Just looking for some confirmation on what I.

Just looking for some confirmation on what I believe will be a huge dent in my wallet. Approximately 6 months ago, I started having occasional issues when starting my engine cold - it would misfire and idle rough for 10-20 seconds, and then run fine. Warm starts were not an issue. The problem eventually became an everyday occurrence, accompanied by the check engine light. I had the code checked and was told to replace spark plugs and coils, which I did. About two days after the same problem came back, accompanied by no heat unless I maintained over 2k rpms. On a friends advice, I loosened the coolant resivour cap to prevent the system from building pressure. The rough idle issue has gone away, but the coolant now has a strange odor. I've surmised that there is a slow leak of coolant into one cylinder (#3 as the check engine code indicated). Is there any way to tell if it is the head gasket or the head itself (crack, warp)?