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A few days ago I started to research auto repair and started with the engine (as you do). I have a question about the engine power and the relation to upstream O2 sensor.

If I understand things correctly, a 14.7:1 fuel ratio is attempted. The MAF combines with IAT and MAP to calculate the air in, and the computer adjusts fuel input accordingly. The upstream O2 reads the air out to the exhaust to see how well the computer did and feeds back, so that adjustments can be made via the fuel trim. If I understand this correctly, it makes sense.

Where it gets unclear to me is: the upstream O2 sensor voltage is a result of measured oxygen. This voltage fluctuates as if it is on a sin wave. This means that the engine must be oscillating the input of air or fuel to produce this result in the O2 sensor?

Is the computer actually continually oscillating the POWER being output by the engine and we just do not notice it because it happens so fast?

I would have expected and increase in fuel injection would correlate directly to an increase in power which would be felt.

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  • Welcome to the site. What is the year/make/model/engine of the vehicle in question?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Feb 17, 2017 at 23:15
  • It was more a research and understanding question, but will be applied in the future to 2008 Mazda CX-9 and 2012 Mazda 3, cheers.
    – Chris
    Commented Feb 17, 2017 at 23:19
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    The unfortunate problem is that the 3 has a zirconia oxygen sensor and operates just like you described. The CX9 has a wide band O2 and has a different mode of operation. While the zirconia is only accurate in a very narrow region. The wide band is accurate over a "wide band" because of this it does not oscillate.
    – vini_i
    Commented Feb 18, 2017 at 2:29
  • Thanks for this comment, you have given me some specific terms to research which I know will end up proving helpful.
    – Chris
    Commented Feb 21, 2017 at 17:00

2 Answers 2

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To use your term, the computer oscillates the fuel trim to keep it right around optimal, based on the O2 reading. While the voltage change may seem drastic, the changes to the fuel trim are more minute.

It's all about the scale. The output of the O2 sensor isn't really the absolute exact number of oxygen molecules in the exhaust, the output is calibrated for how much extra oxygen there is, or how much oxygen is missing. The changes to the fuel trim are too minute to feel as a change to engine power.

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  • I thought perhaps its going to end up a scale thing, but what keeps making me turn away from that is the CAT. Perhaps I need to study CAT more. My understanding is that the mixture going to the CAT has to change to be absorbed in the most optimal way. Which I interpret as different NOx mixtures. I would have assume this means we need to modulate O2 and Gas in a more significant manner to be effective for the CAT to operate correctly, which put me to the thought of engine power modulating. Is my CAT understanding flawed? Thanks
    – Chris
    Commented Feb 21, 2017 at 16:59
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But the answer to "does the computer oscillate engine power" is yes - for example when traction control is fitted - loose traction and the ecu modulates the power and this you do feel.

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