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Agree with Wingerr - poppycock! Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 15 Feb 2005 05:51:18
In Reply to: Alternator voltage OK, amps low - affect O2 sensor?, Marcus, Mon, 14 Feb 2005 15:47:49 Members do not see ads below this line. - Help Keep This Site Online - Signup |
As he said, it isn't the alternator - if it's putting out 13.5 volts, it's working just fine. As to the O2 sensor heater, it only comes on when the engine is cold, and turns off once the engine gets warm. The O2 sensor puts out voltage all by itself as long as it's warm - it doesn't need voltage; it produces it. And after 20 minutes of idling, you bet the exhaust manifold O2 sensor is warm enough. After all, the first O2-equipped cars didn't used heaters on the O2 sensors, they relied on engine exhaust. It was only requirement to drop emissions at startup that drove the heaters.
You didn't mention the year/engine - that helps a lot, as the fuel control system varies from year to year. Earlier cars used the exhaust manifold as the ground return for the O2 sensor signal (3-wire sensor). later cars used a separate signal ground (4-wire sensor)
The issue is probably NOT the O2 sensor. The ECU controls the mixture, which shows up as the amount of oxygen in the exhaust gas. If the O2 sensor reads rich, the ECU reduces fuel; if the O2 reads lean, the ECU increases fuel. But the O2 sensor only works in a Very Narrow range - 0.5 volts is perfect; 0.2 volts is slightly lean; 0.2 volts is lean, and 0.2 volts is incredibly lean.
So say that there is a vacuum leak, causing a lean mixture. The O2 sensor says it's lean, so the ECU richens up the mixture. But the O2 sensor only has so much authority over fuel - only about 20-25%. If the vacuum leak is too big, even when the O2 sensor is still screaming "Lean", the ECU says that it has richened up the mixture as much as it can. So the ECU throws an O2 sensor fault, because the O2 sensor is 'stuck' on lean.
So, could that fault be a bad O2 sensor? Yes, possibly. But since it doesn't die right away, and needs 20 minutes of extended running, I doubt it's the O2 sensor. Most probably, something is getting hot and stops working right. It could be mechanical - something expands, opens up, and creates a vacuum leak. A fuel injector may be packing up, and not squirting as much fuel. Or the AMM (if you have one, depends on year) screws up when warm, and calls for a richer or leaner mixture. And the AMM can call for a LOT more (or less) fuel than the O2 sensor.
So what will it take? It'll take a mechanic that is willing to troubleshoot. Just replacing what the fault code points to is just shoddy work. And the whole 'Alternator' story makes me think that your first step is to find another mechanic. Next, he'll tell you that you need new seatcovers and a wax job (the car, not you.)
posted by 192.249....
, Tue, 15 Feb 2005 05:51:18 <-- Viewing This Message
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