Hope!  If indeed it is just a leaky/loose cap, you will be lucky.
  Your description seems to say...head gasket .
   Have you thought about that?  It matches all the indicators for a leaking
head..
too....
  Don Hanson

On Wed, Dec 23, 2009 at 7:26 PM, Luke Bakken <luke.bakken@gmail.com> wrote:

> 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
>