The o2 sensor affects mixture when the ECU is in open-loop mode. Which is like at 73% or higher TPS and above a certain RPM. Otherwise it's in closed-loop mode and runs for the most part the map stored in memory.

I believe the ambient temp and intake temp sensors can have an effect also but I'm not sure exactly when.

 

what is TPS?

 

Throttle Position Sensor (along side of the throttle body) detects how open the throttle is. i.e. how much lead in your foot :-p

 

so mrfatbooty is just talking about how open the throttle is when the TPS is 73% or greater... right?

also MrFatBooty: you're basically saying then that the car's A/F is USUALLY controlled by the o2 sensor... and only 25% of the time it isn't....

 

No, other way around.

The default mode for the ECU is closed loop where it runs off the stored map. If you go past 73% throttle opening it kicks into open loop and starts reading off the o2.

 

what do you think is the major culprit then?... no one can tell me HOW to clean a o2 sensor...

i'm going to stick in some BG 44k formula once i get my hands on some... lots better for your money!

no sensors are throwing codes

 

See if the fuel system cleaner helps, that's probably what's causing most of the problem.

Sometimes the o2 sensor can just get worn out (like, the voltage it reads is no longer accurate) but not to the point of throwing a code. That can make you run rich but only during the time the ECU is in open loop mode. You can't really clean it, just replace it.

But yeah, my guess is it's something inside the fuel system. Possibilities include a worn fuel pressure regulator that's running a bit higher than normal. Also, any number of spots could be gunked up causing fuel starvation and knock, which causes the computer then dumps more fuel to get rid of.

 

hopefully 44k will fix it

i hear chevron's techron stuff is only 4% of this bg stuff... BG sells it to them... a bottle of 44k is 100% of it... more for ur money

so your bet is on fuel system and a worn o2 sensor? should I bother replacing the o2 sensor?

 

Go one step at a time, dump in the bottle of cleaner and see if your mileage improves.

 

I can't believe no one has mentioned the ignition timing! If that's off, you could be running way rich, and that is a sure fire way to get crappy mileage, burn up your O2 sensor, and your cat.