Is your check engine light on? If so check for codes in the ECM. Are you
sure you arent leeking?
A bad PVC valve could have fouled your plugs. Althought your plugs are
only bad on one side, could be bad ignition parts.(cap, wires, make sure
the wires are snug too.6 mpg seems awfully bad. I had a jimmy with a bad
O2 sensor. Less power, more gas.But not that bad.