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Old 04-24-2008, 10:31 PM   #1
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unplugged o2 sensor now better FE

For the last 3 weeks my MPG has been dropping. I was getting 42-43 and consistently, but it dropped down to 34-35. Well on Monday i unplugged my o2 sensor in front of the cat and my MPg jumped up to 40, I have a scangauge. The scangauge has xgauge and i plugged in the code for the o2 sensor, it was reading anything from 0-97 when it was plugged in and 127 unplgged. It didn't make any difference how i drive on the reading of the o2 gauge, it just kind jumped around. I never got a check engine light. Would the o2 sensor make the MPG drop or some other sensor cause the car isn't going into a closed loop. Also would it hurt anything on the car have the o2 unplugged? I have a 03 civic 5 speed 97k.

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Old 04-25-2008, 03:04 PM   #2
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anyone have any ideas

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Old 04-25-2008, 04:11 PM   #3
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Not good. Not good at all.

( Besides ... you are never going to be able to pass the emissions test here in Texas. )

Sometimes little things can drop your MPG rating. If you do a lot of highway driving, you'll want to take the 'bra' off the front end of the car for sure ( aero drag ) As much as you probably hate to remove it, the air deflector on your sunroof is probably costing you at least 1-2 MPG at highway speeds.

I would put the sensor back on.

Good luck !
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Old 04-25-2008, 05:06 PM   #4
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Some would say you'll fry your cat without the O2 connected.... but if it was that that was going wrong, then it was doing a darn good job of that already and unplugging it is more likely to save it short term.

Hard to say though, whether just forcing it out of closed loop is helping because the O2 is buggered, or some other sensor is buggered.

I'd tend to disagree about the aero, if bras block off some openings like the grille seems a little restricted, and some of them have mesh inserts, then they might gain you mpg, would have to be tested on a per car and per bra basis though. If the thought is about the surface roughness of the thing, forget it until you're going 250mph or have a 0.19 Cd 100mpg car, you're not gonna see a tenth of a percent difference.

Likewise the air deflector should not be dismissed as bad instantly, if it was on a wagon, it would more likely help aero, however, with a fastback style vehicle it may be a toss between no difference and just measurably bad.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
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Old 04-25-2008, 07:06 PM   #5
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My guess is - the O2 sensor went bad but not bad enough for the computer to recognize it yet. Consider getting a new one.

With O2 unplugged the computer is forced to go into open loop mode. That's worse than normal properly working closed loop mode but I can see how it could be better than closed loop using a semi-failed sensor. Open loop can still read the airflow meter's output and throttle position sensor and so it meters out fuel to match airflow as best it can.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 04-25-2008, 09:14 PM   #6
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yup it goes into some factory deault settings, basic crude program to get the engine to run (like BIOS for a computer)

im guessing the O2 is going bad, but not enough/long enough for the computer to reconize it.

those bra things do more harm than good, sure they prevent rock chips BUT they hold/trap water which rusts out the hood. even quicker if you drive with it on in salt...

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