My good friend has a 98 Subaru Forester. It has 120K. Well maintained new timing chain. It is a Northern car and he resides New York so there is exposure to Winter i.e. rust. He has been experiencing a gradual decrease in fuel economy. When he purchased the vehicle he was getting about 20mpg. He is now down to about 11mpg. There has been an accelerated decrease in fuel economy over the past couple of months. He does not drive the car very far. Mostly sporadic 2-3 mile long commutes. He has been running two codes for about a year as well. He needs two o2 sensors and he is getting an "evap" code. He did confirm he was tripping those codes while getting decent mileage. His mechanic also mentioned that the o2 sensors and evap code should not be effecting a drastic decrease in fuel economy. Thanks
As far as I know o2 sensors can affect fuel economy.Some cars use them modify the amount of fuel injected.It could explain why it was getting worse m.p.g. as the sensors got more and more out of range.I am not sure how subarus e.f.i. system thinks.
depends on which O2 sensor it is., I'm not familiar with the forester setup, but if it is like the Impreza, the front O2 sensor being wacky will definitely mess up his fuel trims
Yeah, the rear is only used to compare to the front to make sure the cats are doing their job. Might just be winter gas too. Driving habits can change fuel economy like that and short trips like you mentioned are hell for gas mileage.
2nd o2 The second o2 will also cause the car to stay at 13.5 a/f at idle for about 40secs. This isnt your only problem though. What are the code #s? Evap turns on at part throttle and at cruising speeds only. It sends fuel fuel vapors from your fuel tank to intake. Possible Problems: Leaking injector Clogged air filter Both o2 sensors Coolent temp sensor Tuning Issue Im sure there are more. but those are the main ones. Also if the tech say that o2 sensor does not affect fuel delevery go some where else!
the 98 is completely different fuel control setup compared to 99, 2000~2004 (n/a), 2005+ (n/a) and all the turbo stuff 2002+. The MAF handles 90% of the open and closed loop fueling. Both front and rear O2's handle some of the closed loop fueling. The purpose of the rear is to double check the overall AFR which will show if the cats are working or not if its at 14.7:1 in closed loop under certain conditions. It has a VERY VERY VERY small affect on trims. 99+ the rear had a larger influence on overall long term AFR, aka Long Term Fuel Trim/Correction #3. The front on these ONLY does closed loop fueling if the AFR is within 14.4:1 and 15.0:1. Both the front and rear are known as 'narrow band' sensors. They only read from 0.000 to 0.999 volts. Compare this to the 2000+, which are true A/F sensors, they can read from 5.0 to 1.0 volts. Their scaling is reverse of the normal O2 sensor. 1999 is not a true narrow band, nor is it a true wide band. Its built similar to a narrow band and has the appearance of a narrow band, but it has MUCH better capabilities over the previous designs. Its called an 'A/F Sensor' by subaru, but outside of the 14.4~15.2 range, its not accurate at all. It is MUCH more accurate within that range than previous years tho. Soooooooo.....I would be cleaning the MAF very well and replacing both O2 sensors as well as checking the air filter, fuel filter and the exhaust for clogging (cats, muffler). Usually on those cars its a dirty MAF or a failing O2 sensor. Rarely do the pre 99 cars throw lean/rich codes when an 02 sensor is dead. It will only throw heater codes when there is a short/open in the heater circuit which is RARE on those. My 1996 Outback uses the EXACT same fueling control as the 98 Forester. I recently got 34mpg on a trip from NC to FL. There's no reason for the Forester to not see very high 20's on the interstate and 22~24 around town.