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97 Grand Prix GTP Bad Gas Mileage HELP!!!

06-14-2007, 10:08 PM
hello, i own a 97 grand prix gtp. I get really bad gas mileage from the car, it has 140,000 miles but i get only 15mpg in city and highway it never changes? Can anyone help me out. I bought a maf and replaced it but no change so i took it back. I have a snapon scanner and i have no engine codes. Gas is $3.45 a gallon for premium which my car takes here. Would you think the O2 sensors would cause this bad mileage on this car. Please if anyone has had this problem or can help me. I do not have all kinds of money to spend and right now all my money goes to gas. And i just took out the resonator, replaced wires with MSD, new NGK tr spark plugs, air filter, oil change with synthetic lucas, new 180 degree thermostat which did not change my gas mileage, replaced super charger oil with gm oil...I am at lost for my!!!:banghead:

06-15-2007, 05:43 AM
not getting better highway mileage (unless you drive 90mph) is strange. It sounds like a possible exhaust obstruction or leaking fuel injectors. Normally though, these will cause rich exhaust or cat efficiency codes to pop up. Is your supercharger abnormally noisy?

06-15-2007, 09:51 AM
hello, i forgot to mention i purchased new injectors from ZZP the stock set i think they are #33 injectors or something like that. my supercharger don't seem noisey, it whines like it should and boost guage can hit the end of the meter, car has plenty of power and i can break my 18's with 245/45/18 tires loose to make a set od posi burnouts. Im gonna buy a 3.5 pulley from zzp soon and install that. So do you think changing my O2 sensors and replacing them with Denso would to work ill check back later..good day!

06-15-2007, 10:00 AM
put the stock 16's back on with 225/60/16's, and that might solve some of your problems.

06-15-2007, 01:36 PM
Have you tried "resetting" the MPG on your DIC? I do it ocassionally on the highway and my MPG jumps up to 33-34 mpg. How accurate this is, I'm not sure but it's really nice to see it up like that.

edy figueroa
06-15-2007, 04:43 PM
My 97 GTP has 20 inch wheels and i get 24+ mpg highway everyday.I have changed fuel pump,coils,plugs and wires,o2's,also it works very cool too,no more 200 temps.I would say you start with some tune up..:2cents:

John Kanaly
06-15-2007, 09:44 PM
The oxygen sensor located in back of the eng blocking access to the spark plug is only good for 80,000 miles and will cause bad gas milage when getting old. GM says replace at 100,000 have you replaced it anytime?

06-15-2007, 10:11 PM
hello i have all new tune up that i posted on the begining post i started. How do you reset the dic for mileage? I will buy denso O2 sensors front n back ones...anything else..o ya the stock tires n rims had same mileage as wut i get now with 18's so that aint it..

06-16-2007, 05:12 AM
When you get to the MPG just hold the button down for about 5 or 10 seconds until it resets. Do this while you're cruising.

06-16-2007, 10:13 PM
dude thanks, i reset it while driving and now i hit 24.3 in town..thanks i dont know why it never moved, now it guys rock...what is the highest gas mileage that anyone gets with GTP?

06-17-2007, 05:20 AM
You're welcome, glad I could help. If I don't reset my DIC occasionally the MPG doesn't move either, I'm not sure why it does that. Maybe somebody can chime in and explain it.

06-17-2007, 11:09 PM
ya that is what mine does as well, it sticks...i hit 26.7 doing 70mph today and was pleased....

06-18-2007, 08:45 AM
Maybe somebody can chime in and explain it.

Really no explanation that I know of Paul other than that is how the enhanced DIC works. It is just a rudimentary trip computer nothing fancy.

AVG ECON is just the average fuel economy since last reset. Average fuel economy is viewed as a long term approximation of your overall driving and driving conditions. Thats why to get a current and/or accurate representation of average fuel economy the DIC must be reset otherwise you will see the cumulative average going way back since last reset. Best to reset it after every fill-up. The best way to determine your actual fuel economy is using the good old math way. Total miles per tank divided by gallons put in on the next fill-up.

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