1999 Grand Am SE 4 Cyl Bad Gas mileage

[deathcabforchristina]

Hey, just bought a 1999 Grand Am last week, and I've noticed I am getting horrible gas mileage. I did the math after I got gas yesterday, and it was about 15 mpg. What could it be? My engine light went on the other day, and I had it checked and was told the 2 codes were a loose gas cap and the second o2 sensor... But he said the o2 sensor is probably fine and that sometimes that popped up when it started turning colder... Also said that second o2 sensor really wouldn't have any effect on my gas mileage. Any ideas what this could be!? Should I take it to a shop and have them look? I put a can of sea foam in when I filled up yesterday to see if that would help at all.

 

[deathcabforchristina]

The place I bought it from changed the oil before I got it.... And the spark plugs.... Should I try a fuel filter next?

 

[msmith1956]

What were the exact code numbers? I'm guessing one was a p0442,and the other a p0420?
I'm not sure if it would make your gas mileage be that poor, but you could have a failing fuel pressure regulator. I'm sure some others will chime in with suggestions.

 

[melsg5]

As above, we need the codes. How many miles on the engine? Did you check the condition of the air filter?

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[deathcabforchristina]

Don't have the codes I don't know a lot about cars, there are 88k miles on it. No idea what the air filter looks like.... If I go into a oil change place will they pull it out and look at it for me? What about the fuel filter? Should I have that changed?

 

[msmith1956]

You could change the fuel filter, but I don't think that is your problem. Go to an Autozone, Advanced Auto, etc. Most of those places will pull codes for you free of charge.

 

[deathcabforchristina]

Even if it was cleared like 4 days ago?

 

[msmith1956]

If the codes were cleared, it might still show in history, but if the SES light is off, just drive it a few days and see if it comes back on.

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[Zimgxp]

Front o2 sensor, or the coolant temp sensor if bad will give bad milage. As mentioned, pull the vacuum line off of the fuel pressure regulator. If there is gas there, it needs to be replaced.

 

[deathcabforchristina]

SES light again P0440. Went to a shop, they suggest a smoke test.

 

[deathcabforchristina]

SES light again, code P0440, mechanic says I need a smoke test to find the leak?

 

[deathcabforchristina]

As above, we need the codes. How many miles on the engine? Did you check the condition of the air filter?

And air filter is fine

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[melsg5]

This explains the code but the easiest and cheapest thing to do is replace the gas cap

http://www.obd-codes.com/p0440

 

[msmith1956]

Yeah, it might be a good idea. If you do it, make sure they check the filler neck on the fuel tank, those are known for developing a leak at the neck/tank seal.
http://www.obd-codes.com/p0440

ETA: As above, I would try replacing the cap first.

 

[deathcabforchristina]

I already replaced the gas cap Sunday

http://shop.advanceautoparts.com/we...t_cap-gas-stant_18481527-p?searchTerm=gas+cap

 

[deathcabforchristina]

It is indeed the filler neck, would this be causing my crappy gas mileage? $120 parts + 60 labor + 30 for the test.... All on a car I've had 10 days!

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[melsg5]

The pipe has nothing to do with your gas mileage. Do you have any idea how the car was maintained by the previous owner. How many miles on the air filter how many miles on the fuel filter and spark plugs, etc.

 

[msmith1956]

Well, as stated above, it won't have much to do with mileage , but at least you know now you're not loosing any gas through the neck, if nothing else from evaporation.

ETA: I agree it would be a good idea to have your plugs checked/replaced.

 

[deathcabforchristina]

I replaced the spark plugs on Monday, with AC delco Pro Platinum, the air filter is a little dusty but you can still see light through it, no idea how it was maintained. Would my next step be doing the fuel filter?

 

[msmith1956]

I wouldn't replace the fuel pump without doing a fuel pressure test, and fuel pressure regulator test first. A weak fuel pump isn't going to normally cause poor gas mileage. A bad fuel pressure regulator will cause reduced mileage because if the vacuum diaphragm is bad, it will let excess fuel make it to the injectors causing a overly rich condition at lower RPM's. You can do a FPR check yourself by removing the vacuum line from the diaphragm and putting another piece of vacuum line or clear plastic tubing (like aquarium airline on it, and suck on the tube. If the line doesn't have resistance like sucking on a straw with you finger over the other end, or you see gas coming up the tube (if you're using clear line), you have a bad FPR. BTW, even a good FPR will have a gas smell at the diaphragm, so don't worry about that.