MY 03 PORSCHE BOXSTER OVERHEATS AND LOSES COOLANT, WHAT IS WRONG WITH IT?

Change the cap to a new one and see if that's the problem.

First I would try what joemom recommend and if that does not work I am going to recommend something you may or may not want to try but it worked for me. Lucas Radiator And Block sealer. No promises but it has been working for me for a year now and still no leak. By all means this is not a permanent fix but may hold you over long enough to save the funds to do it right. One more thing if it does not work the first time do not continue to add more hoping it will work it will cause more problems like clogging up your cooling system. If you do decide to try it hope it works for you like it did for me.

It could be your overflow tank it seems to be a weak point in the Boxster cooling system. Heres a video that shows how to test and find the leaks: https://www.youtube.com/watch?v=5WrgWIVFnpw

Heres a link to the OE replament part if that turns out to be the problem. http://www.partsgeek.com/gbproducts/WC/131-01136567.html?utm_content=YN&utm_term=1997-2006+Porsche+Boxster+Expansion+Tank+Original+Equipment+W0133-1647661+97-06+Porsche+Expansion+Tank+2004+2005&fp=pp&gbm=a&utm_source=google&utm_medium=ff&utm_campaign=PartsGeek+Google+Base&gclid=CLmmnsq4pMYCFdcZgQodVIANlQ

Check your hoses and clamps running to the pipes going to the front of the car. On shut down, the engine heat soaks, and raises the pressure enough to cause a leak. The water pump is also a suspect, but it's easy to change. Good luck!

Left carpet in trunk and check if there is coolant. If "yes" you need to change out the coolant tank - a bad design, make sure you have the tank pressurized after refilling coolant.