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Re: o2 questions....again....long.again Posted by Ari [Email] ![]() ![]() In Reply to: o2 questions....again....long.again, paulmanchester, Wed, 31 Jul 2002 15:27:57 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The engine control will detect a disconnected O2 sensor. If it sees no sensor, or an invalid value, it'll make up a value, and for the sake of engine longevity, it'll guess in a way that will make the engine run slightly rich. So you still get good boost, but I can believe a drop in MPG.
The engine control is a pretty sophisticated device, and can maintain performance with bad sensors, depending on which and how many sensors.
It's one thing if the sensor is totally dead, it's another if the sensor is still working, but not very well. That could be the difference in when the Check Engine light comes on.
It's pretty easy to check the sensor. With the sensor still in the circuit, put a good digital voltmeter on the signal, with the low side of the meter to ground. Start the engine, and let it warm up for 5 minutes. The O2 sensor doesn't work until it gets to temp - at least 2 minutes if the heater is working. If all is well, the signal from the sensor should be swinging between 0.2 and 0.9 volts. If it is, you're done - the sensor is good. If it doesn't you're not done - it could be the sensor, or the sensor could be telling the truth and something else is wrong. All the sensor can do is say "rich" or 'lean' within a very narrow range.
It's possible that the sensor output is stuck at one end - 0.2 or 0.9. That could be a bad sensor, or it could be that the engine is actually running rich or lean.
Pull a vacuum line off the intake manifold - create a vacuum leak. This should cause the O2 sensor to read lean - closer to 0.00 or 0.2 volts. Stick the hose back on, and pull the vacuum line off the Fuel Pressure Regulator and cap the hose so it doesn't leak. This should cause a rich condition (0.9 or so).
If you can get the sensor output to move off 0.2 or 0.9 by pulling various hoses per above, chances are the sensor is at least OK. You should be looking elsewhere - vacuum leaks, wrong fuel delivery, etc.
However, as O2 sensors get old, they still put out a signal, they just get slow (like most of us). This slow response will cause the engine control to mis-schedule fuel, but won't necessarily drive a check engine light. There is no really good way to test for a 'lazy' O2 sensor.
The only way the O2 sensor will affect boost is if it's telling the engine to run lean. Lean running will make the engine knock, which will cause it to cut back on boost. However, you could be knocking for a lot of reasons - low fuel pressure causing lean running, ignition issues, carbon buildup in the cylinders, a lot of things.
Good luck!
posted by 140.157.4...
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