My 1997 Expedition (185,000 miles) has started running very rough at idle, and even at high speeds. I put a new fuel filter and air filter on and changed the plugs. I also poured a bottle of fuel injector cleaner in the gas tank. The problem hasn't gotten any better...it almost seems like the new spark plugs have made the problem worse.

I get codes for both left and right bank too lean.

Has anyone ever experienced this problem.

Thanks

 

Check for vacuum leaks (hose off connector or the rubber elbow on the back of the Intake plenum is a good one to check).

The Idle Air Control (IAC) may also be the culprit.