Sudden 20% Drop In Fuel Economy

[jimbob12321]

I have a 2004 WRX (completely stock) with automatic transmission with around 130k miles on it. A few weeks ago, my MPG dropped by about 20% from around 260 miles per tank to around 200 miles per tank. Driving patterns haven't changed, and I checked all the brakes and caliper pistons and all seem to be retracting just fine. There's no difference in how the car feels when it drives and no CEL has come on.

My next suspicion was that the O2 sensors might be shot, causing a rich fuel mixture to burn gas faster, so I got out my trusty OBD2 scanner software and plotted each of the o2 sensors which generally operated within the following voltage ranges:

O2 B1-S1 (WR V): 3.7 Volts +/- 0.2 Volts
O2 B1-S2: Oscillates between 0.1 - 0.8 Volts

My questions are as follows:

Are these voltages (especially the B1-S1 sensor which didn't seem to oscillate like the B1-S2 sensor) within acceptable limits?

If so, does anyone have any other ideas what could cause such a sudden and continuous drop in fuel efficiency?

Thanks!

[lytlec]

Possibly gas quality....

[jimbob12321]

Possibly gas quality....

I guess I could have gotten a bad tank of gas, but I always use 93 octane. The problem has manifested itself over the course of 3 tanks of gas now, so I'd tend to think the odds of 3 bad tanks in a row is pretty low.

Any idea if those voltages are normal?

[zax]

Have you checked your tire pressure?

[lytlec]

I guess I could have gotten a bad tank of gas, but I always use 93 octane. The problem has manifested itself over the course of 3 tanks of gas now, so I'd tend to think the odds of 3 bad tanks in a row is pretty low.

Any idea if those voltages are normal?

I always notice a decrease in mileage during winter months...where are you located?

[jimbob12321]

Thanks for the suggestions. I live near Philadelphia, and do usually see a slight decrease in the winter months, but never anywhere near the amount I'm seeing now. Checked my tire pressure and it is right on...

[06scoobyrex]

When was the last time you replaced you spark plugs?

[jimbob12321]

The spark plugs are original, so they are probably near they end of their life, but wouldn't I be getting cylinder misfires and a CEL error code? Could old spark plugs really drop the gas mileage so dramatically and so quickly?

[06scoobyrex]

Yeah spark plugs should be replaced every 60K or so and even more so if you have moded the car they ware out faster... if they have 140K on them then they should've been replaced about 3 times by now...

[teflon_jones]

Yes, those sparks plugs are WAY past replacement time. I'd start there and see if that helps.

For the record, fuel economy in winter should only drop about 1-2%. I work for the leading oil consulting company in the world so I know a little about this subject. That's due to the extra butane they put in the fuel to keep the evaporative qualities within limits due to the colder air.

[MainFrame]

200 miles/tank is pretty bad (unless you're running e85). I usually get about 320-330 miles/tank. I'd replace those plugs ASAP.

[lytlec]

200 miles/tank is pretty bad (unless you're running e85). I usually get about 320-330 miles/tank. I'd replace those plugs ASAP.

Dang...I'm lucky to get 230!

[Graham46]

Someone pointed out to me earlier in the winter that mileage drops for two main reasons: 1. there are different additives in the fuel in the winter to prevent it from freezing, resulting in an overall poorer quality of gas, and 2. if you warm your car up in the morning before work, that is gas being burned at 0 mpg. Even if its just 5 min every morning, it adds up. I am getting around 300 or so miles per tank instead of around 350 (2006 wrx MT)

[Lengthwise32]

Dirty mass-air-flow sensor?

[MainFrame]

^ would be a good idea to clean the MAF when you change the plugs.