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Ghost of cracked coolant reservoir reared its ugly head
Coolant reservoir leak
Was noticing a whiff of coolant smell for a while and couldn't figure it out. Then was snooping around and found this. Heading to the dealer to take care of it. I would do it but I have a warranty.
Almost same location and failure mode as mine. Kind of a pain job but DIY-able (I did; scary to have to drop the engine). Warranty helps ease the pain. Good luck at the dealer.
I just did my 2005 C2S in July 2016 (56k? mi). As with the OP, I'd get an occasional whiff of coolant and then one morning I saw a saucer-sized puddle almost directly beneath the reservoir (which was different than my water pump leaking (just a few months earlier) which I then replaced).
Did a radiator cleanout and coolant flush. Was about to fire up my Airlift Coolant vacuum for the refill and decided I better take a look at the overflow tank for evidence of cracks. Although I had no signs of leakage I found this. Replacement tank going in today...
Someone f-d up on the FEA during the design phase of these tanks. Most other manufacturers use the same reservoir system and they go 100's of thousands of miles. These tanks can't make it 10 years and 100k?
These tanks were a common problem going back to the introduction of the 996/986.... I went through two (3?) in 197 K miles of ownership of my 2000 Boxster S. This kind of thing really ticks me off as it just goes on year after year. How hard is it to order your supplier to make the plastic a bit thicker? Or of a different formulation?
They design it to last as long as the accountants allow them to. Henri Ford used to gather the carcasses of old model T and A cars from junkyards to see which parts showed little wear, for the ones that did he told the engineers to make it cheaper and not last as long. I bet there are few failures of them in the four year warranty or CPO too. They seem to get suspect at 7 years plus. I'll have the one in my 2009 replaced at next oil change.