If you had to add a liter more than once after a GM flush and fill you have a possibly very large problem. Head gasket anyone.

 

Or a cracked head, time to have the coolant checked for "Products of Combustion", either by using a DIY kit available at an auto parts store, or taking it to a qualified and competent radiator/cooling system tech.

 

Check oil dip stick if oil level is to high you could have coolant in the oil and that means a bad engine block like I did. It's a known problem with early 2.0 eng blocks. Just had my engine replaced 3 months ago. Luckly under my power train warranty. Also water pump can leak coolant into the engine. Again GM had a few engines with bad castings and they know about them. Sorry for the late post but I hope this helps