If you experience difficulty starting the engine when cold, stalling at idle, or hesitations during acceleration, it may be the idle air bypass valve. This valve helps the vehicle idle steady when the accelerator pedal is not pressed.Google+
Car Problem Reports
Mercury Grand Marquis Problem With Idle Air Bypass Valve May Cause Engine Performance Problems
Mercury Grand Marquis Problem
Average Mileage: 150,050 mi (149,000 mi - 151,100 mi)
When accelerating between 1st and 2nd gear, there is a lengthy shift time, also I notice when pressing slightly harder on gas pedal while already in travel, it is not responsive as a v-8 should be with quick acceleration. It seems as either gas is not traveling through the gas pump at the required rate, or after getting my intake replaced, that possibly one of the (pistons/cylinders) could be bad, or may have a compression leak. No smoke or stalls and no problems starting when cold. Thanks!
Visitor, 1991 Mercury Grand Marquis, 151,100 mi
Cold starts difficult (long crank and had to throttle engine to keep idle going.)