When the compressor runs too hot, the oil thins out and vaporizes more easily. Thin, hot oil gets past the rings and into the compressed air more readily. Fix the overheating first, and the oil carry-over often resolves itself.
What you'll see
If the compressor runs at normal temperature and still has oil in the air, the cause is elsewhere -- wrong oil, overfilled, or worn rings. See: Oil Level Too High, Wrong Oil Type, Worn Piston Rings.
How to diagnose
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Check the discharge temperature
The discharge pipe temperature on a reciprocating compressor shouldn't exceed about 200C (390F) for oil-lubricated models. If you have an infrared thermometer, shoot the discharge pipe right at the pump head. Higher than normal = overheating. The exact spec varies by model and stage -- check the manual.
Result: Below max spec = temperature OK. Above spec = compressor overheating. -
Identify the overheating cause
Common overheating causes on reciprocating compressors: poor ventilation, dirty cooling fins, broken valves (recompressing the same air), blown head gasket, excessive duty cycle, or high ambient temperature. Each of these has its own dedicated cause page. Fixing the overheating is the real solution -- the oil carry-over is just a symptom.
Result: Identify which overheating cause applies.
How to fix it
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Fix the overheating problem
Address the root cause of the overheating. See the Overheating section for detailed diagnosis and fixes. Once the compressor runs at normal temperature, the oil carry-over should reduce significantly. You may also want to do an oil change after fixing the overheating, as the oil may have degraded from running hot.
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Change the oil after overheating
Oil that has been overheated loses its lubricating properties and may have started to carbonize. Drain and replace with fresh compressor oil. Check the valve plates for carbon deposits -- clean or replace if necessary. Carbon deposits on valves are a sign that overheating has been going on for a while.
Don't just add a bigger filter to catch the excess oil -- that treats the symptom, not the cause. The overheating is damaging the compressor beyond just oil carry-over: it's degrading the oil, accelerating valve carbon buildup, and shortening the life of every component. Fix the temperature problem, and the oil problem fixes itself.
Infrared thermometer (very useful for compressor troubleshooting). Fresh compressor oil. Valve plate cleaning supplies if carbon has built up.
An overheating compressor has scalding-hot surfaces. Don't touch the head, cylinder, or discharge pipe without first checking temperature.
This issue can also cause
- Overheating / High Cylinder Temperature Compressor running hot or cylinder head too hot to touch: usually caused by valve problems, ventilation issues...