How A/F gauges work

[tiwing]

OK, so maybe this really identifies me as a noob. But I've always heard and read that A/F gauges (unless they are wideband) are basically a light show and don't tell you much. I just found this article explaining what the AF does and why it does what it does. I think I've changed my mind on A/F gauges and now plan on getting one.

From: Stewart Warner Performance website (http://www.sw-performance.com/Tech/0tsQAAFR.html)
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The Air/Fuel Ratio Gauge is essentially a voltmeter that displays the voltage output of an oxygen sensor in order to determine the richness or leanness of the exhaust mixture.
Our Air/Fuel Ratio Gauge features four red LEDs (for lean status), 10 yellow LEDs (stoichiometric), and six green LEDs (rich). The first LED comes on at .05V, the second at .10V and so on, in increments of .05V.
The gauge is wired to an oxygen sensor (a device that measures the presence of oxygen in the exhaust). More oxygen in the exhaust is the result of a rich mixture. The "perfect" ratio (not rich or lean) of air and fuel is called the Stoichiometric (STOICH) ratio. Once the STOICH air/fuel ratio is achieved, the oxygen sensor will be very sensitive (i.e. accurate).
Although it can't measure exact richness or leanness, an Air/Fuel Ratio Gauge provides a good indication of potentially dangerous conditions. For example, a vacuum leak or a drop in fuel pressure could damage your engine (assuming that the air/fuel ratio is too lean). The gauge will reveal this condition before damage occurs.
When used in a fuel injected application, the PCM (Powertrain Control Module) uses the oxygen sensor to keep the engine running at the STOICH ratio. It does this by continually crossing the rich/lean point, leaning the mixture until the sensor shows lean, then richening the mixture until it shows rich (and it contin

[tiwing]

continues to loop rich and lean). This is why you generally see the LEDs move rapidly between lean and rich.
Under heavy acceleration the PCM will keep the mixture rich for optimum power and under de-acceleration the PCM will lean the mixture for optimum economy.
The Air/Fuel Ratio Gauge helps determine whether the PCM is functioning properly, and it can also be used as a tuning aid for carbureted engines. Start with rich-jetting, then gradually lean-out the mixture until the gauge shows lean, then richen the air/fuel ratio slightly until there is a rich reading. This should produce the most power, but as always, the final jetting should be verified by inspecting the spark plugs for proper jetting rings.

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What I learned: If the O2 sensor fails the PCM will not fluctuate between rich and lean properly, and will not richen the mixture on accel and will not lean the mixture on decel. For N/A it doesn't matter as much, but for FI it's a bit more critical. My next 2 gauges: EGT and A/F.

Cheers.

[cruisinblackp5]

af guage is just a light show because the readings are generally based on na cars

[Gen1GT]

Non wideband air/fuel readings are uselss. It does ME no good to know that my air/fuel ratio is 14.5 rich or 14.9 lean(14.7 is stoichiometric). I want to know if I'm 16.5 lean(WAYYYYYY TO LEAN), or 12.5-13 under full throttle acceleration(where I want it to be), or if I'm running 11:1(too rich running NA)

Fortunately Wideband kits are coming down in price. You can get them for like $600 now...

[tiwing]

My interpretation of this article is that they're not entirely useless because it tells you if your O2 sensor is actually working, and if your PCM (ECU) is looping from rich to lean like it's supposed to (which really was the point of me posting this - to explain why it's a light show). I agree that it's not as good as wideband and doesn't give you a numeric reading like you want, but it does provide some information.

I don't understand how being NA or FI will make a difference to the readings..?

[Gen1GT]

Air/fuel ratios are important for every car. But a factory O2 sensor has limited use for tuning. It can really only tell you if you're rich or lean. Lets say your PCM has a base setting to run 14.2:1 at a part throttle, steady state condition(cruising). If the O2 sensor is sending the PCM a lean signal, it will fatten it up til it reaches 14.2:1. It just keeps adding fuel til it's where it needs to be. Then it fluctuates around 14.2:1, adding fuel, or leaning it out as it needs it. It only understands 'more fuel' or 'less fuel'. All at base maps which tend to run as close to stoichiometric as possible(stoichiometric[14.7:1]is where the fuel burns most completely, and gives lowest emissions). Best power is around 12.5:1 and best milage around 16:1. In a turbo application, it could be as rich as 11:1.

So being able to know if you're ONLY running rich, which could be 10:1-14.6:1 is useless. As is knowing that you're ONLY running lean, which could be 14.8:1-20:1 is useless too. Non-wideband O2 is just a lightshow...completely useless....

[tiwing]

That makes sense... but what happens if/when the O2 sensor fails (average of say 80,000km)? Does the car just run super rich all the time? If so, how would you know (aside from gas milage)?

Also, if one were to put in a wideband O2 sensor (like this (http://www.performancetrends.com/wide_band_uego.htm)) that has outputs for normal narrow band for the ECU and wideband for accurate measurement, do gauges like Autometer and such still work, or are there special wideband gauges that are needed?

[Gen1GT]

Usually the wideband kit comes with a guage. I don't know what the autometer one looks like. If your O2 sensor fails, your car runs open loop on default maps. It'll always run rich for safety reasons....I'm not sure if the check engine light will come on or not....

[cruisinblackp5]

CEL should come on as soon as the sensor fails.

[Nutella]

My interpretation of this article is that they're not entirely useless because it tells you if your O2 sensor is actually working, and if your PCM (ECU) is looping from rich to lean like it's supposed to (which really was the point of me posting this - to explain why it's a light show). I agree that it's not as good as wideband and doesn't give you a numeric reading like you want, but it does provide some information.
..?


major bump..

I just installed the 'useless' a/f gauge. And after reading the above posts, I think that the a/f DOES tell you the O2 is actually working. I dont see why its useless? If the O2 fails, what would the a/f gauge show? constant rich and no more pretty LEDs looping??

[evil_y2k_twin]

It would be constant rich. When i had mine installed and not wired yet it would always show RICH, i mean last led rich.