Please explain the easy way to decide a fixed displacement pump or a variable displacement pump is weak.

+ve pump(fixed ) is for oil applications (crane) ,and -ve pump is for ex. irrigation

Unlikely pumps do fails, upstream elements/ controls should be check first.

Variable compensated pumps can be determine easily if not working effeciently through drain case flow.

For fixed displaced pumps can be noticed on loss of discharge pressure, unwanted sounds, etc.

Such failures of this components related to several factors like;

- not good maintenance routines

- improper use of the system (overload)

- aerations( loose suction valves)

- cavitations ( putting suction filters on HF oils is a no) - vaporized and implodes

- mixing same grade oil but different brands (additives etc,) result to lossed of oil properties

- contaminations ( results internal wears), erosions

- failing relief safety valves (overpressure)

- unseen logic errors on power unit operations result to overload overpressure because pumps on continous running (causes heat, internal expansions, etc) - domino effect.

source: my self taught experience accompanied by google and an iphone.

Look at pressure on discarge line if pressure is quickly decrease but flow rate is same, maybe have some problem on your pump like leacking in seal or etc