Poor Mpg

I have a 98 GT with 155,000 i get about 20-24 on the highway any ideas? i have had it for a month and put on 2,500 miles. It is stock other than opening the factory air box alittle. My 98 bonnie got 27-33 its the same motor and tranny set up and that had 280,000 any ideas?

Winter....

Winter....

oxygenated fuel...Gotta love it, er -- hate it.


mark

I use the same fuel in both cars tho and i drove the bonnie in many of winters and never got that poor MPG i was wondering if maybe it was the mass air flow senser or some thing dumb like that.

Seems like they did something they did to the gas this year has a very bad reaction to our Grand Prixs because everyone with a Grand Prix has a lot worse gas milage right around now.

Mine is doing juct fine. All things considered Im getting 22mpg combined highway and city (even including warm up time).

Buff6116 you may want to check out your tire pressures and make sure that OD is working.

20-24 mpg on the highway is low even for winter fuel. Starting with the basics, has it ever been tuned up (plugs, wires, fuel filter, PCV)? If so, were quality parts used? Any CEL on, and if so, have you had it scanned? Have you checked fuel pressure (should be 48-55 psi, key on/engine off)?

Here is what i know i bought from a guy that worked at a GM dealer. He just put in a new upper intake about 10,000-15,000 ago the plugs look good they are AC delcos the wires look fine. It also has a new fuel regulator. I put it on the scaner right after i bought it and there was one code for the ABS mod thats bad and nothing els not even a miss fire. I put in a new air filter in it to. Tire pressure is good with this frozen MN weather i do keep up with air in tires when needed. Yes it goes in to OD. With letting it worm up and driving a few miles i am geting 11 mpg. Fuel filter is on my to do list i will check the fuel pressure next time i am home from college to see how that is. I still have my Bonnie but that died do to a short in the main harness is it worth taking the throttle body and every thing off that and puting on the GT?

Thank you guys for all the ideas so far!!!

I would love to get 20mpg, I'm getting 13.8:disappoin

I get 20-22 around town in my 99 GT. I'm not a leadfoot, but I'm no bluehair on the gas either. I get 32-33 on the highway if I keep it under 75. If I get above that, it starts dropping significantly. Unexplained bad mileage usually goes back to three things.
1. Bad/worn tune-up/emission components (plugs, wires, air filter, PCV, o2 sensors, MAF sensor, etc...)
2. Exhaust blockages (bad converter, resonator, muffler)
3. Fuel pressure issues (leaking pressure regulator, weak pump, clogged filter)

Come spring when its not -20 like it is now i think i am going to get an off road down pipe and a cone air filter set up. Should i get rid of the resonator well i am down there or no big deal. This 10-12 MPG Winter City stuff sucks but i still love the car.

Thanks richtazz

I removed my resonator (stock one had a hole in it) and the only thing I noticed was a nicer sound. I'm not really an exhaust head at all though but it doesn't seem like it hurts the car in anyway.