In the past, I've often been told that the reason for a slow puncture was that the alloy wheels had gone porous. More recently, I've been told that the cylinder head on a certain vehicle had gone porous and that was why it chuffed when cold.
I have always wondered whether this is a standard get-rid-of-him phrase taught as part of the Car Mechanics PhD (Hons) course, or whether alloy does actually start to leak.
Does anyone know? Is there any sort of standard test, like wheel tapping, that can detect this sort of rot?