Two ways and a third possibility: it might not be air at all, but
gases from combustion leaking into the coolant.

It might more/just as likely be air being sucked into the system when
the coolant cools off after driving.

Thirdly, it might have been there all the time, and is working its
way out as you drive. Not uncommon, depending on exactly how the
cooling system is bled.

To determine if it's exhaust gases, try starting and running the
engine for no more than 30 seconds with the lid off so you can get to
the coolant bottle quickly. Do this in the morning when the engine is
cold. After shutting off the engine, remove the coolant cap and see
if you get a burp out of it. If you do, then gases are leaking into
the system since the engine didn't have time to raise the temps to
expansion levels. Don't drive it this way, you're asking for disaster.

Leaking during cooldown can happen an number of ways. Do you have
coolant in the overflow tank behind the tag flap? You should. Check
or replace the expansion tank cap it you can't test it. Cheap
insurance. If it's not those two places, keep looking for a leak
because if air is getting sucked in somewhere, you can bet that
coolant is (or soon will be) leaking out. Keep looking.

For the third scenario, keep checking and see if the air deposit
stops soon, as in the next several days (since you mention you run
the car regularly for short distances). If it does, don't worry about
it. If it doesn't , look more thoroughly.

Jim

On Apr 1, 2006, at 7:05 PM, John Connolly, Aircooled.Net wrote:

> Sister In Law's 85 Vanagon is getting air buildup in the radiator
> after 2-4
> days of errands. We have bled it, and know we are getting all the
> air out.
>
> The question I have is where is the air getting IN FROM? Any usual
> suspects?
>
> We did recently replace the rear heater core.
>
> TIA
>
> John
>