2005 Ford Freestyle • 160,000 miles

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)?
April 3, 2013.

A leak down test will confirm head gasket. After the head is removed, it will need to be checked for cracks.

By no means drive around without a cap, you will damage the motor


Does the cooling system need pressure other than to prevent boiling? I've only been driving with the cap loose since it's been so cold here

Apr 3, 2013.
The coolant will not heat up the motor with the cap off and also it will boil out as well.

Yes, it raises the boiling point as the motor needs to run at about 195-210 warm