Harmon - In a pinch, add 1 oz of oil for the evaporator core. Nonetheless....
GM recommends ester oil. But, don't mix oils (e.g. - PAG with ester).
It's easy to overwhelm an A/C system with excess oil. This is especially true of R134a systems. Excess oil impedes the cooling efficiency of a 134a system. Actually, a compressor can be damaged (hydrolock) if too much oil is added. Yes, a compressor will also be damaged if insufficient oil is circulating, but it's a gamble to routinely add oil whenever the system is opened without knowing
how much oil to add.
Here's Why:
Oil is in an orifice tube A/C system strictly to lubricate the compressor. The oil is carried by the refrigerant through the A/C system between the various components. At any given time, there's no way to know whether there's a concentration of oil in a particular component. So, when you replace a component, you don't really know how much oil you're removing from the system. How do you determine how much oil to add?
Flushing:
The best approach when replacing a major component is to disconnect the major lines, remove the accumulator and the orifice tube, and flush the system completely clean using solvent that assures compatibility with the specific refrigerant. Flushing purges
all the oil and any debris from the lines and major components, so you're starting with a known amount of oil - none. However, NEVER attempt to flush the compressor! Instead, simply remove the compressor and drain it. You'll be pleasantly surprised at how well your A/C system will perform following a thorough flush and recharge.
Adding Oil:
Once drained, the GM R4 compressor needs 6 ounces of oil poured directly into the compressor through its suction port. You may need to turn the clutch by hand as you add the oil. For a C10 pickup, allow an additional 2 oz of oil for the accumulator, and 1 oz each for the condenser and evaporator, poured directly into the accumulator before installing. That's a total of 10 ounces of ester oil for the entire system. For comparison, R12 systems typically used ~12+ oz for a similar configuration.
Additional Considerations:
- Anytime you replace a major A/C component, you should also replace the accumulator. Accumulators wear out - the desiccant that absorbs moisture becomes saturated and physically breaks down from vibration into a fine powder. The powder washes through the walls of its containment bag and circulates through the system as brownish-black colored mud decreasing system effectiveness.
- Inspect and replace the orifice tube as necessary. If replacing the orifice tube, consider using a variable orifice tube (VOT) instead of the original fixed orifice tube (FOT). Although more costly, the VOT significantly improves A/C effectiveness for city driving and at idle in hot climates.
- If significant metal is discovered in the orifice tube screen and the compressor was at all noisy, now is the time to replace the compressor.
- And, don't forget the Schrader valves. They are often overlooked, and often leak.
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