I have an 06 997 C2S that had a coolant leak ever since it was new.  The head gasket was replaced by the dealer at 7k miles, but that didn't solve the problem.  I also replaced the cap a while ago because it had white crust on it, but I still have the leak and the new cap has the same white crust on it.  Over the last 1500 miles, I've used 20 ounces of coolant.  There are no leaks and no smells.  The car has approximately 24k miles on it now.

Anything else I can try?