Is it possible that a Northstar water pump can work when cold and fail when hot? It seems to be happening on my 1999 Seville. :-)

I pulled and extended the return line at the coolant reservoir and it flowed out into a pail when it was cold. After it gets warm however there is no flow.

How likely would an air lock be if I am continually topping it off (with the engine running) with distilled water? My thought is that there should still be flow since there is a bypass around the thermostat where an airlock might occur.

There is no steam coming out of the exhaust. No sweet smell to indicate a blown head gasket. No water in the oil nor in the oil cap. When I tried a block test it did not turn color before it started boiling over. I don't think it is a head gasket. It churns violently when I put it under load and comes out the radiator cap.

I think it must be the water pump. I wasn't aware that it could flow when cold and shut off when hot though. When it was cold the flow rate did seem a little slow though.

Thanks!

 

It is very unlikely water pump would fail that way. One of the rotating parts would come off. What is operating temp at 70 mph? Does water pump drive pulley ( on camshaft) appear melted?