1
Worn piston rings / valve plates
Reciprocating
CheckTime pump-up empty (no load). Compare to OEM new-machine spec.
FixRebuild head, replace valves.
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2
Restricted intake filter
CheckΔP > 6" H2O across filter.
FixReplace element.
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3
Slipping belts
CheckSmoke/squeal under load; check sheave for wear ("valley" worn).
FixRe-tension; replace belts + sheaves as a set.
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4
Internal blow-by past head gasket
Reciprocating
CheckOil whip from breather; cracked head visible.
FixReplace head gasket / head.
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5
Inlet valve partially closed (screw)
Rotary screw
CheckVisual; pilot air signal at full load.
FixService inlet valve.
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Safety
Lockout/tagout before opening enclosures. Verify zero energy. Bleed system pressure before opening any pneumatic connection.