Jordan,
Any advice as to how to check this out before it becomes a big problem? What exactly was leaking?I would like to try to avoid ruining one of my pistons if possible... it's been pressure tested twice and the garage still can't find the leak! (This is a different garage than the one that replaced the radiator cap... long story but I ain't going there anymore).
Thanks for any advice...

 

i wish i had great advice, my garage had to basically pull my car apart before they found the issue at hand, and mine was in the big problem stage. it is a hard problem to diagnose. i feel the best advice i have is to have them run the car untill the can physically track the leak. i know it is a pain but mine ended up in a enginerebuild. hope you can track this down.

 

Has the system been pressure tested? I'd like to think so and that this is also how they found your defective cap. Even so, leaks can sometimes be tricky to run down when they're not constant. The cap may or may not have been contributing to the problem. But if it didn't hold it's proper pressure (or any) it was correct to replace it. Having worked on everything from car engines to 16 litre diesels, I can tell you that cooling system problems in general and even things seemingly as simple as a leak, still can sometimes present quite the challenge. Such problems can be the kind that separate a real technician from a parts replacer. And the difficulty of seeing certain areas on these cars doesn't make it any easier. Hopefully you're in good hands. A methodical approach will eventually find the source.

 

Thanks much to everyone for advice/comments.
C4 ownership is a privilege and a responsibility - I'm learning!

Picked it up today from the shop, and here's the latest:
-They have done pressure tests and looked at it over a two day period but can't find the leak.
- It's not leaking consistently, it's not getting hotter than normal, and the leak is so small that the reservoir has plenty of coolant in it.

Mechanic suggested I keep an eye on it and when it starts leaking more consistently they would find it. He thinks it might be the water pump. I trust this guy (he came on a recommendation and hasn't charged me a dime). Still, I worry that if I wait, it'll be something very serious (see Jordan's post).

So, is it something I can keep an eye on and take back in when it leaks more than once every 10 days~2 weeks, or should I take it somewhere else and let them take everything apart to find the leak now? Any thoughts?
Thanks again...
Mike