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A car at idle runs completely different from one at normal engine speed. At idle, a car must dump more gas in the engine while making the car work so inefficiently as to run at only 800 RPMs. Most pollution occurs during idle. Also why cars sitting in bumper to bumper traffic consume so much gas (another reason why hybrids are so necessary).
IOW a better test of your O2 Sensor and other factors is to be moving at constant speeds as indicated by little or no throttle movement.
Now, some facts. In the first minute, an O2 sensor must get warm before it reports anything. So newer technology sensors have an internal heater to make them hot (work) sooner. In your case, sensor one got to temperature quickly. Sensor 2 seemed to be doing nothing until engine exhaust finally heated it. Sensor 2 may have a defective heater or for some reason is taking longer to get the temperature. (BTW, I am assuming this is a 6 or eight cylinder since fours only need one O2 sensor. If this is a four, then the second sensor may be after the cat converter making this analysis bogus.)
Sensor 2 appears to be reporting inaccurate voltages. An O2 sensor is typically between .6 and .7 volts when the engine is running optimally. When dumping gas excessively, then the voltage is typically about 1 volt. Your sensor two appears to be reporting numbers that are low. Or one side of the engine is using more gas than the other.
However, how electronics measure those voltages might explain different numbers. Your particular car may measure or report those voltages differently. Normal voltages for an O2 sensor must be confirmed from the shop manual. But something is different on both sides of the engine.
Throttle clearly remained closed the entire time. But after 8 minutes, your idle was not yet at 800 RPM? The car should have been hot and at idle after 8 minutes. You might repeat the test while monitoring engine temperature. Your thermostat might not be fully closed (partially sticking) when the engine is cold. Therefore the engine is taking too long to warm up; wasting gas in the first ten minutes.
What you are looking at? Fuel injection is a switch (much like Cloud's idle adjustment valve) that is turned on and off quickly by a microprocessor. Time of each opening (called fuel trim) determines how much fuel is dumped into each cylinder. An O2 sensor measures how much got burned. Microprocessor reads the O2 sensor to determine whether to lengthen or shorten that opening time (adjusts short term fuel trim).
Graph is each O2 sensor number (maybe) sampled only at 5 second intervals. The actual numbers vary significantly in milliseconds. Actual O2 voltage is varying many times more often and more gently. Your chart simply shows some of the highest and lowest voltages. Therefore the steep changes.
Those variations may be normal for idle. But if you see variations that large for normal RPMs, then that may also be symptoms of a problem. An engine at normal speeds should see O2 sensor voltages varying little between 0.6 and 0.7.
Notice that at seven minutes, the variations decrease to what I am more used to seeing.
Voltage should increase when, for example, when a throttle is pressed. An open throttle means intake manifold has no vacuum. So the computer dumps more fuel into cylinders (short term fuel trim increases). More fuel comes out unburned. The O2 Sensor (that I am used to seeing) reports 'more unburned fuel' at about 1.0 volts.
Again, large variation may be normal for your engine. If it was any of my Hondas, I would be concerned. Approaching eight minutes, the variations decreased. Whereas your seven minute voltages to me are too low, still, the O2 sensor varies more like I am used to seeing it.
Meanwhile, the 'Shell' test implies gasoline was irrelevant to your original problem.
You can view a relationship between short term fuel trim, throttle angle, O2 Sensor voltages, and intake manifold vacuum. To see what the microprocessor sees and responds accordingly.
Another parameter is long term fuel trim. I have never confirmed this by doing it. But a long term number would increase if the gasoline is inferior. If gas (ie WaWa) does not burn as efficiently, then engine software may respond by increasing (over a long time) the amount of fuel it dumps into the engine. Long term trim is how a microprocessor learns and adjusts from engine overall performance. Short term fuel trim is how it responds due to immediate road, throttle, load, and other current variations.
O2 Sensor is not the only input. But it varies according to how much gas remains unburned. Other inputs include a sensor that measures atmospheric pressure (are you one mile up in CO or at the shore?), and one that measures manifold vacuum. When does it know to dump more fuel into the engine? When vacuum decreases (due to an open throttle or a sudden air leak). Ignition timing also changes (advances) when more fuel is dumped. You can also view that relationship.
I don't remember what your car's technology is. However older cars adjusted engine timing mechanically (centrifugal force and manifold vacuum). So a sticky or binding ignition timing could be seen on that chart. I have seen where distributor grease got sticky after ten years causing engine timing to intermittently stop changing. That was also indicated by a slight knock. However some auto companies (ie GM) decided knocking is normal rather than fix an inferior engine designs. Knocking alone might not report a problem. But sticky mechanical ignition timing might be seen on the chart. Unlikely. But an example of what the microprocessor can see. But the failure is not serious enough to report. An example of why many detected problems can remain unreported by engine check lights.
Hopefully this successfully introduces simple operations inside a computerized engine. Computerized engines are simpler than carbureted. But involve fancy nouns (ie short term fuel trim) that at first sound confusing.
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