Ryperedo

I have a 2008 Mazda 3 2.3l.
I'm not happy with my mileage. I'm averaging about 19 mpg.
i recently did a long commute about 90 miles all on the highway. now i wasn't running my car hard or anything and i was also only averaging about 24 mpg.
Also to mention i have put headers in my car, a full cat-back exhaust system and a cold air intake. so i believe that i should be getting a lot better than that being that stock my car is rated at 23 city and 29 highway.
just trying to figure out why my mileage is so poor i have already tried fuel system cleans(sea-foam).
maybe some sensors are dirty?
help

MZ3SPORTRUE

how many miles is on ur car? have you done any tune ups? depending on ur cars miles, new plugs can help a TON!

Ryperedo

My car has 44k
so i shouldn't need plugs yet. anything else in mind?

djs2571

adding headers - did you replace the O2 sensor? when putting on the air intake or anything else, did you use silicone ANYWHERE? - that'll kill the O2 sensor and thus kill your mileage.
Try a battery reset, that forces the computer to relearn the fuel mix.
I hope your brakes couldn't be dragging that badly - they should be just warm if you drive a bit and don't stop hard.

Ryperedo

no didnt replace any sensors. and no silicone. ill try the reset then ill let you know

Ryperedo

so tried doing the reset...no luck, made a 50 mile commute and no difference

djs2571

There is a variability between cars, so i've been told - though the vehicles i've had were dead on the estimates, so i'm a bit skeptical that there's a wide range of mileage. I'd assume the 30MPG highway means it'd be 28-32 - something pretty close. Your 19 seems quite low in any case.
Have you had the car from new? keep track of the mileage before and after mods? Do you have the original parts to swap back on to see what made the biggest difference? - yea that one is a pain to do, but a last option to figure it out.
If you get a high end scan tool, you can figure out if the O2 sensors are working properly - giving a good signal.

djs2571

looks like a hiccup in the forum, I replied before your reset...
so how are you measuring your MPG? 50 miles is a short time to figure things out. The trip computer is unreliable, you need to actually refill your tank and measure the gallons added to top it off vs the mileage.
I'd try the scan tool to monitor what's goingon, though without a baseline of a good vehicle it may not tell you much. See the attached, it's from my PC based scan tool (Auterra). It gets a lot of information, but there are some features that i'd like to see which are missing.

Looking at data i've got, it looks like the wideband sensor (#1) should be at pretty much 0mA and both #2 & #3 O2 sensors should read ~0.7-0.9V when in a steady state. #2 fluctuates more with engine accel / decel, and the #3 sensor stays fairly constant. Both can fall to 0V on decel. I'd have to run another test to see what sort of characteristics they have, but it seems to make sense.
The wideband should compensate immediately for engine load, thus no changes - maybe I need to zoom in on my data though. #2 sensor is going to vary more since it's before the cat. #3 sensor should be pretty steady since that's the output of the whole system that goes to the enviornment.