Why Evacuating the System "Cures" Your Imperial's A/C Problems?

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Question from Norm:


Several times in the past, I have had success with restoring cooling function to an AC system by simply evacuating and re-charging the system. Nothing more. Anyone know why this works? Does it dislodge particles which were blocking the freon path in the expansion valve/ screen? If so, maybe I'll try this method to get my rear ac system working in my 63.



From Bob:

Evacuation of the system does two things, it creates a vacuum within the system for a full recharge and reduces the boiling point of water from 212 to around 70, nominal temps., making it possible to remove the moisture.

When an AC system contains moisture, it will frequently freeze right at the expansion valve preventing the flow of freon into the evaporator - no cooling. If you have this happening, you might have a leak in the system allowing moisture to enter. You don't say much else about the system, but if it is leaking, repair the opening and replace the dryer, evacuate and recharge - you're not far off in your theory. Older systems like yours, if not kept up, will succumb to rust and this, too, will block flow at the expansion valve. It's hard to get clean again.

From Frank:

I have no real data to go on, but will hazard some guesses:

* System low on charge, so recharging fixes

* Air / moisture in system reducing efficiency or freezing on expansion valve, so recharging fixes

* Contamination? Doubt it, especially after seeing the condition of contaminated parts

Q: Does the front unit work correctly?

What are the pressures on the system, High, Low, and Port (on the compressor head)?

Is the expansion valve bulb properly routed in the rear unit?

Is the system marginal with just the front unit and sub-par with both on?

Follow-up from Norm:

The front system works very well regardless of the status of the rear. In other words, it makes no difference to the front if the rear is on or not, it just works well.

The rear expansion valve gets cold when the rear system switch is turned on but the discharge air is not cold.

The sight glass is mostly clear except for some occasional large, clear bubbles with no foaming visible.

I have not checked the pressure or done anything else yet.

By the way, when I originally posted the question what I really meant to say was that my experiences were with evacuating and recharging a nearly full system. That's what made it so mysterious to me.

Reply from Frank:

Don't know why evacuating and recharging works, unless your supposition that something blocking a line gets dislodged during this process. Of course, since the contamination is most likely still in the system, the problem will reoccur.

Couple of guesses.

Front unit always ok, sight glass mostly clear w/ occasional bubbles. - System is fully charged. No need to check pressures.

Since the charge is OK it must be: (in order of likliness):

1) Expansion valve partially plugged

2) Air flow bypassing evaporator core

3) Evaporator partially plugged - Should be unevenly cool. Should be very cold near inlet and warm near outlet. (Assuming 2 is not the problem)

4) Return line partially plugged - Should be another cold/cool spot near the blockage

5) Liquid line partially plugged. - Not very likely since the expansion valve would not be cold.

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