The electrical characteristics of a failed electrolytic capacitor with an open vent are the following:
capacitance value decreases to below the rated value
ESR increases to very high values.
Electrolytic capacitors with an open vent are in the process of drying out, regardless of whether they have good or bad electrolyte. They always show low capacitance values and very high ohmic ESR values. Dry e-caps are therefore electrically useless.
E-caps can fail without any visible symptoms. Since the electrical characteristics of electrolytic capacitors are the reason for their use, these parameters must be tested with instruments to definitively decide if the devices have failed. But even if the electrical parameters are out of their specifications, the assignment of failure to the electrolyte-problem is not a certainty.
Non-solid aluminum electrolytic capacitors without visible symptoms, which have improperly formulated electrolyte, typically show two electrical symptoms:
relatively high and fluctuating leakage current
increased capacitance value, up to twice the rated value, which fluctuates after heating and cooling of the capacitor body
When examining a failed electronic device, the failed capacitors can easily be recognized by clearly visible symptoms that include the following:
Bulging of the vent on top of the capacitor. (The "vent" is stamped into the top of the casing of a can-shaped capacitor, forming a seam that is meant to split to relieve pressure build-up inside, preventing an explosion.)
Broken or cracked vent, often accompanied with visible crusty rust-like brown or red dried electrolyte deposits.
Capacitor casing sitting crooked on the circuit board, caused by the bottom rubber plug being pushed out, sometimes with electrolyte having leaked onto the motherboard from the base of the capacitor, visible as dark-brown or black surface deposits on the PCB. The leaked electrolyte can be confused with thick elastic glue sometimes used to secure the capacitors against shock. A dark brown or black crust up the side of a capacitor is invariably glue, not electrolyte. The glue itself is harmless.