An 1174 all by itself can be a real PITA to solve. Lucky you have warranty coverage. I assume they've leak-tested her & reflashed the ECM to the latest version firmware. Your MPG is probably going to be down somewhat due to more fuel being dumped into the cylinders to correct a real or perceived lean condition. Here are some more possible causes:
•Inspect the air induction system for modified, damaged, leaking, or restricted components.
•Inspect the crankcase ventilation system for improper operation.
•Inspect the vacuum hoses for splits, kinks, and improper connections.
•Inspect for vacuum leaks at the intake manifold, the throttle body, and the injector O-rings.
•Test for a restricted, damaged, leaking, or modified exhaust system from the catalytic converter forward.
•Test the ignition system for improper operation.
•Test the engine for any mechanical conditions such as sticking valves, lifters, etc., which could alter the flow into the combustion chamber.
Last edited by greenman; 02-22-2012 at 12:58 AM.