# Both O2 sensors read lean, bad MPG's



## evoL (Jul 9, 2012)

I've had lean codes since I bought the car 8 months ago. But the engine is obviously running rich. MPG's are around 6.8 all city. I have new O2 sensors and they barely create any voltage. I had my car tuned today and had the sensors shut off. I'm actually getting worse gas mileage than before. On the hour trip there I got 14.7mpg, on the way back it was 13.1. I don't know what else I can do about this, and I don't know why both of my sensors would fail at the same time. The stealership and Dyno guys had no clue either. Anyone else have the same problem or have any ideas? 
Mods: SLP Lt headers, SLP exhaust with high flow cats, Vararam CAI.


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## svede1212 (Nov 1, 2005)

It could be the CAI. Either from an air leak or some VRs have had problems messing with the MAF. Turning off your front O2s is not a good idea. I would be willing to wager your fueling is off as pure open loop tuning is very time consuming to do and varies too much with weather conditions. I'd find another tuner


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## evoL (Jul 9, 2012)

I have had this problem months before I installed the CAI, so it isn't that. And my rear sensors are now off, not my front ones. The guy that tuned it came highly recommended by a lot of people in this state. The only other thing I can think of is the O2 extension cables were wired wrong but that wouldn't have any effect on my MPG's right now.

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## evoL (Jul 9, 2012)

Also, after he turned off the rear sensors I was getting a good air to fuel ratio but I am still somehow getting lower gas mileage.


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## svede1212 (Nov 1, 2005)

I could tell you stories about "highly recommended". I probably don't know your guy seeing we don't even know whatever state your in but the problem is people get their cars tuned by someone because they don't know how to do it themselves. That makes it hard for them to assess that it was a good tune. Word of mouth goes around by other people that don't know anything and bingo he's a great tuner. Even great tuners sometimes don't put the effort into it if they don't feel like it. Knowing the state of things I'm extremely glad I took the time to figure out how to do it myself. I've looked at tunes done by "pros" and a lot of them are hack jobs. 

Unless you have done extensive logging while driving around to monitor air/fuel ratios you can't say they are good. Tuners almost universally do a little bit of MAF tuning and then try to nail AFR at wide open throttle to get peak HP. That does nothing for low speed range RPM (driving) and for throttle transitions. GM uses a hybrid system of MAF readings for higher air flows and speed density for throttle transitions and low air flow conditions. 

O2s wired wrong would throw a code. If nothing else go back to your tuner and tell him things are unacceptable. It is not tuned right or there is an identifiable problem that the tuner should be able to spot. If he won't help you you're only other option is to find another tuner.


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## evoL (Jul 9, 2012)

His name is Mike Norris out of Indianapolis. He only works with LSx engines from what i understand. Dealership recommended him. I guess that doesn't really make a difference. It did seem like a rush job though. I'm kind of wishing I would have spent my money on a Diablosport instead. What do you use for tuning? Also, I don't think it would make a difference but I noticed when I was getting codes that my driver side O2 sensor was registering as "sensor 2." Must be wired backwards. I'm gonna clean my plugs and MAF sensor and see if I get any better MPG's.

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## svede1212 (Nov 1, 2005)

You can't swap front and back sensors as they have different plugs and I don't think the wires would reach. You're going to have to get that problem sorted out first. 

I got HP Tuners Pro and an Innovate LC-1 wideband oxygen controller. With those two things and a week or two of reading you can tune as well as the best of them for most things especially fueling. You can of course also read any codes and log for 1 1/2 hrs by pushing a button and tune hundreds of things. Swapping to a FAST intake and BBK fuel rail last year I shortly after was just driving down the road and the car suddenly starting running like crap. I logged and read a #8 fuel injector electrical issue. I popped the hood, went to #8, tried pushing on the plug for the heck of it and heard a "click". That was it and the codes pointed me right to it. Pretty amazing how easy it is to diagnose stuff today.

I run a closed loop speed density HPT custom tune. It uses no MAF and just short term regular fueling is automatically adjusted. The rest of fueling is mainly based off manifold pressure and RPM. Because of that and my general interest I run an AFR gauge full time in a dash pod. There I can easily see (and log) fuel ratios.


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## evoL (Jul 9, 2012)

I didn't swap any sensors, just had the rear ones turned off because for whatever reason they were reading low to no voltage. And now my front two sensors are reading low voltage. Set of the codes p0131 and p0151 at the same time. I don't get it. Maybe a vacuum leak? I'll probably take it in for a smoke test today.


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## GTPpower (Aug 1, 2012)

It's ok to remove the codes from the rear O2's, as that's what most do when they delete their cats. I was under the impression that you shut the front two off.

As you said, check for vacuum leaks, and I would also check your wiring for the front O2's.


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## svede1212 (Nov 1, 2005)

Front O2s are turned off to tune, back on when done. The rears are usually just turned off.


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## The Silver Goat (Mar 18, 2011)

You have avacuum leak somewhere in the system post the MAF. You can take it for a smoke test or attempt to find it yourself. I did a PCV bypass with a breather filter and the rubber caps degraded from heat exposure letting a enough air that I kept changing the duty cycle on my injectors until I did some research. Determined it was the rubber capps and bingo, the settings are back within acceptable ranges. 
arty:


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