The compressor oil has been overfilled. Excess oil overwhelms the separator -- more oil enters the separator than it can handle, and the excess passes through into the compressed air. The fix is simple: drain to the correct level.
What you'll see
If the oil level is correct (between min and max on the sight glass during operation), the problem is not overfilling. Check the separator, scavenge line, or temperature. See: Dirty Separator, Scavenge Line Plugged.
How to diagnose
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Check oil level during operation
The oil level must be checked with the compressor running at operating temperature and loaded. The level drops when running because oil is distributed throughout the system (in the screw element, oil cooler, piping, and separator). If the level is above the maximum mark on the sight glass during running, it is overfilled.Result: Level above max during operation = overfilled.
How to fix it
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Drain to the correct level
Drain oil from the oil drain valve until the level is in the middle of the sight glass during operation. If the machine does not have a convenient drain, you may need to remove a small quantity through the oil filler cap with a suction pump.
Do not check the oil level when the compressor is stopped. When stopped, all the oil drains back to the separator vessel and the sight glass shows a higher level than during operation. If you fill to the top mark when stopped, you will overfill the system. Always check and fill with the compressor running.
Container for drained oil. Oil suction pump if no drain valve. Funnel and rags.
When draining oil, use a suitable container. Hot compressor oil can cause burns.