Probable Cause:
Overcharge of refrigerant: refrigerant added beyond the proper charge reduces capacity by raising the evaporator temperature (on non-receiver systems). An overcharged system is far more likely to damage the compressor than one that is undercharged. This overcharge of refrigerant is returned to the crankcase as a constant floodback during operation, reducing compressor life while dropping efficiencies and capacities.
Oil overcharging should be avoided as this creates the possibility of oil slugs, which can damage the compressor, and it also hinders the performance of the refrigerant in the evaporator. Excess oil in circulation displaces some refrigerant at the metering orifice. Since there is excess oil in the evaporator, the evaporation rate of the refrigerant is slowed down, because the oil acts as an insulator.
Remedy:
Charge the refrigerant to proper levels:
a) Cap tube system, charge by the superheat determined on charts available from valve manufacturers.
b) TEV non-receiver system, charge by subcooling the liquid refrigerant to an optimum of 10°F less than condensing temperatures (at full load, if possible).
c) TEV receiver system, charge by sight glass.
Remove oil and maintain levels according to manufacturer recommendations.