Don't have an O2 Sensor On... Bad Gas Mileage?
 

I got aftermarket 4-2-1 Headers for the lancer and they fit fine with some customization from a muffler shop. 1 leak I had between the header and downpipe but fixed that with some muffler mender. I didnt have a bung for an O2 Sensor so I simply unplugged it. I'm running the stock cat on the system, and only 2.25 inch piping so there should be enough backpressure. I just fixed the exhaust leak today, and I know that would have killed gas mileage, but does the O2 sensor do anything to reflect calculations on the ecu, or is it there simply to tell the ecu to turn on the Check Engine light? I'm getting about 15-20 MPG now.... Help please!

I've got a RRM 4-1 header, RRM testpipe, Magnaflow 2.25in Catback, Injen SRI... and I just got 28mpg city driving.. My check engine light is on because of the improper O2 sensor readings. I don't believe it effects your gas milage if the Check Engine light is on. On the highway I get around 35mpg still too.. I think I might even have a very slight exhaust leak. 20mpg doesn't sound right. Did you reset your ECU after the installs? Unplug the negative terminal of the battery for about 15-20 mins. Then reconnect it, start your car and let it idle for another 10-15 mins. Then take it around the block and accelerate normally.. take it around the block again increasing the RPM range. Then fill up ur tank with gas and see if you get better gas milage.

yes you need the o2 sensor, i forgot who didnt have one on also and they got about the same gas milage as you.

I'm torn on the O2 sensor thing.... on the one hand, I experienced the same crappy mileage you did. On the other, my scan tool says everything's fine, except for the CEL from not having the O2 sensor.... so the ECU shouldn't be in closed loop (which means my eManage wouldn't work when the CEL is on and I've confirmed that it does indeed work). My $0.02 is just get a MIL eliminator- when the CEL comes on for something else, it's nice to know.

Since you're going to ask anyway: bigjoshracing.com

Wow, I'm surprised this is even an issue, EB. I thought you (and others) were adament that simply not using the first O2 sensor was a very bad thing. I'm guessing that this is an Emanage-specific issue? Would the "best" solution still be 1) having a bung welded on, and 2) installing a MIL eliminator?

oh wait, my bad. Even after rererereading it it still sounds like he's not using the 2nd O2 sensor. Um, if you're getting a CEL, that's probably bad. I've never had a problem with the #1 sensor. I guess I'd try just plugging it back in and finding a secure spot to mount it in the engine bay- let us know what happens :D

Oh, and take the battery terminal off to reset the ECU (a few minutes should be fine).

Of course, I'm always a proponent of best methods, so having a proper bunge welded on would be the best route. I wouldn't charge more than $30 to do it, but then I'm the little guy....
and I'm sure if the evom moderators have their way, my post will be deleted for even implying that I might sell goods or services on this site in an effort to make money without first having signed a contract to pay EvoM for the priveldge of advertising the one or two items or services that may be available......
ahem... off soapbox now :)

I have the 2nd one plugged in... but I'm gunna get the first one welded on... Thanks Eboy

I would recommend having the 02 sensors where they can read the exhaust. Make a bunghole (lol) and either thread it or weld it in place. I would be a little cautious of just having it not connected. I'd rather do it the right way. Just my opinion though.

i think the first one would cause the ECU to go to closed loop?...no?...eboy or rhyzin can confirm, bu the second one wont....simple fix for the second o2 is to turn the cat around, so that the o2 is after the cat and not before it.

Yes, if you are still running the stock cat, you can flip it around so the 02 sensor is behind, instead of in front. That might take care of the Check Engine Light as well.