Why does an old viscous coupling stink so miserably?

Anybody who already opened an old VC knows what I mean – the offensive smell of the deep black silicone fluid. But why does it stink so badly?

In the last couple of weeks I have been studying chemical characteristics of various silicon fluids, and so I ran into these:

For all Non-Chemists (including myself): The oxygen in the VC starts at high temperatures (>150°C) a multilevel reaction. At the end of this reaction formaldehyde and formic acid is produced which gets decomposed to hydrogen and carbon dioxide gas.

The emerging gas inside the VC is the explanation for the sparkling silicone fluid – once the screw plugs are open. And the funny smell derives from formic acid and formaldehyde.

Conclusion: The oxygen is not only responsible for the thickening of the silicone fluid, it’s also in charge off the increasing inner pressure and the horrible smell.