Rich had it right the first time but you have a point. The coolant won't
circulate through the radiator unless it has someplace to go. Opening
the bleed screw gives it a place to go, out the bleeder after the air
escapes. With the bleeder closed it won't move through the radiator
until the thermostat opens but that is too long to wait while bleeding
since there may be a dangerous amount of air in the system.

At idle the cooling system relys on a siphon effect to help the
waterpump move coolant through the radiator as the radiator is a high
point. The siphon won't establish if there is too much air in the system
so it is critical that the engine speed be increased and the air purged
from the radiator. Once the air is out the pump will circulate coolant
through the radiator just fine at idle, assuming that the thermostat has
opened.

Mark

Andrew Fox wrote:
>
> Rich left out an important detail:
>
> The coolant won't reach the radiator and radiator bleed valve until the
> t-stat opens.  So you should not try to open the radiator bleed valve
> until the t-stat opens.  I usually run the van at 2000 rpm in the driveway
> until the radiator fan comes on once which is a sure sign that t-stat has
> opened and coolant is reaching the radiator, usually takes about 10-15
> minutes at 2000 rpm in the driveway to open the t-stat and reach normal
> operating temperature.
>
> Andrew Fox
>
> > Follow a basic bleed procedure: cold engine, remove the coolant tank cap,
> > start engine, turn the valve on the thermostat housing to the left, rev the
> > engine up to about 2K and hold it there (might want an assistant for this
> > part), open the bleeder valve on the radiator (you can remove it to get lots
> > of air out faster but be ready to put it back on).
> >
> > If you don't have anything coming out of the radiator bleeder then you have
> > a blockage.
> >
>