pop immediately to mind...
but before even going there... be VERY CAREFUL 275 pounds seems terribly high
for an '89... If something cuts loose at that pressure you best have all your helmets, flak jackets, gloves, glasses & cups on... (I'm just saying).
Pressure line should be hot
and suction line should be cool
... coming back to the compressor.. Yes...??
Lets get to the two things
on my mind:
1. Malfunctioning Heater-A/C baffles cause lotsa grief.
2. Drier: Desiccant saturation by exposure to atmospheric moistures
... replace or attempt recovery by pulling & holding a SERIOUS vacuum before
re-charging your system.
Ice crystals will form at the "ORIFACE" and PROFOUNDLY AFFECT PERFORMANCE...
Pulling & holding a 29 vacuum also certifies your main compressor seal (ergo... refrigerant & lubricant will remain intact)... Any trace of OIL BUILD-UP on the front of the COMPRESSOR (or inside the hood in alignment with your compressor pulley) should be addressed... oil level checked & augmented (only as needed)).
Hope this helps
http://www.rockauto.com/en/catalog/mercury,1989,grand+marquis,5.0l+302cid+v8,1199505,heat+&+air+conditioning,a/c+compressor+seal,6676 More Information for MOTORCRAFT YE57
More Information for SPECTRA PREMIUM COOLING DEPOT 0233168