I think that if you are hearing a boiling sound then your cooling system is not holding pressure or air tight. Since you've been through a couple of coolant caps, this leads me to believe the cap is not the problem but likely the tank or a hose at fault and the reason the system is not holding pressure.

BTW, the pressure is what keeps the coolant from boiling. Going too far back on my chemistry lessons to explain better but temp and pressure are related...

FYI;

A 1:1 mixture of ethylene glycol based anti-freeze and water raises the boiling point of coolant to 225 degrees, from 212 of straight water.

A 15 psi coolant system in turn raises that same coolant boiling point to 265 degrees.

Above those temps the coolant can't cool the engine any longer because the boiling (bubbles) actually insulates the coolant from the metal surface (cylinder) and engine temperature then raises (fast!) to the melting point of the pistons.

In my post above, I meant to say that pressure as well as antifreeze work to raise boiling point.

Aside from the temp/pressure and boiling point discussion, this problem reminds me of an old BMW I formerly owned (84 533i) that used to overheat frequently. After replacing all the obvious cooling system items, which did nothing, it turned out that the cylinder head was cracked and allowing exhaust gasses to enter the cooling system, thereby causing an air break and boil over.

Any chance this Boxster has a cracked cylinder head causing a similar problem?

Its possible but the boiling thing does not hppen all the time. Once in a while. Is it bad when it boils. if it is a craked head that is $4000 or more to fix Can I live with it if it only happens once in a while?

Cap Replacement
 

I bought my coolant cap from Suncoast about 8 month ago (before they released the 03 version). I called Suncoast and found out that if i take the bad coolant cap that i have to ANY Porsche dealer they will replace it free of charge as long as i have the invoice. They also told me that if i didn't have the invoice they would email me one. All original Porsche Parts have 2 years unlimited kilometer warranty on them and these warranties are honored at any Pporsche dealer.

I was very happy about. I did that on Saturday and it did work.

I just bought a new cap from the dealer couple months ago and the cap number ended with 01 so should I bring it back? Is the 03 cap for newer Boxster 987 or for both? I can still smell a little bit of coolant in the heat vent but not as bad as before.

Can someone post the link to a good Coolant Flush DIY? I think i'm going to attempt it over the weekend. I just got my water pump replaced after leaking coolant but they didn't flush and replace the coolant and my levels are way below the MIN mark. I just bought the 03 cap but wanted to find out more about the bleeding.

Check out Pedro's website...seems he's got the best DIY write-ups.

http://www.pedrosgarage.com/Site%203/Install%203rd%20Radiator%20%26%20Coolant-System%20Bleeding%20Instructions.html

I'd like to install the LN low temp t-stat and renew my 7 year old coolant at the same time...just need to figure out how/where to dispose of the old coolant.

Most of you never knew MNBoxster, but if he had read this I am sure he would have countered with all sorts of physics-based opposition just to be contrary!

Jaxon, thanks for this info. I love learning these kinds of things from everyone here on my home away from home, 986forum.com.