will a bad knock sensor always throw a code...

[sspeed]

Here's my situation:

1) Friend had bent valve in '98 Outback

2) In 2005 he started taking it apart, then lost interest

3) Jan 2010 I bought it, pulled engine, had heads redone, did timing belt, headgasket, wheel bearing, etc

4) Put engine back in

5) Tank was 3/4 full so I just started it up with 5 year old gas, amazingly it popped right off and runs great.

Here's my question... I topped off the 3/4 tank of 5 year old gas with premium just because old gas is supposedly so bad... I'm now down to about 1/8 of a tank left and I've only gotten about 230 miles on the tank...

I could chalk this up to old gas, but I'm also wondering if a bad knock sensor would make gas mileage this bad? I have an OBDII scanner/datalogger... I don't see any engine codes, but will a bad knock sensor always throw one? What else can I look for? Or should I just put some new gas in and see what happens next? The car runs GREAT...

[TheFixer253]

It could be the stale gas that hurt the gas mileage or maybe something else. In any case, I would strongly recommend you put some Chevron Techron fuel system cleaner in the next tank of gas. There's a number of things that could cause poor gas mileage and a knock sensor is only one of them. Also, it might take the ECU a while to relearn optimal trim settings to give you the best mileage and power. Another possibility is you have worn oxygen sensors. The car is 12 years old after all and it's easy to change them.

[subiesailor]

I always start with the cheapest thing first in your case gas would be a huge culprit. Shoot my little outboard on my boat runs like crap on gas thats only two-3weeks old.

Given how long the car as sat around. Don't do anything major till you run a few fresh tanks through it - the injector cleaner suggestion is not a bad idea given how long the car sat - this will loosen up any crud thats settled out in the fuel system.

My knock sensor tripped the dash light the second it started to fail and I suspect thats pretty much the normal case so don't go messing with that till you get a few tanks of fresh fuel through it.

[AWDFTW]

Plus whatever to the Techron and logging the next few tanks of gas for a baseline. If the MPG does not start to improve then it would on to the replacement parts.

Also, I've found the gas gauge to be less than stellar in accuracy, I always get my MPG by dividing the miles driven and fuel used.

[ETC]

You don't mention how it runs ?

Is the power decent ?

I'm guessing you changed the plugs, air filter, oil, all that stuff correct ?

like has been said, run some cleaner through it [I probably would have pulled the injectors and cleaned them]. If it doesn't improve let us know and we'll give you suggestions of what to check. Did you set the valve clearances while you had the heads off ?

[sspeed]

Thanks, yep, valve clearances were set... New plugs, wires, air filter... I did a partial cleaning of the injectors with BG44K before putting them back in..

Ok, I finally ran that tank of 5yr old gas out the other night and put in a fresh tank along with some BG44k I had sitting around, I'll try Techron next time. It runs SOOOO much better... The power was ok before and honestly it didn't run all that bad, but it felt "lumpy" for lack of better words... Now it's nice and smooth....

I don't know if it has the power it should, I'd never driven a non-turbo Subaru before this one.. It feels really "peppy" at partial throttle, at full throttle it makes lots of noise and sounds like it's really moving, but doesn't feel all that much faster. My friend has one and says that's about right?