What does it mean by my Ford Taurus - Mercury Sable engine is running lean?

Answer 1

Starting at the beginning...

The engine burns a combination of fuel and air - in fact, gasoline will not even burn all by itself, it requires the presence of some air.

The engine computer controls the mixture / ratio of how much air and how much gas is perfect at any given time.

All those sensors that you find all over today's car are feeding information to the computer - things like the temperature of the air, the barometric pressure, the temperature of the engine coolant, how much oxygen is in the exhaust, and so on.

Based on all this information, the computer will adjust the mixture to more fuel, or less fuel, more air or less air.

Sometimes the computer gets it wrong based on something is starting to go wrong with some part of the system.

Another situation that is very common that results in the mix being wrong is when air is sneaking into the system, bypassing the sensors in a way the computer doesn't know the extra air is even there.

When you hear the term "vacuum leak", that's a reference to air sneaking in through a broken rubber hose, a seal that has hardened and shrunk, or a dozen other situations.

When there is too much fuel in the mix = "rich condition".
Not enough fuel, or conversely, too much air = "lean condition".

• Lean condition is more common than rich
• Lean condition trouble codes:
  
• • 171 - Bank 1 (backside row of engine cylinders - by the windshield)
  • 174 - Bank 2 (front row of engine cylinders)

This concludes the first lesson in Air/Fuel Mixtures 101

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