1998 Mercury Mystique MASS Air Flow Sensor

Hello

Since your engine light is illuminated, you need to have the codes pulled by a diagnostic scanner. Most part stores do this procedure free of charge for you. The codes will point us into the direction of what is at fault here. Post the codes you retrieve and I will follow up.

The MAF sensor would cause bad fuel mileage but to save money you will want to retrieve the engine codes first so you don't spend money on parts that are not actually bad.

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P0174 System too Lean (Bank 2)

P0171 System too Lean (Bank 1)

P1744 TCC (Torque Converter Clutch solenoid)/Escessive Slip Detected

The first two codes indicate air/fuel ratio is correcting rich for an overly lean condition. The possible causes for these codes to appear are as follows :

fuel system malfunction
EGR system malfunction
air intake or vacuum system leak
engine oil level too high
incorrect cam timing
restricted air cleaner
faulty PCV system
excessive internal engine wear

With the symptoms you are experiencing I would start by cleaning the EGR related components as well as either cleaning or replacing the EGR valve. This is also the most common component to fail and cause these codes to appear and will cause poor fuel mileage as well.

The transmission code is for a faulty TCC solenoid or faulty wiring to the TCC which is why you are experiencing the hard shifting.

Let me know if you need further assistance.

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Yes, the MAF sensor is part of the air intake system which I listed in the possible causes.

The EGR system is a Differential Pressure Feedback EGR (DPFE) system, controlled by the Powertrain Control Module (PCM) and composed of the following components: DPFE sensor (also referred to as the backpressure transducer), EGR Vacuum Regulator (EVR) solenoid, EGR valve, and assorted hoses.
The EGR (Exhaust Gas Recirculation) valve is pictured below.

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Not only is MAF sensor a POSSIBLE cause, it's the most likely. It's super easy to diag when you get the "won't idle" symptom. Just unplug it and the computer will substitute a value based on other inputs - not perfect - but if it doesn't die any more you know you've got it. (Of course this 'll set a circuit code.)
You may have to disassmble it to clean - use a spray electronics cleaner that leaves no residue. If cleaning doesn't fix you'll have to replace it - just worn out.
Get factory shop manuals for your car, there's a lot of "description and operation", diag, ect. In them. And lots of cheap ones are available with all the closed dealerships. If you don't mind not having a physical copy they're REALLY cheap on cd/dvd.
The t/c code's a "performance" code - cause could be electrical, hydraulic, mechanical, or deterioration of multiple parts. Recommend replace trans.
Good luck!