The oil return line from the separator bottom is plugged. Oil collects inside the separator and gets carried out with the compressed air.

What you'll see

Oil carry-over that gets worse over time. The separator element might be relatively new but the machine is still pushing oil into the air lines. If you open the separator vessel and look inside, you'll see a pool of oil sitting on top of the separator element -- it looks like a swimming pool in there, when it should be almost dry.
Before you assume this is the problem

Check the separator element condition first -- if it's saturated or torn, that's your primary problem and the scavenge line may be fine. Also verify the oil level isn't too high. See: Separator Element Worn, Oil Level Too High.

Could also be:

See all causes of oil carry-over →

How to diagnose

  1. Open the separator and check for oil pooling

    Stop the machine, depressurize, and remove the separator cover. Look inside. There should be only a tiny puddle of oil at the bottom of the separator element -- a few drops at most. If it looks like a swimming pool, the scavenge system isn't doing its job. Oil is collecting faster than it's being returned.
    Result: Pool of oil inside separator = scavenge problem. Almost dry = scavenge is working.

  2. Check the scavenge line for blockage

    Disconnect the flexible scavenge line from the separator vessel. You should be able to blow through it by mouth. You'll hear bubbles coming from the airend side because the other end connects to the screw element suction. If you can't blow through it, it's blocked. Also check for a restrictor orifice or check valve in the line -- these can clog too.
    Result: Can't blow through = blocked. Can blow through and hear bubbles = line is clear.

  3. Check the scavenge pipe inside the vessel

    The scavenge pipe inside the separator vessel goes down to the bottom of the separator element. Check that it actually reaches the bottom -- but doesn't press against it. If a new (shorter) separator element was installed and the pipe is now too long, it can press against the bottom and get sucked shut by the vacuum. If the pipe is too short, it won't reach the oil pool and can't suck it up.
    Result: Pipe too long or too short for current separator element = adjust or replace.

How to fix it

  1. Clean or replace the scavenge line

    Remove the line, clean it out thoroughly, blow compressed air through it from both ends. If there's a restrictor orifice, remove and clean it separately. If the line is badly clogged or hardened, replace it. The line is just a small-diameter flexible hose -- cheap and easy to replace.

  2. Check and clean the check valve

    Some machines have a small check valve where the scavenge pipe exits the top of the separator vessel. This valve can fail or get blocked. Clean or replace it.

  3. Verify scavenge pipe length

    Make sure the internal scavenge pipe reaches the bottom of the separator element with about 2-3mm clearance. Not touching the bottom, not floating 2 cm above it. Adjust if needed when installing a new separator element -- different brands can have slightly different heights.

Common mistakes

A common mistake after replacing the separator element: the new element is a different height than the old one, and now the scavenge pipe doesn't reach the bottom. Oil pools, carry-over starts, and people blame the new separator. Always check the scavenge pipe length when changing separator elements. Also: install a scavenge sight glass if your machine doesn't have one -- it lets you visually confirm oil is flowing back to the airend during operation.

Parts & tools

Replacement scavenge line/hose. Cleaning supplies. Possibly a replacement orifice or check valve. Scavenge sight glass (recommended addition if not already installed).

Review safety precautions before starting →

Safety

Depressurize the separator vessel before disconnecting the scavenge line.