@GeorgeSanJose curiuosly, my % O2 readings were 0.5 and 0.4. Now all other times these were always 0.0.
The only difference is I went to a different smog place than I usually do, as the machine at my usual place was out of order for testing older cars. Perhaps this is the reason for a difference this time around.
All other measurements (HC and CO) were around the averages listed, sometimes higher, sometimes a bit lower depending on the year and if something was fixed; Cat converter the one year, a visual failure of PCV ( whatever that is) and vacuum line to sensor back in 2013.
Thanks OP.
For the experts here, I wonder what the typical range of exhaust O2 reading is when they do emissions testing? Does the 0.5 reading OP got indicate a possible small exhaust leak?
@MichelleZ The reason the machine was out of order, is because it’s an antiquated piece of junk, which still uses a desktop style PC, CRT monitor and a 3-1/2" floppy disk drive. I wish I were kidding, but they’re still in use
Whoever is performing your smog inspections is doing a good job. For him to fail your car for PCV and a vacuum line, means that he’s actually spending time on the visual inspection, versus just entering “pass” on the screen without actually looking
I didn’t know about the back pressure induced EGR activity. Thanks Tester.
Nitrogen and oxygen only combine at high temperatures to form NOx. Introducing “already burned” dead exhaust gasses via the EGR dilutes the incoming air-fuel mixture to keep those peak temperatures from occurring.
Exhaust gas recirculation is not a concern at idle, since idle combustion chamber densities are so low that peak temperatures, where nitrogen and oxygen combine, are not a concern.
And as a side note, reducing peak temperatures also means reducing peak pressures, which means reduced “push” on the piston. But that’s another topic.
I love that people are explaining things.
Just a bit of frosting for the cake: “already burned” exhaust gasses simply means that the oxygen atoms are already bound to hydrogen, carbon, and (some) nitrogen. Since the process of combustion is the release of heat energy when the hydrogen and carbon atoms of the hydrocarbon molecules (gasoline) split apart to bond to free oxygen atoms, if the oxygen atoms are already tied up and not free, as they are in the exhaust stream, there’s no contribution to the combustion process. By displacing a bit of the incoming air with exhaust gas, the explosion isn’t as intense and less heat is released.
It’s sort of analogous to closing the vents in a woodstove. By reducing the oxygen available to the fire, the fire’s temperature drops. Conversely, blowing on the fire with a bellows adds oxygen to the process and the fire gets hotter.
I hope I didn’t just make things more confusing.