The outlet (discharge) valves must seal to prevent compressed air from flowing back into the cylinder on the suction stroke. When they fail, air that was just compressed leaks back, and the pump has to re-compress the same air over and over. Performance drops and the head runs very hot.
What you'll see
If air is pulsing out of the intake filter, it's the inlet valve, not the outlet. If both inlet and outlet valves are bad, you'll get both symptoms. See: Broken Inlet Valves. If the compressor pumps fine but pressure drops when you use air, the leak is downstream, not in the valves.
See all causes of not building pressure / air blowing out inlet →
How to diagnose
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Check the pump head temperature
After running for a few minutes, carefully touch the pump head (quickly -- it may be very hot). Compare to what's normal. A pump with leaking outlet valves runs significantly hotter because compressed air keeps leaking back and getting re-compressed. On a two-stage compressor, compare the temperature of both stages -- a dramatic difference points to a valve failure in the hotter stage.
Result: Normal warm = valves probably OK. Extremely hot or much hotter than the other stage = outlet valve leaking. -
Check for air at the intake while watching the discharge
If you don't feel pulsing at the intake filter, the inlet valves are OK. But if the compressor still can't build pressure, the outlet valves are leaking. Compressed air flows back through the outlet valve into the cylinder on the suction stroke, then gets pushed out again on compression -- an endless cycle of recompression that generates heat but makes little progress.
Result: No intake pulsing but still poor performance and excessive head heat = outlet valve failure. -
Remove and inspect the outlet valve plate
Same procedure as inlet valves: remove the head and inspect the outlet valve assembly. Look for the same failures: cracks, warping, carbon buildup on seats, broken pieces. Outlet valves often fail from carbon buildup first because they're exposed to hot, compressed air and oil mist which bakes into carbon deposits.
Result: Clean, flat, unbroken = OK. Carbon-fouled, cracked, or warped = replace.
How to fix it
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Replace the complete valve kit
Same as inlet valve repair: replace all valves (inlet and exhaust) at once with a complete valve kit. Clean the seats, use new gaskets, torque in a cross pattern. Pay particular attention to the exhaust valve seats -- carbon buildup is more common here due to the heat and oil exposure.
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Consider why the valves failed
If carbon buildup was the cause, think about prevention. Using the correct compressor oil (not engine oil) reduces carbon deposits. Running the compressor at the correct temperature helps too -- an overheating compressor bakes oil onto valve surfaces faster. Regular valve cleaning at service intervals can extend valve life significantly.
Don't confuse outlet valve failure with a blown head gasket -- both cause overheating and poor performance. The head gasket leaks between the cylinder and the outside (or between stages on a two-stage), while the valve leaks between the cylinder and the discharge passage. You'll find the answer when you remove the head. Also: if carbon buildup was the issue, investigate your oil. Engine oil produces far more carbon deposits than proper compressor oil because of its detergent additives.
Valve kit for your compressor model. Head gasket. Carbon scraper or gasket scraper. Torque wrench. Clean rags. Proper compressor oil (if oil type was contributing to carbon buildup).
A pump running with leaking outlet valves gets dangerously hot. The head and discharge pipe can cause burns. Let everything cool before touching.
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...
- Unusual Noise / Knocking New or unusual sounds from the compressor: often caused by loose parts, worn bearings, or valve problems.