My 2006 Pontiac Grand Prix 3.8 engine light flashing and sputtering when accelerating cylinder 3 misfire code sweet exhaust smell rough when starts
Short answer, Blown Head Gasket or Intake Gasket.
Sweet Smell
The sweet smell is an indication of engine coolant burning and being passed through the exhaust. Checking your coolant level will give you a better indication of how much you are going through.
Check Engine Light Flashing
Anytime the check engine light is flashing you should pull over and discontinue running the engine until it is repaired. This is to prevent permanent damage that WILL occur otherwise. When the Check Engine Light is Flashing the fuel injector may be disabled for the misfiring cylinder to protect the catalytic converter.
Pontiac Code P0303: Cylinder 3 Misfire Detected
The engines computer uses signals from a couple of sensors to determine when an engine misfire is occurring. it uses the Ignition Control Module and the Camshaft Position Sensor signals to do it. A misfire rate the is high enough can cause the catalytic converter to overheat under certain conditions.