Darin-- thanks for the insightful reply. I've been wondering the same thing about my overflow as well. I'm fairly certain it works, because when I loosen the radiator cap slightly when it's under all that pressure, air is forced through the tube into the overflow bottle. I'm still wondering if something is not right with the brand of cap that I have. I bought a Stant 13 PSI (equivalent to 0.9 bar) cap a year ago, and it seemed to work fine. This was the one I was using up until this morning, when I put on another Stant 13 PSI cap. Think I should try a Toyota cap? I might have to just to make certain that the system is working the way it is supposed to.

I am using Havoline Dex Cool as my coolant. I flushed the system fairly well of the green (that had turned to brown) stuff in the spring of last year - which was about a year after I bought it. I wasn't happy with how the coolant was looking, so I completely flushed the system several times in late November, and put Havoline Dex Cool in there again. I had some debate in a red vs. green coolant topic about whether this stuff is right for our engines or not, and I came to the conclusion that I probably wouldn't know until things went bad over the long term or not. The point of that is that it needs to be maintained, and it wasn't done that well before I got my hands on it I'm presuming.

Back to the main topic at hand though-- troubleshooting this thing. I don't have tools to do a leakdown test-- I suppose I would have to take that into a shop to do it. I don't even have a compression test gauge. <img src="/forums/images/graemlins/baby.gif" alt="" /> By a quick search, I did find a good FAQ on leakdown tests (catered to FWD cars for the how-to) : Leak down test FAQ . <web browsing> <web browsing> Oh hey, it looks like if I have access to compressed air (which I do), I should be able to thread a fitting into the spark plug hole, turn engine to TDC for that cylinder, and turn the air on to see if any bubbles start coming up through the radiator. I guess that would work for other parts of the test, too, if someone was in a pinch without a leak down tester.