Is an 02 sensor necessary?
 

what is everyone's thoughts/experiences on running without an 02 sensor? my 911 has custom heat exchangers and bursch muffler. the previous owners had done the exhaust work and left out the 02 sensor and i took out the 02 sensor light bulb in the dash bc it was blinding red at night.

is it necessary to have one to get the right air/fuel mixture? and what set up should i go with if i need to add one? i am getting ready to drop motor/trans for various other projects. i might add i've adjusted the mixture just using my ears to set the rpms around 900-1000


thanks,

james

Yes. You don't say what injection you have CIS or Motronic or ? but it is needed. Search is your friend.

Larry

i have a CIS system and all i know is that the vac retard is capped, with a golf tee...the local shop said this is a common fix. i guess this is so the rpms drop faster once you let off the pedal?? my father swears i need an 02 sensor to run correctly but from what i've searched and read it's a mixed answer. i need to replace my muffler and didn't know if i should order one with a 02 bung or not.

sorry for the lack of knowledge, im a newby and trying to learn as i go along.

on Motronic seems to run fine unplugged, not sure if will pass smog regs if unplugged, probably not, Steve Wong will know, but at WOT the DME may not look at the O2 signal

do you have an 85?

not that familiar with 911 EFI, but the EFI cars i have messed with, i cant see anyone setting the mixture by ear. again not sure of the 911, but i have seen AFM's with the adjustment on them, so i dont k now if you are setting the mixture that way or what. most EFI cars have a ICV that controls the idle, so i dont see how setting it to the idle would work.

and yes, i would use the O2.

if you have a CIS car, then setting the mixture by ear can work, i have done it and checked it with an LM2. there is also an idle adjustment. dont use the little screw on the throttle body, its the big one.
a lot of people dont use the O2 on CIS, me, i woud try to make it work with it.


we need the year and setup.

James has an '83SC. I'd order the exhaust with a bung for an O2 sensor. You can always plug it if you don't use the sensor.

I ran an '83 both ways, back in '83-'86. I'd suggest running the O2 sensor connected. Reset the O2 sensor counter. Set the mixture with a CO meter.

What shop are you using?

JR

its an 83 911sc

I've had a few minor things done at Rennsport here in town. Prob going to take them the trans to replace synchros while my dad and i fix clutch, oil leaks and 02 sensor issues at the house.

i've adjusted the bid screw with just using my fingers while listening and watching the rpms while adjusting screw. seems like a half-ass way of doing it... but it runs smooth, strong, no backfires. I don't have to worry about passing emissions in oklahoma so if all an 02 sensor does is help exhaust emissions im not so sure i'll worry about hooking it back up. but does it have an effect on engine hp, torque, spark plugs etc...

i had mine disconnected last summer by a shop in denver. my o2 sensor system got fried somehow. he set the mixture to ~3.0% CO (if that is the correct term?) either way it runs fine, spark plugs show normal wear when i check them, and im still getting ~25mpg highway. feels plenty powerful too.

The big screw is the idle adjustment screw. Adjusting that while looking at the tach is fine. The mixture is adjusted with a 3mm allen wrench. You can't see the adjusting screw for that one. It's buried inside the airflow sensor housing.

It's been years since I had to think about one of these systems. I think you'll get better driveability with the O2 sensor functioning. I seem to recall there was some acceleration enrichment using the frequency valve and the computer. You'll also get better mileage.

JR

The O2 on my '82 SC has been disconnected for years. Car runs great and managed 180 RWHP on dyno 2 years ago.

James, just to emphasize (JR mentioned it also)

There's a little button behind the speedo that you can push to turn off that OXS light. It's just a reminder, triggered by the odometer.


My buddy with an '81 SC was flabbergasted when I turned off his OXS light with a flashlight and a little screwdriver. Shop was going to charge him $300 to do it.

lol thats exactly how i turned mine off. flashlight and needle nose.

well from the posts i've received so far, it sounds like they aren't necessary for a cis system. i will post pics here soon of my set up to see if anyone has suggestions. and to also show my hideous oil leaks!

there is no reason to run with an unplugged 02. the 02 only works at part throttle, (below the 35 degree throttle swith) on cis and mot. It give you a perfect mix for idling and cruising. when you go past the 35 degree switch the system goes into open loop (not working), so it has NO effect on performance. with it hooked up, your engine will be more responsive at low throttle, great fuel milage, run cleaner and LAST longer. before the 02 system, 80 and earlier, most engines where gone at 100k (no compressiion from ring wash) now, with the perfect mix you can get from an 02, 300k plus is not uncommon at all anymore.

i'm spending an inordinate amount of time trying to get the lambda sensor system from my 83 motor into my 78 chassis. what a hassle. if I thought the motor would run just as well without it, I would eliminate it.

Don,

You are correct. However, my concern is being too lean at WOT. Currently I run without the sensor with the mixture set at 2.5% CO or 13.56 AFR (Perfect Power - CO to Air Fuel ratio conversion).

If my baseline AFR is set to 14.7 in order to work with the O2 sensor, won't I see overly lean conditions at WOT? My WUR vacuum is atmospheric on the 82SC. So it seems like a richer baseline would allow for richer mixtures at WOT.

Unfortunately you can't set your baseline at 13.56 AFR when using the O2 sensor because it will create hesitation / hitching at constant acceleration. Very annoying to me.

Thoughts? Suggestions?

I do have a new sensor on the shelf and a new connection bung for the engine bay (old one cracked and disintegrated). I've been thinking about hooking it up again for the reasons you stated in your post. But I want to make sure that it is the right decision. Hell, I don't even think my new JE pistons will last 100k miles.

Thanks,
Craig

craig, your freq. valve does more than correct your mix below the 35 degree switch, when you go past the switch, the 02 goes into open loop, and your F/V goes to a 90 percent duty cycle to richen the mix by lowering press. in your lower chambers of you fuel dist. You should never dissable your 02 system by pulling the relay, as it make the F/V inop, and then you will be in a lean situation at WOT. On setting basline afr, it gets tricky with a wur that is not calibrated perfectly, as i have discovered. spec is 50-55 lbs of warm press. on your car, when playing with my manual adjutable wur, i noticed that 50 gives me 13.9 to1 (with the 02 unpluged) and 55 gives me 15.5. the 02 job is to correct to 14.7 to one at all times regardless if you are 50 or 55. with it unplugged, this is not going to happen.

My relay is always plugged in and operable. I'm talking about the sensor.

So are you saying that WOT mixture will be the same regardless if whether or not I'm using my o2 sensor?

if I thought the motor would run just as well without it, I would eliminate it.


I did the swap on my 75 and thought the same thing, but once i installed it driveabilty was fantastic, engine ran so much smoother ,milage was awesome ( 28+ on the hwy) it is well worth the effort. its a simple harness and not that bad to install http://forums.pelicanparts.com/uploa...1293198126.jpg

So are you saying that WOT mixture will be the same regardless if whether or not I'm using my o2 sensor?


yes, as the ecu goes into open loop as soon as you break the contact in the 35 degree swith, the 02 is inop. and has nothing to do with wot mix. You are at the mercy of whatever your warm control press. is set at.

If my baseline AFR is set to 14.7 in order to work with the O2 sensor, won't I see overly lean conditions at WOT


No, that is when an operable freq. valve comes into play, to richen the mix past the 35 degree swith. it gets a 90 percent duty cycle when this happens, thus bleeding of press. in the lower chambers, letting the more fuel flow to your injectors.