It is a sealed system when not being bled. The energy needed to lift up
to the top is "recovered" by the coolant falling down the other side. It
is called a siphon effect.

 Of course coolant flows through the radiator at idle!

That is the primary reason for the radiator fan, cooling down the
radiator when hot coolant is circulating at idle but the van isn't
moving. Then air flow through the radiator must be provided by the fan.

Mark

Andrew Fox wrote:
>
> > I believe that is backwards.  It should come up the side and down
> > through the cooling veins and then out.  This is the direction of the
> > thermodynamics, colder water (relative) will sink....
>
> This is correct.  I will add that the coolant has to raise about 2
> feet alongside the cooling fins before "waterfalling" into the fins.
> Unfortunately the h20 pump does not generate enough pressure at idle to
> get the coolant "over the top".  This is why if you remove your bleeder
> screw at idle nothing will come out.  The engine must be running at about
> 2000 rpm to get enough h20 pressure to make the radiator actually cool the
> coolant.  Makes me wonder about the effectiveness of this cooling system.
> You would think the VW engineers that designed this cooling system would
> have realized this and built a shorter, fatter radiator or a better
> waterpump.
>
> Andrew
> 86 Westy (has new H20 pump and radiator)
>