so over the last couple of months my gas mileage has gone down and im not really sure why. i have not changed my driving habits at all. even my sisters 94 Grand Cherokee with the 5.2 V8 is getting better gas mileage than my car. any idea what i can do to fix this. i have cleaned out the throttle body and still no change. im going to put in a new fuel filter this week. im wondering if i get a fuel injection flush would help any.... so if anyone can give me some suggestions as to how i can get my mpg back up again. only mods on the car is a 3 inch FWI.

Sounds like you might have a bad Engine Coolant Temp Sensor; over time they get gunked up with antifreeze (like the Low Coolant sensor in the radiator), which insulates the sensor, and can give you poor mileage. Autozone sells them for around $13(TU178). I'd replace it along with the thermostat, especially with colder weather on the way. The Engine Coolant Temp Sensor is in the Lower Intake Manifold, below the thermostat.
Give it a tune up (plugs, wires, filters) and check your tire pressure. Hope that helps!

ya do the coolant shit, possibly flush. and do a good tune up. and for injector cleaner i use lucasoil injector cleaner you really can feel the differnce

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Behold the majestic beast as it strides

so i have done the coolant temp sensor. new plugs and wires. fuel filter. air filter. injection flush. hmmm. just about everything i can think of. and i have driven the car about 260 miles and its right above empty....any ideas???

here is a shot in the dark
I think you told me what mileage and did you check that cat. the only other thing I could say is the maf give it a through cleaning and see if that helps. I get 290ish on a tank on the 03 and I am not nice to the pedal in it . another thought is change your gas source, I can tell you I get good mileage with base gas but gas back home that amanda runs that is laced with ethanol is horrible for mileage.

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03 gs just ur avg bolt on car

Daily driver gone wrong
99 gs beater... the slippery slope car cam/heads blah blah blah

i was getting HORRIBLE gas mileage before i had my outer tie rod ends done....i was getting about 12-14 city mpg before and now im getting about 16-18+ city.....i didnt touch anything else, thats why i know it helped with mpg's....

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I checked the injectors today. and they are fine. sooo.... i think tomorrow i will check the cat. I have cleaned the MAF. doesnt seem to help. Im going to start going to the same gas station and using the same pump. so that way i know its not the gas.

Could be exhaust leaks throwing off the O2 readings?

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2005 GT owner [Non S/C] with a "big DIC".
Two years later and I still love the car.
Thanks Pontiac...Bleen is keen!

Are you starting your car & warming it up?
(letting it sit for 10-20min, just running?)

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i usually just drive it right after starting it. I think im going to check the cat today. if i have time.

Just how bad is your mileage? Any real numbers to go by? I just filled mine up, it was just a hair under 1/2 tank. Took 9.662 gal. but I went 269.1 miles. Totaling 27.86 mpg.

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2004 Grand Prix GT1, F&R STB's, GMPP R. Sway Bar, GMPP Trailing Arms, 180* stat,

15 mpg.

I pull 15/19mpg in the winter & 18/24mpg summer

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i get 15/16 in the winter city driving, if i ever go on the highway, i get about 20

then summer it bumps up to about 18 in the city

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SLP Intake|SLP Powerflow|ZZP DP|ZZP PCM|GMPP Handling Kit +Rear STB|ZZP Powerlog|ZZP Stage 1 TB|Immaculate Original Paint

I get 15-17 in the city and 28-33 on the highway.

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Pontiac Garage:
01 Pontiac Grand Prix GT/ 96 Pontiac Grand Prix SE/ 90 ASC-Mclaren Pontiac Turbo Grand Prix

Same here.

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1999 GT|SLP CAI|Borla Catback| Shift kit|18" Koblenz|GMPP Lowering|GMPP handling|Blk Headlights|ER Rcokers|HV3|HV TB http://www.fquick.com/The_Guz

I dont get very good gas mileage either.
Well It doesnt seem like i do maybe its because i used to have a 4 cylinder,
But my mom has a 05 gp and she gets better mileage and im pretty sure my gas mileage should be about as good as hers.
I changed the air filter, used injector cleaner last fill up.
Hasn't seemed to change anything yet.
Its a mystery i tell ya!

same problam here i get around 15 in the city year round no matter how i drive, i have noticed that if i get on it that it smells like rotten eggs sometimes...would that be the cat going bad..?

Yup. Sure would.

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One Big Ass Mistake America

Rotten egg smell could only be three things. Bad cat, bad battery or rotten eggs! Assuming that there is no rotten eggs that only leaves the cat or the batt. If it was the batt. it would smell all the time. So now we are back down to the cat. So yea, probally a bad cat.

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2004 Grand Prix GT1, F&R STB's, GMPP R. Sway Bar, GMPP Trailing Arms, 180* stat,

thats what i thought, i wish i could find somewhere to cut it out and put a straight peice of pipe in there...oh well

Take it to a "ma & pa" shop, tell them you are setting it up for a circle track.
Since you won't be using it on the street they should have no problem putting in a "test pipe"
I'm sure they won't give you a receipt nor a warranty... But who cares? Right?

FYI You CAT has a 100k Warranty!
So, if you are under 100,000 miles, take it to the dealership and reap the free repair.

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well my mpg still sucks ass. i did replace the front o2. but i have noticed the car is running really rich....any ideas?

If it is running rich you will have a SES light and at least one code pointing us in the right direction. Without that it is all a shot in the dark.

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Six-speed top-swapped GT, sold.
2006 F-150 4x4 for a daily driver.

nope. no light or code. kinda weird.