A vacuum leak can cause a lean burn.
basicly its changing the mixture of fuel/air to improve performance
P0171 is System Too Lean (Bank 1), which means that the oxygen sensor detected an excessively lean air fuel mixture.
yes
yes
turn clockwise or inward to lean the mixture.
If your code is PO171 or PO174 then there is a fuel delivery issue, If your fuel filter hasn't been replaced yet then do that first before replacing the fuel pump[The volume of sending fuel is insufficient]
Not necessarily. An excess of air causes the fuel/air mixture to become lean, and it doesn't have a positive impact on efficiency.
A motor will die when put in gear if there is a major vacuum leak. This causes an excessive amount of air to enter the engine and lean out the fuel mixture.
51** Fuel System Lean A lean air/fuel mixture has been indicated by an abnormally rich correction factor
"Rich" and "lean" refer to the air/fuel mixture. Rich means a higher proportion of fuel than is needed and lean means less. An overly rich mixture can foul plugs, cause sluggish performance and promote carbon buildup in the combustion chambers, as well as decrease fuel economy. An overly lean mixture can cause jerky, hesitant performance, overheating and in extreme cases even melt a piston crown.
It's when there's a little less fuel for the amount of air than the ideal condition.