still getting abnormal gas mileage..
 

when i first got my alero, it was getting 11 mpg (mostly highway)

since then ive:

changed oxygen sensor
changed air filter
cleaned throttle body
cleaned maf sensor (got rid of oxy sensor code)
changed spark plugs to ac delco iridiums
changed spark plug wires
changed oil to mobil1 synth

now im getting about 16 mpg highway, which is still not acceptable. i do have a low cat efficiency code but there are no signs of it being clogged (loss of power, rotten egg smell)

so is it really the cat converter thats causing my bad mileage? if not what else is there to check out?

also i check tire inflation almost every week, and i store NOTHING in my car to weigh it down. so those couldnt be part of it..

cat would become a problem in mpg, idk how much of a loss but i know there is some, upgrade exhaust system and add a CAI you'll also see a little better mpg.

my car has pacesetter exhaust, hi flow cat, and cai. works pretty well with great gas mileage.... until i get on it sometimes lol..

thats the thing though. i know it might damage the mpg like 2-3 mpg but the 3.4s are supposed to be getting like 25 mpg mixed right? something isnt right here.

Yeah somewhere in there. I got a 3.4.. but that's weird... maybe someone else will key in cause you got me stumped... lol

when was the last time the fuel filter was changed?

Ignition coils maybe?

fuel filter will be changed shortly. hasnt been changed since i bought the car though.

didnt even think of ignition coils. would those hurt mileage pretty bad?

if they arnt working properly and you would probably have missfires. change the fuel filter first and see what happens then.

The ignition coils on my rotary have to be changed every 30k - 40k kM and it's a noticeable difference which is ridiculous but I haven't had to change it yet on my 2.4. The easy way to test it is to take out spark plugs one at a time and hold it with an insulator (don't electrocute yourself) and have someone crank the engine. If the sparks are bright enough to be seen in daylight, then your coils are fine.

id still do the fuel filter first. if the coils were bad enough to cause that kind of fuel millage he would be complaining of more then just gas consumption.

yeah im not getting any sort of misfire or any performance issue. idles beautifully and has plenty of power

Probably running rich, which would lead to the cat going bad, run a couple bottles of injector cleaner through the fuel and see what that does for you.

A bottle of injector cleaner and if that doesnt help time for a new cat. I would reccomend removing the after cat o2 sensor clip and see if that changes anything you will pop a code but it should run fine since actual afrs only come from the first o2 sensor pre cat

i forgot to add that i ran a bottle of techron through about 1k miles ago.

and yes, the car was running rich due to an upstream o2 sensor issue for quite awhile but i fixed that, and thats probably why my cat is dead lol

im still betting on fuel filter. if the cat wasnt working it would probably set a light. if the cat was plugged you would have crappy power and possibly missfires.

I am thinking that either your new O2 sensor is bad or the one in the back is bad. I always change all of them at the same time, and with Bosch exact fit replacements.

bosh sucks and will go bad quickly buy denso also the cat code will cuase it to run rich too fix cat and get MAP sensor checked at part store it will run pig rich too