18 replies to this topic

[BIGBLUE1212]

ok i have a weird fuel question. When i do practically 100% highway driving i get 25mpg but even if i do 10% city driving my fuel economy gets cut in half. For example, i was driving to my cottage and i got 150kms on the first quarter of a tank and now when i was driving to school which is about 80% highway driving im only getting 100kms on the first quarter! What is going on!....oh and the bad bad fuel economy continues even if i do 100% highway after that!

[ericem]

I know!!! mine is the same way, i dont understand, i only use 1/4 of the tank and did 156 KM then when i got fill up i only did 270km Im getting 350KM max before i fill up???? What is wrong here? It runs just fine, and my mom is the main driver, most of the time, and she drives well, you know how a women drives Pretty good but slow, but she doesn't have a steady foot u can say:rolleyes: Maybe the fuel indicator is dropping off, and i don't see it, because when i went downtown once(lots of traffice and 40km drive) i drove tehre with a empty tank and then came back still empty and made it home. Must be a fuel indicator problem.

I know nipper suggested something you pour in the gas tank, to clean the sensor.

[Manarius]

You both know that you should ignore the gas gauge and use the Tripometer. Drive it down to empty, fill it back up, drive it until the low fuel light comes on, refill and then divide the number of km's by the number of gallons or liters that you put in. Then, repeat this a few times, then complain about your gas mileage and we can go from there.

I repeat: IGNORE THE GAS GAUGE, IT IS INACCURATE.

[nipper]

Chevron has a product for cleaning gas gauges. i forget the name.

And as it was said, go by mileage since the cars are pretty consistant, and the fuel light always goes on at the same level. Its independent of the gauge so you can trust it.


nipper

[ericem]

Chevron has a product for cleaning gas gauges. i forget the name.

And as it was said, go by mileage since the cars are pretty consistant, and the fuel light always goes on at the same level. Its independent of the gauge so you can trust it.


nipper

i guess your right, but im still filling up about 40 liters when i go fill up:confused:

[Manarius]

i guess your right, but im still filling up about 40 liters when i go fill up:confused:

If your gas mileage is bad over several (several being more than 1) tanks of gas, check out your O2 sensor, knock sensor, and air filter.

Also, run a bottle of techron through your gas. The Legacy gas tanks hold 15.9 gallons or 60 liters. The low fuel light should come on at 13.5 gallons or 50 miles before you would potentially run out of gas.

[BIGBLUE1212]

the only thing i havent replaced is the knock sensor......are there any signs where you can tell you need a new one?

[Manarius]

the only thing i havent replaced is the knock sensor......are there any signs where you can tell you need a new one?

If the sensor is cracked or the ECU is throwing a CEL.

[ericem]

If the sensor is cracked or the ECU is throwing a CEL.

how difficult is it, to get at? And the o2 was replaced not to long ago.

[BIGBLUE1212]

where is the knock sensor on the 2.5 on a 97

[Manarius]

how difficult is it, to get at? And the o2 was replaced not to long ago.

The knock sensor is right under the intake manifold. It can be accessed with a very long ratchet extension. That's where it is on the 2.2 - I think it's the same place on the 2.5.

[Reveeen]

A knock sensor failure will put the "check engine" light on. The knock sensor is bolted to the block kinda right of center (standing in front looking in) directly below the intake manifold. They look like the one pictured here with the wire attatched:
http://rb-k.bosch.de...kopfsensor.html
I *suspect* that Subaru buys them from Bosch, because there is no other reason they are "crazy expensive", in excess of $200 for my Legacy, while the same damm thing for my Saab is $80. The one on my Legacy went electrically "open" and had a crack you could see in the housing. When this happens your timing auto-retards and will cost you fuel mileage. I measured a new one @500 ohms and used a resistor to fool the computer, this is something I don't recommend, it is there for a reason, but I always use premium fuel, so I do not feel threatened.
It is more likely that the current cold weather is fooling with your gas mileage, keeping your engine cold longer, than a knock sensor failure.

[BIGBLUE1212]

what would the difference in fuel economy be between a good and bad knock sensor?

[Manarius]

what would the difference in fuel economy be between a good and bad knock sensor?

A few mpg...3-5 on the high side I think. I really think though that you should go through a few tanks, figure out your MPG, and then come back to us. Don't use the gas gauge, figure it out the correct way, came back with your stats, and we'll go from there. It may just be a winter gas thing or it could be something more. Before you go and throw money at it, we need to know hard facts.

[Reveeen]

FIRST: you have to determine the one you have is "bad", as I said earlier, a bad knock sensor will throw a code, then you would unplug it from the harness (connector up by your throttle body) and check it to ground, you want to see 400-800ohms, but wait, you are not done yet, if the value is not within spec the sensor grounds through it's body to the engine block, so maybe checking the bolt tightness and/or cleaning under it will bring it within spec. It is a go, no go, situation, there is not an adjustable value here, so maintence replacement is not a consideration other than wasting money.
IF you have a code (check engine light on) then you potentially have something to fix, no light, no "fix". I *guess* I'm trying to say "if your computer doesn't "see" a fault then there is nothing that a new knock sensor is going to fix", look elsewhere for your potential lost fuel mileage.

[ericem]

I just checked it and it is cracked, but no CEL, so what now? subaru told me 151CDN for the knock sensor and its in stock........ Should i go get it? I just checked this website and its $51US which is still ALOT cheaper then going to the dealer.

https://www.subaruge..._parts_cat.html

[nipper]

I just checked it and it is cracked, but no CEL, so what now? subaru told me 151CDN for the knock sensor and its in stock........ Should i go get it? I just checked this website and its $51US which is still ALOT cheaper then going to the dealer.

https://www.subaruge..._parts_cat.html

If its cracked it has to be replaced. Being cracked will affect its function, not enough to throw a code, but enough to affect performance.


nipper

[Manarius]

If its cracked it has to be replaced. Being cracked will affect its function, not enough to throw a code, but enough to affect performance.


nipper

I agree with nipper. Mine was cracked and not throwing a code. I think that having a new knock sensor this winter has kept me in the low 20's for mpg even with 20-50w oil and winter blend gas.

[ericem]

I agree with nipper. Mine was cracked and not throwing a code. I think that having a new knock sensor this winter has kept me in the low 20's for mpg even with 20-50w oil and winter blend gas.

ok, im guessing bosch makes these sensor's so can i go to NAPA? because my dealer is crazy!! 150 PLUS 14 percent tax. Online its 50 bucks.