Piston on evaporator coil which way does it go

Since you stand to loose all the food I would call a service tech ASAP. But here are the most common reasons for the problem you have.
Cause 1
The Evaporator Coils are Frosted Over The defrost heater assembly turns on a few times throughout the day to melt away any frost that may have accumulated on the evaporator coils. If the defrost heater assembly is defective, frost will continue to accumulate on the evaporator coils, and the coils will become plugged with frost. If the evaporator coils are frosted over, the airflow through the coils will be restricted, causing the refrigerator not to cool. Check the evaporator coils to determine if they are frosted over. If the evaporator coils are frosted over, test each component of the defrost system.
Cause 2
Evaporator Fan Motor The evaporator fan motor draws cold air over the evaporator coils and circulates it throughout the freezer. If the evaporator fan is not working, the freezer or refrigerator will not cool adequately. To determine if the evaporator fan motor is defective, try turning the fan blade by hand. If the fan blade does not turn freely, replace the fan motor. Additionally, if the motor is unusually noisy, replace it. Finally, if the motor does not run at all, use a multimeter to test the motor windings for continuity. If the windings do not have continuity, replace the evaporator fan motor..
Cause 3
Damper Control Assembly The air damper control opens and closes to let the proper amount of cold air into the refrigerator compartment. If the damper does not open properly, it won't let enough cold air into the refrigerator. Check the damper control to determine if it is broken or stuck closed.

That's correct. If the piston opening is too large it will allow more frozen through than designed and flood the evaporator coil.

the evaporater coil's piston acts as a orfice. the size of the piston hole allows a certain amount of refrigerant through at design pressure. it produces a designed temperture drop at designed cfm of air across it.

That piston is the metering device. On the liquid line, before the metering device it is always warm (this is sub cooled, high pressure liquid), but should flash to low pressure, lower temperature vapor once it passes through the metering device. The suction and liquid line never connect together.The liquid line brings liquid from the condenser to the evaporator. The suction line brings super heated vapor from the evaporator to the compressor. Since the unit does run, I'd have to put a gauge on it,but if I had to guess, I'd guess you have high head pressure. Check for dirt or debris on the condenser, check for proper clearance of the condensing unit and check for operation and directional rotation of the condenser fan. This could also be the result of an over charge.

if it is a heat pump you could have a clogged actuator piston or a stuck reversing valve depending on whether the unit is energized in the heating or cooling cycle. Try pulling the wire on the reversing valve coil on and off a few times and that may force it to unstick. There is a lubricant that you can pour directly into the true suction line above the reversing valve itself. Supco oil is the name i believe. Also can try removing the actuator piston at both the condenser and the evaporator and checking to see if there is some trash in the line, especially if the lines were brazed without using nitrogen.

.078 is Shipped with Coil
Note: For a properly matched system and piston sizing information, refer to Goodman piston kit chart of the corresponding Goodman
outdoor
unit.

No, it simply has to do with the fact that the conditions are exceeding the innate capabilities of the unit. Need more refrigerant and larger condenser and evaporator. Basically, a bigger, more powerful unit for the conditions encountered.

This mixmatch will work but during times of low temps ( during rain or at night) you may liquid flood the compressor, Watch your super heat at the compressor or your (line temp verses press relation).  http://www.1coolair.com/houston_ac_news/blog.php?BlogID=10

it goes high pressure line evaporator coil if it call for it see instruction it will say.

If you purchased it as a package the right piston may already be installed in the indoor Evaporator coil. When shipped the extra piston if needed is packaged with the outdoor section condensing unit.