Check for unmetered air ... check PCV hoses, check your intake tube, check it all. Everything from the MAF to the cylinder head. The P0171 happens because the PCM has detected a lean condition and has trimmed out. This is almost always due to unmetered air (air which enters the system behind the MAF). Vacuum leaks are a big source, but it can happen before the throttle body too.
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1988 9C1 - Modified LM1 @ 275HP/350TQ - TH700R4 - 3.08 8.5" Disc Rear - see it at
http://www.silicon212.org/9c1!
2005 Crown Vic P71 - former AZ DPS - 4.6 liters of pure creamy slothness!
1967 El Camino L79/M20 old school asphalt raper
Remember - a government that is strong enough to give you everything you need, is also strong enough to take everything you have.