1999 Grand AM SE 3.4L Horrible gas mileage

Hi everyone.

I bought this car about 4 years ago and I used to get approximately 400 KM per full tank. I have kept up regular maintenance on the car but now the car only gets approximately 264 KM per full tank.

while I am driving for even 15-20 minutes I can actually see my fuel guage going down... thats how rapid I am burning gas. I need to fill my tank once a week (40 litre tank) and I live 12 min away from work!

I have replaced the air filter with a K&N filter
Had a tune up and replaced wires, plugs (which were not wet), fuel filter pcv valve.
Used a fuel treatment/cleaning system
Tried a higher octane gas (usually use 89)

I also let the car idol over a fresh bed of snow to see if there were any gas leaks in the lines..... none detected.

Still the problem exists.
I just had the mandatory drive clean test which we have here in my province which checks emissions to ensure that the levels are good, so I do not assume that my oxygen sensors are failing.

I do not have a service engine light on which all but eliminates the chance of it being caused by the oxygen sensors.

I do not smell gas and have recently had my nozzel which leads into the gas tank replaced.

I am at a total loss and do not want to spend a few hundred on new oxygen sensors when nothing really indicates that they are faulty.

Any suggestions would be greatly appreciated.

Thank you!

Feel free to email me if you have some suggestions to try.

I will try to look at this forum often as well.

Welcome to AF.

Ok, i did the craxy conversions..
40 liters-15 gal
400 km = 248.548 mi
to 264 km = 164.041 mi

That is terrible !! Even the 400 km is not real good.

How many miles on the car ?
If thier is over 100,000 miles then its time to change your O2 sensors. That is what's eating your fuel.
Plus during the winter your MPG drops.

My Se gets at least 300 miles per tank, even right now when its cold. No more then 350 per tank though.

Thank you very much for your reply.

The car has 114 979 KM on the odometer.

I would have assumed that I should have had a service engine light come on, but I guess I was wrong.

The car does not lag, and seems to have full power, with no studdering or choking, but I guess changing out the O2 sensors (front and rear) could not hurt.

I truly appreciate your feedback.

You dont always get a DTC for O2's unless they are totaly failing.
You have to figure too, they are probly 10 years old now, if they are the stock ones.

Here is a link if you want to test them manualy.
Oxygen Sensor- Testing and replacement (http://www.automotiveforums.com/vbulletin/showthread.php?t=672110)

Thank you for that link it was extremely helpful.

I went out today and purchased both the front and rear oxygen sensors.

Getting the front one out was a bit tricky but I was able to get it out successfully. I replaced it and let the car warm up a bit more, then tightened it a quarter turn so it was snug.

After about 15 min the service engine light came on but I assume this is expected behavior. The car sounded great had nice power and at idol was running about 700 rpm which is normal for my car.

When attempting to replace the rear sensor, there does not seem to be enough space to get the ratchet in between the heat shield, so I guess I will have to use a suitable wrench to over come this.

My questions are these.
If I replace the front and not the rear, should the service engine light come on due to one reading one value and possibly the other one complaining about it?

Is it normal to see a service engine light appear after replacing these sensors even though the car seems to be performing better?

I assume I will not see any considerable gas mileage changes for a few days as the computer will need to gradually alter the mixture of o2 and gas.

Turns out the "Service engine light" was not caused by the O2 sensors, when I pulled into work today and parked my car, as soon as I put it in park, the bloody thing dropped down to 0 revs and stalled.

I turned it right back on and kept it in park. Again she died.

Called the tow truck, had it sent to the shop, and found out my fuel pump decided it had enough.

New O2 sensors are now installed, and not seeing any difference in the gas mileage, but I assume it should take a few days for the pc to reset itself, as I read somewhere that it will not be a noticeable difference in gas mileage immediately after changing these sensors.

I get the car back from the shop tomorrow and will update in about a week with my findings.

Well it is going from fun to even more fun.

The dealership can not find any leaks in the vaccum hoses so now they seem to believe it may actually be the computer.

So... changed oxygen sensors, put in a K&N air filter, fuel pump and possibly the computer if deemed fubared.

I sure hope to heck that my fuel economy gets better now.... Im going to cry soon,,,, went from a 200 dollar job to 2000...

Yeah you shouldnt have got the SES light from changing the O2 sensors.... though if you testes the O2s then you might trip the SES .
Did you ever get a code for that ??

I always hate it when the stealership says "We cant find anything wrong" or "it might be the PCM". :disappoin
I'd just buy a DHP PCM for $100 !! (http://www.pfyc.com/pc/GA3010/GAUNDER/Digital+Horsepower+PCM+-+Grand+Am+SE-GT.html)

When was the last plugs & wires installed ??

I would recommend AC delco Iridiums. Pricey but well worth the extra money you'll save in replacement and gas mileage.

Good Luck hopefully you wont need to replace the PCM.

I had a complete tune up this summer and had the bosch platnum plugs and wires put on.

They apparently do not have codes being seen and are going hook up a new pcm to see if this resolves the issue.

Right now they even called up yet another foreman to come and look at the car as they are completely perplexed with what is causing the car to choke. I did get the "we cant find anything wrong so it may be your pcm"

If they are perplexed, I am in awe because when I sent them the car it stalled on me while in park but was still drivable (drove it about 100 yards to the tow truck) and completely died at the dealership.

I gave them a nice lashing on the phone as I can not comprehend why the car shows all these symptoms now when it didn't when I handed it to them.

the bosch... that very well could be your troubles.

I've never had them last more then 1 season(summer) and they would be burnt up and damaged.

Might pull 1-2 of the front plugs out and see how they look.

I would tell them that you wont pay for items replaced that arent needed or dont solve your troubles, this tends to happen often. :rolleyes:

Good Luck

Well the verdict is in.

They did not need to replace the PCM but the new oxygen sensor I bought was faulty, and kept things wide open.

They tuned up the car, and did a data fuel trim at 23%.

I had a bad wire leading to the O2 sensor which they re-wired for me, and changed the fuel pump.

Since they had the car for 3 days and could not locate the initial problem in a reasonable time, they decided to knock off several hundred dollars off of the replacement parts and not charge me taxes or labor! They showed me a print out from the testing on the fuel pump and it was only at 29.9 psi when it should have been about 41 so it was going for sure.

So all in all, I got a new front O2 sensor, new fuel pump, they installed the rear sensor for me, tuned up the car and modified my fuel distribution for a price of 971 CAD.

The car sounds and runs amazing now. I havent noticed any huge difference in gas mileage yet, however Ive only had the car for a matter of hours and have only driven it about 25 kilometers.

Yes tuning through the PCM is essential with a tune up to realy get things back into spec.

Glad to hear you got it straightend out !! This should improve you MPG, or KMPG in your case. :D