im still having fuel milage problems, and am wondering if it is neccisary to replace the o2 sensor. will it have that big of an effect??

 

"Way too low. Do a full tuneup as per the FAQ and see how things go after that. The O2 sensor really doesn't effect mileage all that much."

Aaron Cake

 

Dont you run pig rich when it is not functioning right? Just currious.
Chris

 

If your O2 sensor is bad, you will get a "check engine" light.

I'd start by checking the TPS.
If it is set high, the ECU will not go closed loop to use the O2 sensor at all, or will not be giving a deceleration fuel cut.
Also check that the pressure sensor (on the right strut tower) is plugged in.

Also the water temp & intake air temp sensor plugs may be bad.
If so the ECU sees "cold motor" and adds extra fuel.

 

no....slighty richer......certainly not pig rich (pig rich of course being a technical term for gas running out the tail pipe - lol)

 

ok cool thanks

 

Not on the S4s, it won't. Only on the S5s.

Good points. This would all fall in the range of "Full tuneup", in my book.

A bad/disconnected O2 sensor won't have a huge effect in town, and on the highway will probably be worth 4-7mpg. However, with a SAFC-II or such, you can actually get similar or slightly better economy with the O2 sensor disconnected - slightly lean of stoich is a good place to cruise if you don't have cats to melt.

Also consider getting your fuel injectors professionally (out of car, ultrasonic reverse flow flushed, blueprinted) cleaned.

-=Russ=-

 

In my personal experience, the difference in measured fuel consumption between a brand new O2 sensor and no sensor at all was ~5%. That's with a mix of city and highway driving. The more time you spend on the highway, the more difference it'll make.

 

I was just checking the FSM - 86-88 - fuel & emissions - NA - pages 21-22.

Code 5 = bad O2 sensor

 

Yes, but you read that code from a plug in the engine bay, not from a light inside the car.

 

Bingo. The S4 does not have a check engine light.

-=Russ=-

 

you'll notice backfiring shifts and flooding exhaust, and flames