[sdiy] How do old electrolytics go bad?

[Ingo Debus]

Hi,

we all know that electrolytic capacitors often go bad when they are  
old. In the past I had replaced some that had a significantly lower  
than rated capacitance. But now I got two that show a higher than  
rated value on my DMM: one is rated 100uF/3V and shows 270uF on the  
meter, the other is rated 16uF/16V and shows 23uF. Of course I  
desoldered them from the circuit before measuring.
Is this normal? Or did they have the higher values from the  
beginning? Or are they leaking and thus fooling my meter?

I estimate these caps are about 40 years old.

Ingo