High fuel consumption, 12.1 mpg on vehicle computer in city, hesitation on acceleration.

Changing the fuel filter is no bid deal so you may want to go ahead with that but it would not cause a huge drop in gas mileage. Check for fuel leaks. Do an actual mileage test by filling the car up, writing down the mileage and then again at the next fill up. Calculate the mileage after a couple of hundred miles to make sure the computer is not off for some reason.

Thanks for the response, but I failed to mention that fuel consumption has been a problem for a few years now, and nothing I have done seems to help. I will check for fuel leaks, but what about the hesitation, this is something new, and it has never hesitated. That is why i was concerned about mass air flow sensor being dirty, or the throttle body. ???

Maybe your fuel injectors are leaking or the mass air sensor is off. I would have a mechanic check to see if your emissions show that the car is running really rich. What did the spark plugs look like when you changed them? Sooty?

I just had transmission rebuilt, because I had lost overdrive, was doing 3000 rpms at 60 mph. Now, 15-1600 rpms. TTC was defective until 2003 when manf. corrected. The spark plugs still looked good, like they did not need to be changed. It has always passed smog on first test. I am going to clean the mass air flow sensor, that I have not done. Thanks.

What trouble codes came up when you had it scanned?

Have not had it scanned. Drove a 150 miles yesterday on freeway, hesitation on acceleration, almost seems like it is starving for air or fuel. Okay once I get to the desired speed. I averaged about 20 mpg. Thanks.

I have a 2002 Pontiac grand Prix. I stopped at a gas station and put possibly watered down. My car wont drive past 25 mph and my r p m stay near 2 x 1000.. I was told to siphone out that possible bad gas. Then attempt to start my car and if it does not start change my plugs and wires. Does this sound proper ?

No, that sounds like advice from idiots. First of all how do you water down gas? Gasoline and water don't mix. If the gas was that badly contaminated with something, or the tanks were so full of water that you got water instead of gas the same thing would have happened to the other cars also there filling up. Worn out plugs and wires don't all of a sudden cause a car to stop running to the point where you can't drive it. It sounds like your car has gone into reduced power mode (limp mode) and needs immediate servicing to avoid some sort of engine damage. First double check that your gas cap is on tight. I don't think a loose or missing cap will put it into limp mode but it might. The fuel system has to be closed properly for everything to function correctly. Now... Take it to a reputable shop. They will first scan it for trouble codes which will usually give them some direction as to what is needed. Tell them to call you with a diagnosis and a repair estimate before performing any repair work. It's an old car and you don't want to get too carried away sinking money into it. Good luck. Let us know what happens.

We had a guy with a video on YouTube how he was getting 50 MPG, his gauge was defective, so dont believe that factory gauge. Everyone is talking about the injection like a bunch of back yard Mechanics didn't get the code now it's the injectors lol. Look it could also mean a warn engine, so you decide on how much you want a mechanic poking around the car. Anything can make a car run rough, bad gas, vacuum leak, bad computer, down to a bad fuel pump. This is why I hate computer cars for. So good luck on finding your Poor M.P.G.