Lubricating oil for AC system with lost refrigerant

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Does anyone have any experience with adding PAG lubricating oil to a system that lost all its refrigerant? My system in a 2008 GM (R134A) was lost to the atmosphere from a leak at the high pressure connection to the evaporator. I plan to fix the leak, replace the accumulator and orifice tube, pull a vacuum and recharge. What about the lube? Any rule of thumb on how much to add? Thanks.
 
You'll need access to an FSM for the proper amount to add. I'm not familiar with GM products; however, in my Chrysler manuals it spells out quantity by component. The below info is from a 2005 Dodge Caravan manual, which I'm posting as an example. You'll need to investigate the data for your vehicle:

REFRIGERANT OIL LEVEL CHECK
When an air conditioning system is first assembled at the factory, all components (except the A/C compressor) are refrigerant oil free. After the refrigerant system has been charged with (R-134a) refrigerant and operated, the oil in the A/C compressor is dispersed through the lines and components. The A/C evaporator, A/C condenser, and receiver/drier will retain a significant amount of oil. Refer to the A/C Component Refrigerant Oil Capacities table. When a component is replaced, the specified amount of refrigerant oil must be added. When the compressor is replaced, the amount of oil that is retained in the replacement A/C compressor. The oil capacity of the system, minus the amount of oil still in the remaining components (refer to the oil capacity chart below) can be measured and poured into the suction port of the A/C compressor. When a line or component has ruptured and oil has escaped, the receiver/drier must be replaced along with the ruptured component. Example: On this A/C system the front A/C evaporator retains 50 ml. (1.7 oz.). The A/C condenser retains 50 ml. (1.7 oz.) of oil, and front A/C system oil capacity is 180 ml. (6.1 oz.). 180 ml. – 100 ml. = 80 ml. (2.7 oz.).

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Varies wildly by car and compressor. You'll need the service manual which should tell you how much in each component. If that's not available then blow the whole system out and add the correct total amount to it.
 
Anyone know if some oil typically leaves with it?
With any other than a catastrophic system blow-out, the amount lost with a refrigerant leak should be negligible. A few years ago I caught a large stone in my grille while on the interstate. Gashed quite a hole in the condenser and blew oil like a gunshot would while the R-134 leaked out.

For a joint/O-ring leak I wouldn't worry about oil loss.
 
What kind of car exactly? Did it come out under high pressure where you could see the oil?
This is the 2008 Chev Suburban. The leak seems to be at the seal to the front evaporator. The area is a a bit dirty but not soaking in oil. Here is the shot under ambient light and another under the black light. A check in the other area including the pipes to the rear evaporator did not indicate any other leaks.

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This is the 2008 Chev Suburban. The leak seems to be at the seal to the front evaporator. The area is a a bit dirty but not soaking in oil. Here is the shot under ambient light and another under the black light. A check in the other area including the pipes to the rear evaporator did not indicate any other leaks.

View attachment 123709View attachment 123710
It didn't lose very much. Since that system is so large you don't run the chance of oil fouling the system unless you really go overboard on the oil. A couple years extra ounces of oil won't effect the cooling. I would add about 3oz if I did this repair.
 
One quick way to add oil is to pull a vacuum, then place clear vinyl tubing over the charge port, fill the tubing with the desired quantity of oil and depress the schrader valve using something like a clean long screwdriver or welding rod down through the oil. The vacuum will immediately draw in the oil.
 
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