. Car starts up fine.
BUT just as the temp gauge goes over mid way thermostat opens and all hell breaks loose, car just boils, floods overflow bottle and it then looks like a rather bizarre water feature !
The pump impeller is all attached to the pump but if you remove the thermostat, put it all back together and start the car there is no flow of water visible in