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Final sounds so, well, final Posted by Ari [Email] ![]() ![]() In Reply to: Final question ( hopefully ) for Ari, John Daikh, Thu, 27 Oct 2005 02:39:15 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The O2 sensor and the ECU work together. If you see the O2 voltage swinging from 0.2 to 0.9, then a lot of things are working just right. It means the O2 sensor is working, the ECU is reading the O2 signal and adjusting fuel flow, and the injectors are responding to the ECU commands properly. It also means the Fuel Pressure Regulator (FPR) is probably OK, and you've got no major vacuum leaks. It's like a house of cards - everybody does it's job, and it works just fine.
The only thing that could still be wrong with a car if you see the voltage swinging is an old, 'slow' O2 sensor. As they age, they respond slower (same with all of us, I guess). The swinging frequency doesn't change, it just overshoots. This usually shows up as failing emissions, as the effect on driveability is pretty minor. Very hard to tell even with the best equipment.
If you don't see the voltage swinging, it means there are a lot of possibilities to look at. Just like that house of cards, just one card out of place and it's a mess on the living room floor. As mentioned before, the O2 sensor only works in a narrow mixture range. Get out of that range, and the sensor simply says 'too rich' or 'too lean.' This could be due to a bad O2 sensor or a bad ECU, but more likely it's a vacuum leak, a bad FPR, or a flaky AMM.
The ECU is pretty dumb when it comes to O2 sensor failures - if it sees the O2 sensor stuck at 0.2 or 0.9, it will throw an O2 sensor code. That doesn't mean the O2 sensor is bad. It does mean that more troubleshooting is required.
If an O2 sensor output is stuck at 0.2 volts (lean running), the first thing I do is look at the spark plugs. If they show rich running - fluffy black - then the O2 sensor is probably bad. If they show lean or normal running, I keep looking. The first thing I do is remove the vacuum line from the FPR, and plug it at the throttle body end, leaving the FPR vacuum side open to air. This will richen the mixture. If the O2 sensor signal comes off of 0.2 volts, then the problem likely ISN'T the O2 sensor - most likely a vacuum leak, but could be the AMM. Always look for vacuum leaks - all cars have a few.
If the O2 sensor output is stuck at 0.9 (rich), again, go to the plugs. If they look lean/normal, then I suspect the O2 sensor; if they look rich, the sensor is telling the truth. I then pull a vacuum line off (any but the FPR) to make a vacuum leak. If the O2 sensor comes down off 0.9, the sensor is OK, and something is causing excess fuel. This could be a leaky FPR, a sticky injector (one plug will look richer), or a bad AMM.
I'm located in Central Connecticut, but I don't own a shop. I'm just a guy that likes working on my cars.
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