When a pump does not perform well, one of the first steps toward finding the difficulty is to install a vacuum gauge in the suction port and a pressure gauge in the discharge port.  Readings of these gauges will often give a clue as to where to start looking for trouble.

High Vacuum Gauge Readings (above 15 in. Hg.) indicate:
1)  Suction line blocked, valve closed, strainer plugged
2)  Liquid too viscous
3)  Lift too high
4)  Line too small

Low Vacuum Gauge Readings (0-5 in. Hg) indicate:
1)    Air leak in suction line
2)    Pump is worn or pump should be primed

Erratic or fluttering Vacuum Gauge readings indicate:
1)    Vibration from cavitation, misalignment or damaged parts
2)    Liquid coming to the pump in slugs, possibly an air leak or insufficient liquid above the end of the pipe

High Pressure Gauge Reading would indicate:
1)    High viscosity and small and/or long discharge line
2)    Valves partially closed or other restriction in discharge line

Low Pressure Gauge Reading would indicate:
1)    Relief valve set too low or poppet not seating properly
2)    Too much extra clearance / pump worn

Erratic or fluttering Pressure Gauge Reading would indicate:
1)    Cavitation
2)    Liquid coming to pump in slugs
3)    Air leak in suction
4)    Vibration from misalignment or mechanical problems

Common issues associated with noisy installations.

1)  Pump is being starved.  Viscous liquid cannot get into pump fast enough considering pump speed
2)    Pump is cavitating.  Liquid is vaporizing in the suction line
3)    Misalignment of piping or drive
4)    Relief Valve Chatter.  Pressure setting too low
5)    May have to anchor base or piping to eliminate vibration
6)    Bent rotor tooth or broken bushings