The compressor intake valve is only partially opening, restricting the amount of air entering the screw element. The compressor runs but produces less air than it should. Pressure builds slowly or cannot reach the setpoint.

What you'll see

The engine runs at full speed, the compressor is clearly working (you can hear it compressing), but the pressure builds very slowly or stabilizes below the normal setpoint. When you close the outlet valves, pressure does eventually build -- but much slower than normal. The engine is not overloaded. If the intake valve were completely stuck closed, you would get no pressure at all (see Not Building Pressure). A partially stuck valve gives you some pressure but not full capacity.
Before you assume this is the problem

Close the outlets and see if pressure builds to normal unload pressure. If it does (just slowly), the control system is working and the intake valve is partially restricted. If pressure never reaches normal, check the control system and regulating valve. See: Engine Not Reaching Full RPM, Air Leaks.

See all causes of low pressure / low air output →

How to diagnose

  1. Check control air pressure

    The intake valve position is controlled by control air pressure. When loaded (outlets open, pressure below setpoint), the control air pressure should be low, allowing the intake valve to open fully. Measure the control air pressure at the inlet valve actuator. If control air pressure is higher than it should be during loading, the valve is being partially held closed by the control system.
    Result: Low control air during loading = valve should be open. High control air = regulating valve or control system issue.

  2. Inspect the inlet valve mechanism

    Shut down and depressurize. Remove the inlet valve cover or access panel. Check if the valve moves freely through its full range. Look for carbon buildup, sticky deposits from degraded oil, or mechanical damage. The valve piston or butterfly should move smoothly from fully closed to fully open. Check the return spring (if fitted) -- a broken spring can prevent full opening.
    Result: Valve moves freely = control air issue. Valve sticky/restricted = clean or repair.

How to fix it

  1. Clean the inlet valve

    Remove the valve assembly and clean it with a suitable solvent. Remove carbon deposits and sticky oil residue from the valve piston, bore, and seals. Check the seals and o-rings -- replace if hardened or damaged. Reassemble and check that the valve moves freely.

  2. Check and adjust the control air system

    If the valve is mechanically OK but the control air is holding it partially closed, check the regulating valve adjustment. The regulating valve may need recalibration. Also check for leaks in the control air lines that could cause incorrect pressure readings at the valve actuator.

Common mistakes

Do not force the inlet valve open manually while the compressor is running. This bypasses the control system and can cause the compressor to overload the engine or overpressurize. Also: a dirty compressor inlet air filter can be mistaken for a stuck inlet valve -- both reduce capacity. Check and clean the inlet filter first, as it is much simpler.

Parts & tools

Solvent for cleaning valve deposits. Replacement o-rings and seals for the inlet valve. Pressure gauge for measuring control air. Basic hand tools.

Review safety precautions before starting →

Safety

The inlet valve is under the influence of control air pressure and internal compressor pressure. Shut down the compressor and release all internal pressure before inspecting the inlet valve.

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