On Tue, 2 Sep 2003, Vanagon Man wrote:

> But there still is a wierd problem.  It seems that the pressure cap
> allows coolant to leave the system into the overflow, but it does not
> suck it back in.  So, if you have the overflow filled to max, after a
> drive you look at the overflow and it is 1 1/2 " higher than high, and
> the regular expansion tank ( the one with the fitted pressure cap) has a
> good sized air bubble in it.  I have bled the system more than once, and
> this same thing keeps occuring.  Tested again for exhaust gasses, none
> present, and no coolant leaks on this vehicle.

Test the pressure cap and make sure it's releasing under vaccuum.  Then
replace the hose between the overflow and expansion tank, making sure the
new hose fits tightly.  (I think I used a chunk of fuel hose I had lying
around, but I don't know what size it was right now.)  If the hose has any
pinholes in it they may break the siphon and prevent the overflow tank
from pulling coolant back.  Check for trash in the bottom of the overflow
tank that might be acting as a check valve, too.

If none of this works, you may want to loosen the radiator bleeder with
the engine hot and see if any air comes out.  Any air bubbles in the
system will tend to prevent a vaccuum from forming and liquid from being
drawn back in.  If you routinely get air at the radiator you have a leak
somewhere.  Some people have told stories of pinhole radiator leaks that
weren't obvious under pressure but let air in under vaccuum.

David Brodbeck, N8SRE
'82 Diesel Westfalia
'94 Honda Civic Si