IronMan0002005

So my car (03 v6 manual) has been running at less than optimal lately (for anyone who doesn't remember or didn't see my previous posts: slightly rough idle, almost stalls occasionally, slight (but noticeable) loss of power, and a pretty considerable loss in gas mileage). The car has a bad O2 sensor, and offroad pipes to go with it. It has been suggested that the bad sensor is the cause of a lot of these problems, but I have gotten conflicting reports. I took it to a performance shop around here that my friend goes to asking the guy what I could do to get more out of my car, and brought up my concerns with the O2 sensors while I was there. He said something to the effect that they didn't really affect the engine that much in my car, and they were there basically just for environmental purposes. He also said something like "You could unplug the O2 sensors, bundle the wires, and plug the holes and you would only lose 1-2 mpg."

Now this seems strange to me because my dad's maxima had a dirty O2 sensor and was throwing a code, but because of the "rich" reading from the O2 sensor, it actually leaned the fuel mixture out which cut his power but he got better mileage out of it.

Now my plan here is to get the car tuned to "shut off" the O2 sensors so it wont consider readings from them anymore, and I found a place locally that claimed to do it for $299. Do you guys think that could be the source of my problems, and would it be worth it to get the tuning done? Other options as far as the tuning goes, like the possibility of an online company? Thoughts are greatly appreciated.

ladywelder

I just read some where on this site...replacing the back 2 o2 sensors wont make any difference in the car's performance

warthogdriver

is there a check light on? most times a bad O2 sets the light off. have you check the vacuum lines for leaks? (that usually trips the light as well) but a vac leak will effect your idle as well. easy to check for vac leaks. also try cleaning out the IAC if you havent.

petrock

That perf shop guy has no idea what he’s talking about, and I would go to a different shop from now on. He’s probably used to carbonated motors or pure track/drag cars, not street cars where milage is more important then performance. A bad o2 sensor can cause severe loses in gas milage and dramatically effect the motors performance. The O2 sensors are used by the computer to determine the amount of fuel it puts into the motor in order to keep the air/fuel mix balanced (there is a term for it, but I can’t remember it right now) so the motor isn’t running lean or rich. Too lean causes heat which can damage a motor. Too rich wastes fuel, foul your plugs, bog down a motor and clog up your cats.


Yeah, that can happen with a bad O2 sensor. The reverse can also be true, where it tells the computer that the motor is running lean, when its not, and the computer will dump a bunch of fuel into the motor to try to correct the lean condition but it won’t ever be able too. This symptom can also be caused by a vacuum leak or an exhaust leak, btw. A lean or rich condition is rarely caused by the O2 sensor.


I wouldn’t do it. It would be much cheaper to replace the sensor. It can be done in your driveway with basic hand tools (although there isn’t a lot of space under there). If your state/county has emissions testing you’ll fail if you shut off/remove the o2 sensors.


I wish people would stop keeping that myth alive. The rear O2 sensors most definitely effect your cars performance. Granted, not as much as the front O2 sensors, but they do have an effect in gas milage. The computer uses both the front & rear sensors for fuel trim calculations. It also uses the rear O2 sensors to determine if your catalytic converters are working properly.

IronMan0002005

I talked to another shop and they said on a street car the o2 sensor it's only allowed to create a 5% variation, to prevent a bad sensor giving a super rich signal and then burning holes in the pistons or adversely messing up spark plugs because of a super lean signal, all because of a non-existent problem caused by a bad sensor.

also, I have off-road pipes so changing the sensor will only continue to result in a rich reading.

petrock

Yes, it is true that there are limits to how much the computer can add/subtract fuel from the motor. Its not necessarily to protect the motor against a bad sensor, but it does have that result. Different cars allow for different amounts of adjustment. So saying its 5% is kind of arbitrary. 5% only has meaning when you know what 100% is.


By off-road, you mean you don’t have catalytic converters. Correct? If so, that could be contributing to your problem. The motor needs a little bit of back pressure to run properly and it gets that from the headers & cats. Also, if the lack of cats is causing a rich condition that will definitely effect the cars performance and gas milage. I’d go back to a street setup with cats.

I remember when I was reinstalling the motor in my wifey’s ’98 3.8L a few months ago, I wanted to fire it up at least once before bolting the exhaust to the headers to see what it sounded like. OMG it was loud and the car also ran like crap. I had to keep the RPM’s above 1.5k to keep it from stalling and even then the RPM’s weren’t stable. Once I put the exhaust on, it ran like a champ. Smooth as silk. Just sayin’...

Good luck...