I have a 1993 E8D Enduro (mostly the same as the earlier 8B) with an overheating problem - from cold there's a flow at the telltale but as it warms up,the flow diminishes until it's just steam. The block and head are too hot to touch at this point. I've been through the usual routes - water pump kit replacement, flushing the system with weak acid - I've even had the head off to check the gasket, then the powerhead off to poke wires through the waterways. I thought I'd found the problem - an external water hose at the bottom of the block had a blockage at one end, but on clearing this and putting it all back together, the overheating problem was the same, if not worse.

I've just discovered that if I take the lower leg off and attach mains water to the leg inlet pipe, the engine will run perfectly - no overheating no matter how hard I run it - what on earth can be wrong with a water pump which has had a new kit installed? I notice that if I submerge the leg and use an electric drill to turn the shaft, there's not a terribly impressive amount of water coming out of the rubber seal to the leg inlet pipe and also that there's an equivalent amount of water coming out the the top of the housing around the shaft. The impellor I'm using is the correct part - the same part number in both E8D and 8B parts diagrams. The parts diagrams show no kind of seal between the shaft and the housing - can anyone think why this is happening?

Thanks in advance.