When I filled my 968 cooling system, I tightened everything up except the top of the large passenger side coolant hose coming from the water pump. I then put a long neck funnel into this...and poured in coolant until it was full. then clamped it onto the Water Pump. Only then did I fill the coolant recovery receptacle. run it a bit, open the air release scren on top front of manifold area.. massage the driver side hose, etc. .and it's been fine ever since.
It's much harder (for me) to get air out of my 928.
Harvey
