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A little more info Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Tue, 19 Nov 2002 09:22:29
In Reply to: O2 sensor warm-up, JJ, Mon, 18 Nov 2002 16:32:11 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The O2 sensor doesn't start to work until it gets up to temperature (about 200 C). When the O2 sensor is warming up, the AMM (Air Mass Meter) controls the fuel flow.
After the ECU thinks the O2 sensor is up to temperature, it schedules fuel flow with BOTH the AMM and the O2 sensor. The AMM has 100% authority over fuel, and the O2 sensor 25%. The AMM measures the airflow, and gets the fuel flow into the right ballpark. The O2 sensor does the 'fine tuning'.
Here's where it gets tricky. The O2 sensor converts mixture (rich/lean) into voltage. The sensor is very sensitive. 0.5 volts is the perfect stochiometric ratio. Just a little rich is 0.9 volts. Just a little lean is 0.2. That's fine. Just a little rich is 0.9 volts, rich is 0.9 volts, and very rich is 0.9 volts. So the AMM has to be adjusted to get the O2 sensor into its 'sweet spot.'
The ECU is designed so that the mixture actually swings slightly rich to lean and back again, about once a second. So the voltage swings between 0.2 and 0.9 volts, spending about 50% of the time above 0.5 volts, and 50% below.
So what? If I stick a voltmeter on the O2 sensor and it reads 0.2 volts, then it's bad, right? Well, no. 0.2 volts is lean. The sensor may be bad, or it may just be telling the truth. If the AMM is adjusted too lean, the O2 sensor could be telling the truth, but it doesn't have enough authority to richen the mixture. Or there could be a vacuum leak, letting in air the AMM doesn't know about.
If you have good evidence that the O2 sensor is bad - for example, you know the engine is running rich, but the O2 sensor is saying it's lean, then the O2 is probably bad. But you need to (1) make sure there are no vacuum leaks, and (2) that the AMM is properly adjusted, before going after the O2 sensor.
To make things even more tricky, the usual failure mode of an O2 sensor isn't to just croak. Instead, the response gets slow. The output doesn't change as fast as a new sensor. It would be like someone giving you directions as you back up your car, but he delays his answer 30 seconds. You can get out of phase in time. The output of the O2 sensor will swing, but it's swinging at the wrong time.
If after you've checked for vacuum leaks, and adjusted the AMM, and you still can't work it out, then look at replacing the O2 sensor.
Good luck!
posted by 192.249....
, Tue, 19 Nov 2002 09:22:29 <-- Viewing This Message
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