Date:         Wed, 23 Dec 2009 19:26:35 -0800
Reply-To:     Luke Bakken <luke.bakken@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Luke Bakken <luke.bakken@GMAIL.COM>
Subject:      Expansion tank pressure cap - how tight?
Content-Type: text/plain; charset=ISO-8859-1

Hey everyone, Merry Christmas!

I've had an interesting issue with my coolant system that I think may
be due to me not putting the expansion tank pressure cap on tight
enough. When checking the expansion tank, the past 8 or so drives I've
had to put in about 1 to 2 cups of coolant into the tank before
driving, which fills the tank to the top. I then put the pressure cap
on and reattach the overflow hose. When I get back home after tooling
around, I'll check the overflow tank to see if the level has increased
- sometimes it does, sometimes it doesn't (hmmm). The expansion tank
never has any air in it after a drive, but, once the system cools all
the way back down again, there is an air pocket representing about 1
or 2 cups of lost coolant.

Now, I did some major work on the coolant system this summer - I
replaced every single hose in the van as well as put in stainless
pipes. I have monitored and tightened the clamps as necessary since
the overhaul and am (mostly!) confident that there are no leaks in
that part of the system. I also replaced the pressure cap about a
month ago and still see this behavior. I have not tried the old cap
again (yet). The expansion tank itself appears relatively new (white
plastic, not yellowed). I'm assuming it was replaced when the engine
was converted to a Subaru in '04.

Today I wanted to get to the bottom of this so I drove the van around
a bunch to get it up to temperature and then immediately started
checking the entire system, starting at the front. Everything looked
fine until I got back to the engine compartment and lo and behold I
did see that it looked like some coolant had "sprayed" around the
expansion tank. I had the engine on and revved the motor while
watching in the compartment to see if I could reproduce it while
watching but did not see any more leaks! But, I did see trails that
appeared to show that the coolant had sprayed somehow from underneath
the cap. In addition, coolant did flow into the overfill tank this
time.

Since the system was hot I was able to tighten the pressure cap a
little more - maybe this will help. What concerns me is that maybe the
replacement cap I got isn't working correctly and not allowing the hot
coolant to expand into the overfill tank. Or, I'm just not tightening
it enough to allow it to build pressure - this would also explain the
air bubble after cooling since it would be sucking air from around the
threads.

Thanks for reading this far ... any ideas or descriptions of just how
tight that cap should be are appreciated.
Luke