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Re: Check engine light - Lambda?
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 19 Jan 2001 16:09:55 Share Post by Email
In Reply to: Check engine light - Lambda?, T.J.Bramhall, Fri, 19 Jan 2001 13:45:53
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The fault code can only bring you so far, then troubleshooting begins.

Under normal operation, the signal from the O2 sensor swings between rich(0.9 volts) and lean (0.2) volts about once or twice a second (depends on LH system). The average is about 0.5 volts, which should be the ideal stoichiometric ratio. If the mixture is too lean, the fuel control adds fuel, and if the mixture is too rich, the fuel control puts in less fuel. The fuel control uses the O2 sensor to adjust the mixture by adjusting how long the fuel injectors stay open. But the signal from the O2 sensor can control only about 25% of the full authority of the fuel.

The check engine light will come on if it thinks the O2 sensor is bad. How could it do this? Well, if the O2 sensor signal appears to be stuck rich (0.9) or lean (0.2) volts, the Check Engine light comes on.

So what the heck does that mean? Suppose you have a big vacuum leak, and despite all the fuel dumped in, you're still running lean? The O2 sensor will be telling the truth, but the fault code will come back as O2 sensor. Say a fuel injector is stuck open, and you're dumping fuel in and running a very rich mixture? Same O2 sensor fault.

The fuel control isn't all knowing. It just knows what it's told. So the first thing you should do is see if the O2 sensor is telling the truth. Pull all the spark plugs and look at them. Are they white, indicating a lean mixture? Start looking for vacuum leaks, or perhaps kinked fuel hoses Are the plugs black and fluffy, or even more, wet with fuel? Look for a fuel injector stuck open.

Either way, look for a a fuel-pressure regulator problem (did the vacuum line get hooked up?). If the Air Mass Meter is set wrong or bad, it can cause the same problem. The O2 sensor controls 25% of the fuel flow, but the AMM controls 100%.

So don't assume the O2 sensor is bad. It might be, but it could be just fine. It will require more troubleshooting.
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