Coolant tank leaking only one driven hard

[78F350]

The worst case would be a blocked coolant passage in the engine, causing a hot-spot and steam. Hope it's not that. I'm pretty sure that's the case with one of my engines that had a water pump impeller come apart. Easy driving, no problem. Hard driving on it loses coolant. Tank was replaced air purged, and leak checked. At least in my case, I never liked that engine anyway.
Do the pressure test as JFP suggested. That way, you will know if it is a true leak or an over-pressure problem.

[brybart]

If your Boxster has more than 75,000 miles on it, I'd recommend changing the tank, it is not the easiest replacement to undertake but it is well worth the effort. The tank will fail eventually. Besides the coolant in your trunk and carpet, look at the color of the tank, if it's yellow it is on its way out. You'll see your new tank will be bright white. I had the same indications, many times the level would be fine, then I'd have periods when I'd have to top off weekly then it got worse and I had a coolant in my trunk. Get a ring clamp extension tool, order a new tank and some coolant from Pelican, get a six pack of beer (German of course you'll need it when your done) and a buddy, do some stretching and replace the tank. Then every time you open your trunk and see a nice new tank and pink coolant at the perfect level you'll get the satisfaction of a job well done.

[Netguy]

My 2000 Boxster S has 104k miles on it and it's in the shop today for a pressure test. If it's the tank, it will be the third time it will be replaced. Is there a good replacement part? I've been using Porsche parts, and this is getting quite expensive. This is crazy, I've never had to replace a overflow tank on a vehicle before, caps yes, but not tanks.