mileage slowly getting worse...

[coldfusion21]

90 legacy. Used to get 260-280 miles out of a tank. granted it needs a knock sensor but it was being pretty consistent. Over the last two or three tanks i have watched my miles per tank drop to near 200-220 or so. I have swapped on some other tires and they may have slightly altered the fuel mix for the colder weather but that seems like a bit much. The mileage change has also been gradual, didnt happen all at once when i swapped the tires on or anything.

 

O2 sensor? could it be bad without throwing a code? I'm gonna do the knock sensor this week for sure and go from there i guess.

[89Ru]

that's a considerable drop- 20-25% or so...worsening knock sensor is likely cause, retards the timing, combined with maybe 10% drop in winter mpg's could explain it

[subynut]

I'd go after that knock sensor first, then the O2. When was the last time the plugs/wires/pcv were done? Is it running rough? Quirky idle? How's the power output? What kind of primary driving? Maybe it's begging for a nice run out in the country.

[coldfusion21]

Power output seems ok. maybe a bit on the low side, but ok. Plugs were done recently, wires i haven't done, so atleast 10k miles or so. Probably been a long time on those guys.

 

Idles ok, probably could use a IAC cleaning though.

 

Its my DD, but i drive my cars pretty hard... Ran a can of seafoam through the fuel system shortly before changing the fuel filter, that was done in the last 4k or so.

 

Will do knock sensor soon, run a tank through and see what happens. Will report back

[Mike104]

I had a back knock sensor on my 99 OBW CEL was coming on and off. When it stayed on I checked the code and it was the Knock Sensor code (P0325?). The gas mileage had dropped to less than 150 mi per 1/2 tank. Changed knock sensor and went up to 200 mi per 1/2 tank (on guage).

 

Winter mileage is less too.

 

Try the knock sensor and make sure you don't have any dragging brakes too.

 

Good Luck

[charm]

I'm not sure measuring mileage in terms of miles per tank tells a good story. Miles per gallon is the story needed. MPG is a more reliable number because it removes the variability of how many gallons of gas you used per tank. Although you've been consistent in the past, your drop in miles per tank could be due to some other random factor having to do with when you filled the tank or, I don't know, but something other than just the mileage. As anal retentive as this makes me look, I actually built a spreadsheet that calculates this for me. I enter my mileage, the number of gallons purchased, and the cost (so I can see how much I'm spending on gas) and the spreadsheet not only tells me my MPG, but, it also graphs it so I can see the trend. My mileage is incredibly spikey even though I fill up right around 300 miles per tank. My MPG has ranged from, off the top of my head, 20 MPG to 29 MPG...all from ~300 miles per tank. Your mileage may not have dropped off as severely as you think it did.

[nipper]

Lets get that knock sensor replaced first and do a good old fashioned tune-up. If the O2 sensor has more then 80K on it be prepared to replace it, but wait untill you get a few tanks of gas in the car to see if that knock sensor was the issue or not.

 

 

nipper

[johnceggleston]

does this need to be an OEM part??

 

$75.73 plus shipping at: https://www.chaplinssubarugenuineparts.com/oe_parts_cat.html

 

$88.99 at my local autozone.

 

$78 - $153 at napa online.

 

well that's a no-brainer.

[nipper]

heheh knock senser can be from anywhere

[Olnick]

Been searching for a knock sensor for my sons '92 Legacy. Check this out for price:

 

http://www.autopartswarehouse.com/search/?N=0&VN=+4294964890+4294967234&Nr=AND(make:Subaru,part:Knock+Sensor,universal:0)

[lewisd]

sounds like a good price. I put in the bosch one recently. I was looking at the o2 sensors. is the subaru picky when it comes to sensors or would a universal one work? o2 sensors seemed to range from 50 - $111.

[nipper]

Sooby's that are OBDII need an OE O2 sensor for the front one. On an OBDI I havent heard any complaints about using universal ones, just use a brand name.

[coldfusion21]

Ok. Changed knock sensor. Averaged 20 mpg over 300+ miles.

 

Sounds ok, but not what the car was like when purchased. 24-26 mpg was common.

 

20mpg for a first gen with studs sound normal?

 

O2 is next on the list.