There's a TSB issued in April '00 regarding P1171.

Service Information

Condition: Some customers may complain that the engine runs rough or MIL light on. In some cases, vacuum leak noise may be heard as well. [Intermittent rough idle and/or the MIL is on with DTC P0171 (Fuel Trim System Lean Bank 1), P0174 (Fuel Trim System Lean Bank 2), or P1171 (Fuel Trim System Lean During Acceleration)].

Probable Cause: It was found that the inlet manifold gasket(s) were cracking due to excessive stress caused by heat expansion/contraction.

Correction: Replace old inlet manifold gasket(s) with new inlet manifold gasket(s).