The oil vapor is mixing with the antifreeze and backing up into the antifreeze reservoir. What is wrong?
From the information provided about your Saturn L300 it appears that oil and coolant are indeed mixing and this is not a good thing. Usually when oil and coolant mix, this is an indication of a leaking or blown head gasket. However, this is a major repair. To be certain that this is indeed the problem your Saturn is experiencing, it is best to enlist the assistance of a certified mechanic to come inspect your antifreeze reservoir, drain it, and locate any leaks that may exist.
Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing