2005 Saturn Ion 2 2 Liter Had Sudden Drop Power Using Gas Excessively Only Code

2.2 Liter had sudden drop in power and using gas excessively. The only code is random misfire . We already tested fuel pressure replaced plugs,boots,coil also disconnected the exhaust system without improvement. The compession seems a little low in the 80's.It ran fine then sat overnite and within a few seconds of starting it in the morning the check engine light came on.What else seems likely to cause this ?