Ive been noticing that my 92 CX hatch with D16z6 swap is getting poor gas mileage lately. I have replaced plugs,wires,cap,rotor,fuel filter and checked ignition timing. The engine was completely stock when I first noticed the poor mileage about 230miles to a tank 10gal. I just put on a RS*R header, Magnaflow Catback and intake and the mileage is still poor about the same as before the mods. So my ? is can a O2 sensor be bad and not throw a CEL? My exhaust smells like it is running rich and the inside of my muffler tip is already sooted up with only about 200 miles since I installed it. I dont really want to go buy a 02 sensor if its not the culprit. Thanks in Advance.

 

It can...
Why not just borrow a sensor from someone and give that a shot?

 

I dont know of anyone that has a O2 sensor here local to borrow from. I have the old CX one but its 1 wire vs. 4 wire so thats not gonna do me any good.

 

don't mean to jack the thread but how bad can an o2sensor really affect the gas mileage? like in mpg, is it really really bad or just slight? the reason i ask is bc mine isnt even hooked up and i only get a engine light for it every so often, but i get about 230 miles a tank like SlowCrx, with my b18c with bolt ons in my civic ex sedan

 

it make a pretty substantial difference in MPG, if the computer is not gettting the correct reading fuel trim will be thrown off resulting in poor mileage, a "sluggish" O2 does the same thing as no O2 sensor but does not throw a code

 

A bad O2 will KILL the MPG. I almost a 10MPG increase when I replaced mine.

 

oh wow i definately should try to make it a priority to hook mine up then... I knew it had to have made a difference but 10mpg is A LOT... especially since i drive about 400miles or so a week.

 

i went from 20 mpg to 27 mpg when i changed out my bad O2 sensors

 

I was getting between 22-25MPG, I now avereage 32.28MPG

 

when u guys replaced ur o2, did u buy from the dealership or did u buy used?

 

sup man... definetely sounds like your o2 sensor. I had the same problem when i changed my down pipe. I cant remember if it threw the light on though. I went to the junk yard for mine and it only cost me about $45.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by egg_baLLa &raquo;</TD></TR><TR><TD CLASS="quote">when u guys replaced ur o2, did u buy from the dealership or did u buy used?</TD></TR></TABLE>
Dont buy it used

Depending on how many 02 sensors your car has, usually one you can get at say autozone for real cheap (generic O2 sensor) and the other one is usually more expensive and is either in stock or special order. the dealer should have it

 

I bought a Bosch OEM replacement unit, but I am apparently the only success story, everyone else seems to have problems w/ theirs.

 

DOT says an 02 sensor can affect your MPG by as much as 40%

 

I have an autometer a/f gauge and and it likes to jump all over the place from lean to rich, but I never see the motor run stoich. Sometimes my O2 sensor light will come on, especially when I use premium gas. But the answer to your question, yes, the O2 sensor can give faulty signals while still not triggering a check engine light(1 blink). I just replaced mine.

 

Now, would a 2ndary o2 sensor cause bad gas mileage as well? (2ndary o2 sensor on OBDII cars, right after the header.)

 

No, an O2 sensor that is close to being shot DOES NOT have to show a CEL. My O2 sensor has been shot for the past 6 months and I still have no CEL. Yet, my gas mileage is in the *******. 250-300 mix of highway/city on a stock D15B7. That SUCKS. Stock D15B7s should be at around 350-400 per tank, like when I first got it.

Yes, a faulty O2 snesor can throw your mileage to the *******. Your ECU will be reading the O2 sensor, if it's shot, it will be reading a shitty signal. Once it's shot, for the most part, mileage goes south since your engine is running rich.