Update: The guy I had to check the codes for me suggested I put a can of SeaFoam in the tank, saying if it was an O2 sensor, this may help. I don't know if it was an O2 sensor, fuel injector, or what. All I know is that after running a tank through with a can of SeaFoam added, I'm back to my usually bad 12-14 MPG, which is a heck of a lot better than the horrible 7-9 MPG I was getting before. I'm a believer. It does make me wonder if the plenum gasket failure somehow contributed to getting things gunked up.