Ok...I just purchased an 01 Oldsmobile Alero with 150,000 Miles. It has the 3.4 3400 Engine in it Automatic Transmission.. I just purchased it to get good fuel economy on a 30 mile commute I do a couple times a week. Ive drove it about 100 miles and it took 6 Gallons??...Im thinking I should be getting way better fuel mileage than that. The Only real problems I have had with it is, occasionally when you are driving like going from 1st to 2nd and 2nd to 3rd slowly it Shakes and Stutters. If you push the gas when it starts stuttering... it comes right out of it and Drives Fine. What could cause my fuel mileage to be so bad. There is no Service Engine Soon light?...

Does it feel like a misfire, or a transmission slipping?

Was it scanned for codes?

Did it do this from day 1 when you got it?

Does your transmission have a dipstick? If yes, fluid level/condition ok?

it does not feel like transmission is slipping because if I give it a lot of gas it shifts through gears Smooth. It almost seems like the Fuel is cutting in and out. It never stalls. What could cause this car to get 18mpg....and not throw a code?...or atleast I don't think it has thrown a code... no check Engine?..

02 Sensor was the Only thing I could think of that'd cause these problems and Fuel Mileage to be terrible....Would it be the Updraft or Downdraft 02?

A ruptured diaphram in the fuel pressure regulator will cause fuel to be drawn into the intake system, and can have an effect on gas mileage. Its a good place to start because it is so easy to check. Just pull the vacuum hose off and smell for gasoline.

As suggested, check the vac line for fuel on the f/p regulator......the pre O2 sensor controls fuel, the post is for catalytic efficiency....

Was it scanned for present/history codes?

No high idle(vac leak)? No power problems when accelerating?

Those look like Bosch or NGK plugs......I prefer OEM'* over those....

Typical mileage is 19/29, around 23 mpg combined....

real world mileage is always over 30 on 3400'*. unless you drive it strictly city, and plugs are not going to drop it 30 percent. it sounds more like possibly, fpr, od is not engaging, o2, maf, something like that. you inspect the air filter?

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Air filter appears to be a k&n, i cant smell for gasoline in vacuum line because i have a terrible cold. Rpms are 1800 at 60 if that helps for the overdrive. The only problems driving is at low rpms like barely on the gas running through the gears it just like its surging but if im giving the car medium to high throttle no problems what so ever. Hills or flat doesnt make a difference.

In my experience these car never got good fuel economy. My 01 Grand am was around the 22 mpg mark and 17 city. One thing for sure the fuel tank isn't very big. Also need to look at tire size, smaller diameter tire will drop fuel mileage. Throw that K&n in the trash, if its the oil type, it will mess with the sensors thus affecting operating conditions.