I was pretty regularly getting 22-27 mpg (i check every time i put gas in) but about 2 months ago my mileage dropped down to about 16mpg. I've since had the air filter and fuel filter replaced with an oil change but it hasn't fixed anything. I've also had the compression in the engine checked and it's perfect. The tires are all at a standard pressure and my driving habits haven't changed at all. Around the time the drop occured the battery was disconnected for a bit, not sure if that has anything to do with it or not.

 

I really have no clue what would have caused this... any thoughts?

I was pretty regularly getting 22-27 mpg (i check every time i put gas in) but about 2 months ago my mileage dropped down to about 16mpg. I've since had the air filter and fuel filter replaced with an oil change but it hasn't fixed anything. I've also had the compression in the engine checked and it's perfect. The tires are all at a standard pressure and my driving habits haven't changed at all. Around the time the drop occured the battery was disconnected for a bit, not sure if that has anything to do with it or not.

 

I really have no clue what would have caused this... any thoughts?

 

grrrr time to snarl about people listing thier cars mileage again.

 

Winter with a combination of a tired/bad o2 snesor is my bet. Espeically since your not complaing about anything else. Have you checked your tire pressures?

 

nipper

Yeah i've checked the tire pressure... they're all fine...

 

How do i go about getting the O2 sensor checked/replaced? Is it an expensive item?

Yeah i've checked the tire pressure... they're all fine...

 

How do i go about getting the O2 sensor checked/replaced? Is it an expensive item?

 

What is the mileage on the car? How long ago were the plugs and wires replaced?

 

Steve

The o2 sensor is about $100 bucks. If you replace it do yourself a favor and buy an OEM sensor. Don't even mess with aftermarket on this one. It'll be a 5/6 wire wideband. Some of the '00obw's had recalls on the front o2 sensor for tip cracking. How long has it bee since the battery was disconnected?

I dont know when the wires and plugs were last done, it may have just been 6 months ago, i can't remember if the mechanic did it then or said to wait till next time. mileage is around 107k

it's easily been two months since the battery was disconnected.

I would question the o2 sensor. In modern cars, plugs and wires last about 100k (subaru is way to aggresive at 36k for a tune up). At 100K or there abouts I usually do plugs wires and the o2 sensor (front) if i am having any performance or gas mileage issues.

The o2 sensor is the nose of the FI system. If that is lazy, it wont throw a code, but will dump the mileage.

 

One more question, does your car war up properly?

 

nipper

to quote the Nipper

"does your car war up properly"

 

I believe he is refering to:

it may be your FI coolant thermosensor.

 

 

to quote the porcupine

" It'll be a 5/6 wire wideband."

 

May I please have a link verifying this bit of info?

to quote the Nipper

"does your car war up properly"

 

I believe he is refering to:

it may be your FI coolant thermosensor.

 

 

to quote the porcupine

" It'll be a 5/6 wire wideband."

 

May I please have a link verifying this bit of info?

 

Actually i was referring to a stuck thermostat. If the car doesnt reach at least (i think) 170 degrees, it stays in open loop. this means the car is running off a program and not the actual value of the sensors. This can lead to dramatically lower gas mileage. A bad temp sesnor would throw a code, or make the car stall. A bad T-stat wont.

 

nipper

dragging brakes can also affect gas mileage. i've done a brake job and notice an increase in MPG afterwards. be sure the caliper pins are greased when the brakes are addressed.

Sorry Nip I guess my guess at your

reference was incorrect but thats

what we get for making references and guesses.

 

By the way it was a good point -

you could have also ask if the heat from the heater had changed?

 

Also

I guess I am the only Suby owner that had a coolant thermosensor that did not thow a code

since the resistance was not out of range.

 

But it did tell the ECU the engine was cold

and thus kept the mixture rich => the fuel mileage fell

dramaticly.

 

I do admit this was on an OBD I car.

The OBD II may be more sensitive.

So I guess thats a guess and not a reference??

Are there any codes? When our knock sensor went bad the mileage dropped pretty badly also.

to quote the porcupine

" It'll be a 5/6 wire wideband."

 

May I please have a link verifying this bit of info?

I have an '00obw with a 5 wire front o2 sensor which I assumed meant wideband. I saw something about it in this post but maybe it isn't right.

Thank you very much for the link porcupine.

 

Here is the actual link you refer to

Wide Band sensors

 

 

I need to read some more of the reference links

but the Subaru in question

is not listed.

 

Thanks again, I have an extra and would like to build

a wide band A/F meter.

The sensor was always the big buck stumbling block.

Great discussion... thanks for all the info.

 

1. The heater seems to be the same as always.

 

2. Is it easy to install the 02 sensor myself with limited car knowledge but decent wiring and mechanical general knowledge... can a car part store test the 02 sensor before removing it? Does it pay to "upgrade" to an aftermarket product rather than OEM?

 

3. I will keep an eye on the thermostat, i haven't noticed anything out of the ordinary with it, but i'll keep my eyes on it.

aftermarket is NOT always an upgrade on any part. At best its an equal. Auto parts mfg just copy or buy parts from the same suppliers as OE (if the can). Parts mfgs also try to use as many one size fit all parts as they can.

I worked for a parts mfg, and they do limited testing on most parts, unless they are critical to keeping the engine from seizing.

 

OE o2 sensor for the front o2 sensor is a must. The theromostat may come back to get you in the summer.

 

nipper

2. Is it easy to install the 02 sensor myself with limited car knowledge but decent wiring and mechanical general knowledge

It's not too hard; sometimes breaking the old o2 sensor free to remove it can be a little tricky. But if you use good penetrating oil a couple days before the job and warm the exhaust up for a minute before the removal it should come right out.

...can a car part store test the 02 sensor before removing it?

I don't think so. I mean they could read the voltage signal with an advanced scantool but your average parts store isn't going to have or do that.

Does it pay to "upgrade" to an aftermarket product rather than OEM?

No, most aftermarket parts are not an 'upgrade'. Stick with OEM on this one.