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My 2000 Taurus started throwing a code that translated into an unexpectedly lean engine. I suspected air was getting in someplace it shouldn't. Eventually, we found a small 3/32 inch slit in the crankcase breather hose coupler and when it flexed, it allowed a little air to enter. We replaced it with a complete stock Motorcraft hose. Hey, the last one lasted almost twenty years... Also replaced whatever other vacuum hoses we could with by cutting bulk tubing to fit. So far, so good.

I had a similar problem with my 94 Explorer years ago. The plastic duct that runs from the MAF to the engine was rubbing on the radiator and was sucking air through the worn spot. A new duct fixed the problem,

Question: When the system senses a lean condition in a 2000 Taurus and reports it, does it also try to enrich the fuel mixture to compensate?

Jonathan
 

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I believe that the system sets the air-fuel ratio by measuring the air intake with the MAF sensor and adds the correct amount of fuel using injector timing. The results are tested using the O2 sensors. If the results are not correct, the system makes adjustments to the long and short term fuel trims. The trim values are used in the calculations for the injector timing. If the trim values get too far from nominal, you’ll get a fault.

Don’t rule out an aging MAF sensor under reporting the actual amount of air being taken in. When this happens, too little fuel is added and the O2 sensors report a lean mixture.
 
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