I have a 1988 Mercury Grand Marquis. This is kind of a strange situation. Every time I press the accelerator, the A/C blows hot air from the dash vents. It blows cold when coasting or idling. Any suggestions?
The various flaps in the AC/H ductwork are probably vacuum operated. When you quickly press down on the accelerator pedal, engine vacuum falls and this is affecting the positioning of the flaps (blend doors). Some automotive systems have a small vaccuum accumulator to help carry through momentary drops in vacuum. For this to work, there is a one-way valve to keep the accumulator from drawing air through the vacuum line when the vacuum falls. This valve may be in the accumulator or in the vacuum supply line. It's not working or is missing. There may be ported fittings that are missing. Some of the vacuum lines are ported, in other words, an orifice is present to throttle maximum allowable flow. Often, the orifices are built into the various fittings. Therefore, if a broken fitting was replaced by an ordinary, general purpose fitting, the orifice would be missing. you'll need to consult a good repair manual to sort out where the orifices are located. If a fitting is described as a ported fitting, it has a restrictor built in. If the one-way valve is in the accumulator and it no longer works, replace the accumulator. The accumulator may be spherical and about the size of a softball.
If there are serious vacuum line leaks between the accumulator and the blend door actuators, the supply system can not keep up with the leaks when vacuum reduces. A common leak is a bad diaphram in an actuator.
Of course, if you pull a long grade, the vacuum will fall and the accumulator will exhaust itself and the blend doors will return to their "normal" positions. In this case, the engine may be overloaded (running full time at near open throttle). It needs a downshift to a lower gear to pull the grade. A general rule-of-thumb is that if the engine can not accelerate, it is "lugging" and needs a shift to a lower gear.
Let us know what you find.