99 Grand AM GT = Crappy Gas Mileage

[FleshAndBlood1990]

I recently traded a 95 grand am 3.1 liter in for a 99 grand am GT 3.4 liter. My 99 has been getting crappy gas mileage!! Averaging maybe 16 MPG, one tank was just under 14 MPG. The car is supposed to get 20 - 28 according to the original windows sticker and fueleconomy.gov. What could be the problem?? Anyone have any idea? Makes me mad, my other grand am averaged at least 22 MPG. I was actually expecting the 99 to get better gas mileage. Thanks for your reply in advance!!

[GTP Dad]

Welcome to AF!!

There are many causes of bad fuel mileage in a car but one that usually comes to mind besides a lead foot involves the O2 sensor. If the sensor is going bad it does not properly read the O2 in the exhaust stream and causes the computer to allow more fuel into the engine and thus reduces the amount of mileage you are getting. I would change the front O2 sensor on the car and see what happens. Also watch your driving habits and idle times on cold mornings. You should see an improvement.

[daman]

Like Dad said,new O2 plus a compleat tune up and injector clean,
check tire pressure add a few more pounds than whats recomended.

and try to keep your foot off the floor..

[FleshAndBlood1990]

Originally Posted by GTP Dad

Welcome to AF!!

There are many causes of bad fuel mileage in a car but one that usually comes to mind besides a lead foot involves the O2 sensor. If the sensor is going bad it does not properly read the O2 in the exhaust stream and causes the computer to allow more fuel into the engine and thus reduces the amount of mileage you are getting. I would change the front O2 sensor on the car and see what happens. Also watch your driving habits and idle times on cold mornings. You should see an improvement.

Thanks for your reply's! I ended up talking to a mechanic at the local pontiac dealership and he said the bad gas mileage was extremely common during the cold months. He said also said the "check engine light" should be coming on if the 02 sensor was bad. Maybe I will wait a while and see what the mileage is like when it warms up a bit more. So far it hasnt seemed to make that much of a difference though...

[daman]

Originally Posted by FleshAndBlood1990

He said also said the "check engine light" should be coming on if the 02 sensor was bad..

Thats not always true..you could have a "lazy" sensor thats not
reading right and not throw a code..

[xeroinfinity]

I would also agree with the O2 not always throwing a code.

But upon testing the sensor with a multimeter,
you can see if thier is a problem with it not working with in its set perameters.

My GT has low signal on bank 1, so I get about 18-20mph n
Which is wierd because my SE moded out the wazzoo still gets 24-26mpg

[FleshAndBlood1990]

Well, its warmed up and I am not letting my vehicle run during cold mornings. I dont drive it crazy or anything either. Last take I got a whopping 18 MPG. Do you all think I should take it in to get looked at, or is that normal for a 3.4 liter grand am??

[crazy Jim]

I have a '99 3.4 with 140K miles, I get 25.5 MsPG Spark plugs/wires are fresh, air filter is fresh, fuel filter is old, I burn 87 octane gas tires are reasonably inflated and the brakes are in good working order, and I do not baby the car.

[xeroinfinity]

Well, its warmed up and I am not letting my vehicle run during cold mornings. I dont drive it crazy or anything either. Last take I got a whopping 18 MPG. Do you all think I should take it in to get looked at, or is that normal for a 3.4 liter grand am??

When was the last time you changed your air filter?
How about the oil, you change it every 4-5k miles?
Are your plugs and wires fairly new ?
Probly wouldnt hurt to change the fuel filter too if that hasnt been done.

If all those are in order,
then you might test or get the O2 sensors tested
as GTP Dad suggested.

But I wouldnt expect much more then 22mpg with the GT.

[alan &marge pepper]

thanks guys for your info.
i do not have to ask when the answer is in front of me.
i have a 1995 grand am. going to have to re place heater core soon!
alan