Water pump replaced, still loosing coolant
 

I picked up a 2000 2.7 986 Boxster a few months ago, its been a great car bar the fact it slowly looses coolant. I took it to a local garage that I know well, they diagnosed that the water pump needed replacing and did so. However it is still loosing coolant, albeit slightly slower. It is booked back in, in a few days with this garage but I wanted to do some research of my own first.

It looses about 1cm on the meter on the side of the expansion tank in the rear boot over the space of about a month. The rear boot stinks of coolant after the car has warmed up and you can smell it sometimes if you drive with the windows or roof down too.

I pulled away the carpet to check for an expansion tank leak, but was all bone dry and seemed fine. I do have some condensation above the expansion tank on the inside of the boot lid? I wondered if this was normal or maybe the lid isn't sealing properly? Other than this im really hoping its not the head gasket or something bad like that....

Thank you for any suggestions.

If you have condensation on the boot lid there is a good chance your expansion tank screwon filler cap seal has gone. The latest filler cap part number from Porsche ends on xxxx04 (blue in colour) - you may find your cap part number ends with xxx01.
Go to your nearest Porsche dealer or try www.design911.com and buy a new one.

Another possibility...
 

That or the vacuum break bail needs replacement. Do a forum search, it's been posted here as a DIY.

Also remove your bumper and check your radiators (they leak from the end tanks). I think it not as common on the 986 but they go bad on the 996tt at this age. The radiators would also make sense that you are losing the coolant slowly with no boil overs.

Rather than change the top of the coolant tank, just replace the entire thing. I made mine worse when I had a similar problem by changing out the bleeder valve gaskets. If it's not a lot of condensation don't worry about it. I hated the boxster coolant system. TT's have problems too though, just not as difficult to diagnose.

i started to loose coolant because one of the metal joints along the coolant pipe started to break apart (just near right rear wheel)

And while you have the cap off remove the oil cap and pry up the plastic cover panel and make sure the bleeder valve is CLOSED- that is, that it is folded over to the side. This is used to bleed air out of the system and someone may have forgot it in the open position.

If you've condensation in the trunk, a good bet would be to replace everything associated with the cooling system in the trunk. That way everything that has failed, or is likely to fail soon, is out of the picture, and not going to bite you.

There are O-rings under the bleeder valve that go bad also.

Thanks for the replies, it seems I do have the latest expansion tank cap already (ending in number 4), but it may be worth buying a new one still as they are fairly cheap. I took it back to the garage and they have checked the radiator and piping again and ran a compression check but can't find any leaks. Still got condensation on the boot lid. I will look into some of the suggestions you guys have listed here, thanks!