O2 Sensor on a Carb Engine Question

[71GSSDemon]

Has anyone bared witness to an O2 sensor giving a lean number when it is actually pig rich. I believe this is a possible case as it is only looking for unburnt O2, yes? In my research I thought I had come across this, but can't find it. I am not running an O2 nor am I having running issues, just have this question to help those that believe the O2 sensor is the God gift to tuning. I understand in a fuel injected engine, with more sensors, variables, and tuning maps it shouldn't be the case. But this question is for CARB induction use.

 

[coloradohill]

I use one on a carb engine. You calibrate them before use and at least once a year. I use it as a guide because I’m running twin carbs at 5k feet altitude.
I watch my plugs but it’s nice to have the afr. gauge when changing jets and other carb tuning.

No issues so far, it’s a good tool.

 

[ProjectBazza]

I use one on a carb engine. You calibrate them before use and at least once a year. I use it as a guide because I’m running twin carbs at 5k feet altitude.
I watch my plugs but it’s nice to have the afr. gauge when changing jets and other carb tuning.

No issues so far, it’s a good tool.

I have one on my '68 F250 (390 FE motor with an 1850 Holley), but I too only use it as a "tuning tool", as I don't have a clue what's in that engine or what it needs.

 

[remytherat]

If there is an upstream exhaust leak it will read more O2 than it should and therefore produce a leaner number.

 

[MomsDuster]

The only time I’ve gotten a “false” reading is like stated above, when I had an exhaust leak upstream front the sensor.

 

[mderoy340]

I've had mine read pig rich when the engine was starving for fuel. Heat soaked motor and fuel boiling off in the fuel pump. Car would barely run at idle with fuel bowls empty. Acted like the motor was drowning in fuel until I saw the fuel level.

 

[ir3333]

Installed properly they are fool proof. They cannot give an indication of something that isn't there. Even on a carbed engine they read 5 gasses. Saved my bacon twice...once when flooding and once when lean...knew right away where to look. Carb problems both times. Flooding was dirty carb inlet screen and the other was a stuck float.

 

[dano]

I've seen the opposite when the sensor is fouled and needs cleaned. As the other said either leak or bad sensor or gauge.

 

[MopaR&D]

If there is a cylinder misfiring or incomplete fuel burn it will show as lean on the gauge since not all the O2 is being consumed by the engine and is just passing through. So yes if the engine is running so rich that it can't properly combust the air/fuel mix the sensor will show a false lean reading.

 

[sarguy01]

I use an older innovate wideband I had on an SRT4. It needs to be calibrated every once in a while but I think it's pretty accurate. I find it helpful on my 87 Ramcharger.

 

[MV8]

Great for tuning carbs or efi, but they can show what isn't there; the further outside the design pressure of 11-18 psi the greater the error. Also, the further it is from 14.7:1 (when outside the design pressure), the greater the error. No annual cal with heat or no temp compensation circuit increases error too.

Even when reasonably accurate, what if anything should be done about it varies including if you have a lot of overlap (effectively an exhaust leak).

 

[Hysteric]

Has anyone bared witness to an O2 sensor giving a lean number when it is actually pig rich. I believe this is a possible case as it is only looking for unburnt O2, yes? In my research I thought I had come across this, but can't find it. I am not running an O2 nor am I having running issues, just have this question to help those that believe the O2 sensor is the God gift to tuning. I understand in a fuel injected engine, with more sensors, variables, and tuning maps it shouldn't be the case. But this question is for CARB induction use.

That's why you read plugs. Its the only real witness to the combustion event.

 

[Newbomb Turk]

YES IT WILL LIE TO YOU.

Learn to read plugs and then use the O2 sensor.

 

[71GSSDemon]

Thanks everyone. It seems I was correct in my memory.

 

[TrailBeast]

If there is a cylinder misfiring or incomplete fuel burn it will show as lean on the gauge since not all the O2 is being consumed by the engine and is just passing through. So yes if the engine is running so rich that it can't properly combust the air/fuel mix the sensor will show a false lean reading.

I was wondering if anyone was going to mention that.

 

[71GSSDemon]

If there is a cylinder misfiring or incomplete fuel burn it will show as lean on the gauge since not all the O2 is being consumed by the engine and is just passing through. So yes if the engine is running so rich that it can't properly combust the air/fuel mix the sensor will show a false lean reading.

I was wondering if anyone was going to mention that.

This was the statement I remembered and needed reconfirmation of. I am hoping for dyno data to prove the case. It would maybe take an O2 in a header head pipe for a cylinder and force the cylinder to run very fat and see. Like increase the duty cycle of just 1 injector. Read how that cylinder reacts and total exhaust also. Or a cylinder drop test to turn Ignition off on a single cylinder. I wonder of I could get this data at work....

 

[Johnny Mac]

I use an AEM on my 408. If the O2 fails, it seems to default to stoich and freeze there. As others have pointed out, it reads unburnt O2, so any misfire, or a fouled plug, can cause false lean readings. This is why the bolt on TBI systems get a false "Bad" diagnosis. They run in a default open loop when cold (rich) and if you start it and move it 10 times, and shut it off cold (think of moving it around a garage in the winter) it soots the plugs, runs worst, soots the plugs more, and when it finally kicks to close loop when warm, it reads the unburnt fuel from the fouled plugs, and pours even more coal to it.

 

[TrailBeast]

This was the statement I remembered and needed reconfirmation of. I am hoping for dyno data to prove the case. It would maybe take an O2 in a header head pipe for a cylinder and force the cylinder to run very fat and see. Like increase the duty cycle of just 1 injector. Read how that cylinder reacts and total exhaust also. Or a cylinder drop test to turn Ignition off on a single cylinder. I wonder of I could get this data at work....

I use an AEM on my 408. If the O2 fails, it seems to default to stoich and freeze there. As others have pointed out, it reads unburnt O2, so any misfire, or a fouled plug, can cause false lean readings. This is why the bolt on TBI systems get a false "Bad" diagnosis. They run in a default open loop when cold (rich) and if you start it and move it 10 times, and shut it off cold (think of moving it around a garage in the winter) it soots the plugs, runs worst, soots the plugs more, and when it finally kicks to close loop when warm, it reads the unburnt fuel from the fouled plugs, and pours even more coal to it.

My experience with it was when Holley sent me a bad ignition module.
I had random constant bad misses and my afr was showing lean as hell.

It wasn’t lean, but just reading unburned 02.

 

[Johnny Mac]

My experience with it was when Holley sent me a bad ignition module.
I had random constant bad misses and my afr was showing lean as hell.

It wasn’t lean, but just reading unburned 02.

You wouldn't believe how many calls my tech dept gets that start with "yeah, it ran amazing before it went to the body shop" unbenounced to them....the shop started it and moved it 10 feet at a time, about 30 times, over the course of having it lol

 

[Newbomb Turk]

You wouldn't believe how many calls my tech dept gets that start with "yeah, it ran amazing before it went to the body shop" unbenounced to them....the shop started it and moved it 10 feet at a time, about 30 times, over the course of having it lol

You should write a book with all the customer complaints and explanations they come up with.

You’d sell millions of copies.

 

[TrailBeast]

You should write a book with all the customer complaints and explanations they come up with.

You’d sell millions of copies.

I was a professional certified mechanic for decades.
Caddy needed head gaskets and the computer was dumping huge amounts of fuel into the engine.
I did the head gaskets and had him sign an agreement to NOT drive the car until the computer was fixed or replaced.
He drove it anyway and blew it up.
Then sued me.
He lost of course.

 

[AJ/FormS]

I am not running an O2 nor am I having running issues, just have this question to help those that believe the O2 sensor is the God gift to tuning.

I agree. I had one.
it lied. then the gauge quit working.
I got a complete replacement set, under warranty.
it too lied. then the gauge quit working.
I threw it on the wall of shame, and have never missed it and my plugs were replaced 22 years later with about 110,000 miles on them, NOT because they needed to be changed, but because I thought the car was sold, and so I gave it a tune-up. Buyer backed out.
My AFR may not be perfect, but by the plugs, I doubt an AFR could do better., Plus, you know, the car has, with one size smaller cam, and a lil more cylinder pressure, gotten UNBELIEVABLE (to some of the smartest guys here on FABO), fuel economy.

IMO;
RPM/Load/AFR/ and Ignition Timing, all go hand in hand. If you cannot change the timing on the fly, Nor change the AFR on the fly, as is the case with a carburetor; what's the point of monitoring the AFR?
I mean, that's like watching your wife gain weight, and thinking you can talk to her about it. If yur stupid enough to try, as I was, stand well back, and be prepared to rough it for a week or a lifetime.
Just let her/it, be, Cuz just like my wife goes around complaining about how fat she is, that AFR will do the same.

 

[Newbomb Turk]

I agree. I had one.
it lied. then the gauge quit working.
I got a complete replacement set, under warranty.
it too lied. then the gauge quit working.
I threw it on the wall of shame, and have never missed it and my plugs were replaced 22 years later with about 110,000 miles on them, NOT because they needed to be changed, but because I thought the car was sold, and so I gave it a tune-up. Buyer backed out.
My AFR may not be perfect, but by the plugs, I doubt an AFR could do better., Plus, you know, the car has, with one size smaller cam, and a lil more cylinder pressure, gotten UNBELIEVABLE (to some of the smartest guys here on FABO), fuel economy.

IMO;
RPM/Load/AFR/ and Ignition Timing, all go hand in hand. If you cannot change the timing on the fly, Nor change the AFR on the fly, as is the case with a carburetor; what's the point of monitoring the AFR?
I mean, that's like watching your wife gain weight, and thinking you can talk to her about it. If yur stupid enough to try, as I was, stand well back, and be prepared to rough it for a week or a lifetime.
Just let her/it, be, Cuz just like my wife goes around complaining about how fat she is, that AFR will do the same.

If you have to ask why monitoring more data is bad, you are missing huge pieces of information.

 

[AJ/FormS]

Yeah yeah, I've heard that before;
I think you completely missed the point,
We had nearly a hundred years of carbs and never an AFR to tune them, the guy doing the tuning was the AFR man.
Besides
The statement you are concerned about was the lead-in to the fat-lady joke.
If I have to explain everything, there are not enough hours in the day.

 

[Newbomb Turk]

Yeah yeah, I've heard that before;
I think you completely missed the point,
We had nearly a hundred years of carbs and never an AFR to tune them, the guy doing the tuning was the AFR man.
Besides
The statement you are concerned about was the lead-in to the fat-lady joke.
If I have to explain everything, there are not enough hours in the day.

Whatever makes you feel better but you’ll never convince me that more data is bad. I mean, that’s an ignorant statement.

You don’t have to live and die by it, but it’s best virtue is it cuts my tuning time by half or more.

So not knowing what the AFR is and not using an O2 meter you lose knowledge and waste time.

I don’t care what you say or don’t say to your wife, but I appreciate humor for sure.