Coolant Expansion Tank Cap

[Carmo13]

Hello,

My coolant expansion tank cap seems to be weeping a little, should the cap screw down onto a rubber washer?
If so then I must have lost mine at some point.

Many thanks

[N-Gen]

It should indeed have a rubber washer in the cap.

[Carmo13]

Well I wonder where that has gone!

Thanks

[N-Gen]

The rubbers can easily slip out after they get worn.

This is what you need to be seeing.

[Brianmoooore]

Flat rubber washer at the top has nothing to do with keeping coolant in. The O rings on the side are the pressure seals.
The cap in the pic. is from an iS, which uses a different (2 bar) cap from the 1.4 bar cap fitted to most other E30s. Unless your car is an iS, the original cap should have been replaced under a safety recall around seventeen years ago, which is still active. Most replacement caps have a yellow disc at the bottom instead of black, so if yours doesn't appear to have been changed, visit your dealer for a free replacement.

[N-Gen]

That's some good info. Wish I've known about it before replacing my black ring one with a new yellow ring one.