Gas mileage problems in 99 GT [Archive] - GrandAmGT.com Forum

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bigtard
10-16-2005, 01:22 AM
Can someone please help me figure out what's wrong with my 99 Grand Am GT?

First, please look at my gas mileage log. It's here: http://img422.imageshack.us/my.php?image=gasmileage4wd.gif. "Alternator" designates the fill-up immediately after I got my alternator replaced. "HWY" means I drove on the highway during that period. "O2, Mass" and "Spark Plugs" refer to when I got those parts replaced.

Second, can anyone help me figure out why my mileage is so bad? I drive almost the exact same route every day. Even when I don't drive the same route, I never drive on any hills. All the roads around me are the exact same terrain (flat, major roads every 1 mile). The temperature is quite constant.

I thought there might be hope after I replaced my plugs last week. For about 76 miles I had marginally good gas mileage. Then tonight I noticed that my gas mileage dropped to it's previous -- and wholly unacceptable -- level.

Why? I've wasted over $1000 trying to fix this car, but nothing seems to work.

Any suggestions?

bigtard
10-16-2005, 02:19 AM
what do you get to a tank?!

I think my tank is 11 gallons. I'd probably get about 200 miles on the highway.

This is what my fuel mileage *should* look like: http://www.fueleconomy.gov/feg/noframes/15027.shtml

You'll notice that for a few days after my spark plugs were changed, I got the correct mileage. Then it dropped again.

H.O. Driver
10-16-2005, 07:18 AM
Does it feel like its sputters or has a lack of acceleration? have you changed the fuel filter? That can cause some bad mileage or a stuck open fuel injector. The other thing to look for would be does it smell like rotten eggs if you get into the gas at all? I would look at the fuel filter and cat at this point, both or one or the other might be clogged, that would cause bad mileage.

nascarnate326
10-16-2005, 08:16 AM
Does your intake gasket need to be changed? I gained 8 mpg when mine was done.

Panacea
10-16-2005, 11:15 AM
Change your oil, oil filter, air filter, fuel filter, get your injectors cleaned, take the lead out of your foot.

The 99 Grand Am's have a 15 gallon tank. I get 240-300 per tank. With little to no highway driving. I will get around 200-260 per tank in the winter time. THere are a lot of variables to why your gas isn't consistant.

Also, how are you figuring out your milage? The correct way to do it is to fill up your tank. Then thake the total gallons you filled up your tank with and divide that into the total number of miles driven since the last fill-up.

bigtard
10-16-2005, 02:29 PM
Does it feel like its sputters or has a lack of acceleration? have you changed the fuel filter? That can cause some bad mileage or a stuck open fuel injector. The other thing to look for would be does it smell like rotten eggs if you get into the gas at all? I would look at the fuel filter and cat at this point, both or one or the other might be clogged, that would cause bad mileage.

The idle when stopped and in gear is a little rough. If I put it in neutral, it smooths out, though.

It does not smell like rotten eggs.

I replaced the fuel filter and PCV valve on September 14 when I had a new O2 sensor and mass air sensor put in.

bigtard
10-16-2005, 02:29 PM
Does your intake gasket need to be changed? I gained 8 mpg when mine was done.

I don't know. I don't even know what that is.

bigtard
10-16-2005, 02:35 PM
Change your oil, oil filter, air filter, fuel filter, get your injectors cleaned, take the lead out of your foot.

I've done all of that except get my injectors cleaned. The price I was quoted for that was nearly $200. Sounded like a phenomenal rip off to me, particularly since no one can tell me what's going on. I'd gladly pay it if a competent mechanic could properly diagnose the problem. But I am not interested in paying for their hunches.

Also, how are you figuring out your milage? The correct way to do it is to fill up your tank. Then thake the total gallons you filled up your tank with and divide that into the total number of miles driven since the last fill-up.

That's what I do. (I reset the trip meter every time I fill up for just that purpose.)

Panacea
10-16-2005, 03:02 PM
Go to the dealership. It is $100 plus some tax. I highly recommend it! If your injectors have never been cleaned, I would say you could benefit from it.

DrFabulous
10-16-2005, 04:39 PM
You should try taking some measurements after driving a few more miles. Don't refill all the time, drive it all the way down to empty a few times, and see how much gas you burn.

Injectors could be the problem, though.

Mike Jung
10-16-2005, 04:47 PM
You should try taking some measurements after driving a few more miles. Don't refill all the time, drive it all the way down to empty a few times, and see how much gas you burn...
IMO, I would NOT regularly run your gas tank down to empty :nono:
(Fuel pumps are not cheap to replace. & fuel tank sensor sending units are known to give false levels, leaving you running out of gas)

Just use a few fill-up periods & add them together. & calculate the #'s

DrFabulous
10-16-2005, 04:50 PM
IMO, I would NOT regularly run your gas tank down to empty :nono:
(Fuel pumps are not cheap to replace. & fuel tank sensor sending units are know to give false levels, leaving you running out of gas)

Just use a few fill-up periods & add them together. & calculate the #'s

I shouldn't have said EMPTY, I just meant more than 3 gallons burned off. Down to a 1/4 tank or so. My mistake. :doh:

bigtard
10-16-2005, 04:51 PM
You should try taking some measurements after driving a few more miles. Don't refill all the time, drive it all the way down to empty a few times, and see how much gas you burn.

I can't fathom how the refilling frequency would matter. Driving 150 miles and putting in 10 gallons results in the same mileage as driving 75 miles and putting in 5 gallons.

DrFabulous
10-16-2005, 04:58 PM
I can't fathom how the refilling frequency would matter. Driving 150 miles and putting in 10 gallons results in the same mileage as driving 75 miles and putting in 5 gallons.

Well, not really. Depends on your driving habits. If I measured mine every 3 gallons or so, it'd be pretty crappy, because most of my driving is to work and back, 15 mins of stop and go driving each way. I get bad milage when I do that. If I wait for a week or so to go by, I'll have a few longer drives in there, and it will make my milage more accurate. What I suggested isn't a solution to your problem, more of a general idea.

Panacea
10-16-2005, 04:59 PM
Regardless, get your fuel injectors cleaned at the dealership and see if that helps out.

DrFabulous
10-16-2005, 04:59 PM
Regardless, get your fuel injectors cleaned at the dealership and see if that helps out.

Agreed.

bigtard
10-16-2005, 05:46 PM
Regardless, get your fuel injectors cleaned at the dealership and see if that helps out.

How does that compare to using the fuel injector cleaning additives found at automotive stores?

DrFabulous
10-16-2005, 05:57 PM
How does that compare to using the fuel injector cleaning additives found at automotive stores?

It's the difference between running soapy water over a crusty dish vs. scrubbing it.

bigtard
10-16-2005, 07:11 PM
Do they only clean the injectors, or are other parts of the fuel system cleaned?

Panacea
10-16-2005, 10:12 PM
When the dealer cleans your injectors they shut off the fuel and run a cleaner through the fuel rail all by itself. It will clean all of your injectors out better than any other method.