o2 spacer...
 

Iv heard of using a spacer on the back o2 for non cat applications. You know, racing, etc. I was looking for one on EBay, and found 2? They had a long one, for the 2nd(back) o2 and no cat, and another short one to be used on the front o2 that claims to increase mileage a bit. I did a forum search, but couldnt find anything. I used the terms "o2 spacer" . A search with those terms on EBay will turn up both kinds. Any thoughts on this short version as it would apply to mileage?

Search for extender instead of spacer. That's the terminology that's predominantly used here.
https://www.gassavers.org/showthread.php?t=7091
https://www.gassavers.org/showthread.php?t=10121
https://www.gassavers.org/showthread.php?t=8513
https://www.gassavers.org/showthread.php?t=8316
https://www.gassavers.org/showthread.php?t=8346

Thanks.....

I tried using one on the 02 sensor on my '88 Escort. but it didn't help my mileage any, actually I think I lost a little mileage. I've heard some people claim mileage gains using them, but I don't think I did so it may be car specific. What most people use are spark plug anti foulers drilled out so the tip of the 02 sensor can fit into it. You can probably get the anti foulers at about any auto parts store, then just drill out the center hole to about 1/2". If you want a longer one to use on the post converter sensor you can screw two of the anti foulers together to take the sensor out of the exhaust stream. I have 2 or 3 extras if you are interested in them, I'll take $4. each for them. I think that is what I paid so I'm just looking to get my money back. If you are interested just drop me a PM.

The o2 spacer is for cars that have a rear o2 sensor after the cat. It spaces the o2 sensor out away from the exhuast a little. From where you will not get a check engine light when the cat is taken off the car...

Re: o2 spacer...
 

Use a spark plug de-fouler, found 2 per package at the auto parts store. I've seen them most commonly in two sizes, I can't remember off hand but the larger one works for the o2 sensors on my Civic.

What you do is drill them both out to make room for the part of the o2 sensor that sits in the exhaust stream. Thread the sensor into one de-fouler, then thread that de-fouler into the other. Thread into the exhaust pipe.

Guys use it to fool the ecu when they're no longer running a cat, so it doesn't throw any codes. Basically it works as a spacer, pulling the o2 sensor out of the richest part of the stream.

I recently did this trick on my ex-girlfriend's car because she started throwing a code for a bad cat. $2.00 fix that works is much better than the hundreds it would have cost for the new cat.

Re: o2 spacer...
 

Oh, as for fuel economy, I have no idea whether it helps. I've only used this to stop the ecu from throwing codes.

Re: o2 spacer...
 

You pretty much said what i said in the post above....