When intake or exhaust valves leak, the compressor re-compresses the same air over and over. This double (or triple) compression generates excessive heat in the affected cylinder. The pump head becomes extremely hot -- much hotter than the other cylinders on a multi-cylinder machine.

What you'll see

One or more pump heads are significantly hotter than normal. On a multi-cylinder compressor, you can feel a clear temperature difference between the affected and healthy cylinders. The compressor also takes longer to fill the tank. You might hear a clicking or rattling noise from the hot head. In severe cases, the heat can scorch paint on the pump or cause the thermal overload to trip. The discharge pipe from the affected cylinder may be cooler than expected (less air getting through).
Before you assume this is the problem

If all cylinders are equally hot and the compressor is in a poorly ventilated space, the problem is likely environmental, not valve-related. If only one cylinder is dramatically hotter, that points directly to a valve problem in that cylinder. See: Poor Ventilation, Dirty Cooling Fins.

See all causes of overheating / high cylinder temperature →

How to diagnose

  1. Compare cylinder temperatures

    On a multi-cylinder or two-stage compressor, check each head temperature. A significant difference (one cylinder much hotter than others) pinpoints the problem cylinder. Use an infrared thermometer for accuracy and safety. On a single-cylinder compressor, compare to what's normal or to the manufacturer's maximum temperature spec.

    Result: Even temperatures across cylinders = likely not a valve issue. One much hotter = valve problem in that cylinder.

  2. Check for intake pulsing on the hot cylinder

    If the inlet valve on the hot cylinder is leaking, you'll feel air pulsing in and out at the intake of that cylinder. This confirms the valve is the cause of both the overheating and the poor performance.

    Result: Pulsing = inlet valve leaking on that cylinder. No pulsing but still hot = outlet valve or head gasket.

How to fix it

  1. Replace the valve kit on the affected cylinder

    Remove the head from the overheating cylinder. Replace the complete valve set (inlet and exhaust). Clean all carbon deposits from the valve plate and seats -- the excessive heat has likely baked oil into hard carbon. Use a new head gasket. Torque head bolts in a cross pattern.

  2. Check for carbon buildup prevention

    If carbon buildup caused the valve failure, use the correct compressor oil going forward. Engine oil produces far more carbon deposits. Also check if the compressor was running too hot before the valve failure -- overheating causes carbon buildup, which causes valve failure, which causes more overheating. Break the cycle.

Common mistakes

Don't assume the thermostat or overload is 'too sensitive' when it keeps tripping. It's doing its job -- the compressor is genuinely overheating because of the recompression cycle. Fix the valves, and the temperature returns to normal. Also: don't replace valves on the wrong cylinder on a multi-cylinder compressor. Use the temperature comparison to identify which cylinder has the problem.

Parts & tools

Valve kit for your compressor model. Head gasket. Infrared thermometer. Carbon scraper. Torque wrench.

Review safety precautions before starting →

Safety

Overheating pump heads can exceed 200C. Do not touch. An infrared thermometer is the safe way to check. Let the compressor cool completely before disassembly.

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