I would pull a spark plug and see if it shows rich fuel mix.
Your '98 would have the higher pressure "returnless" fuel system.
You would need a pressure gauge to test for leaking injector, 50-60psi would be expected pressure with engine off.
The returnless fuel system doesn't have a fuel pressure regulator on the fuel rail but it does have a Pulse Damper with a vacuum hose attached.
These rarely fail but if they do leak the vacuum line is there to suck any leaking fuel into the intake to prevent a fire.
Remove this vacuum line and check it for fuel or fuel smell.
MAF sensor is the main sensor the computer uses to set fuel/air mix, this sensor can get dirty over time which effects MPG, but not usually in a few weeks.