The glass hatch on the fortwo, the boot on the Roadster and Forfour and the bonnet on the Roadster are all
helped to open by a pair of pressurised gas struts. They hold the weight so the boot or bonnet stays open
but they also allow a smooth opening and closing by maintaining a balance of the weight.
As with many things, they wear out. The seals inside them fail and the pressurised gas escapes.
With no gas in them, they are useless and do nothing to support the weight of the boot or bonnet.
Even if only 1 fails, the boot or bonnet will not stay open. This is because they are precisely matched to
slowly lift a set weight but not to be too strong that it makes it hard to push against to close. The last
thing you want is to put a lot of stress through a glass hatch as you try to close it.
You can't fit "any old gas strut". They have to be the correct length, correct throw and correct strength.
You can pick these up from smart, most car motorfactors and on eBay. Depending what you require, search for:
Fortwo tailgate strut
Roadster tailgate strut
Roadster bonnet strut
Forfour tailgate strut
You don't necessarily have to replace both. More often than not it is only 1 that has failed.
If you disconnect 1 end of both struts as shown below, you'll be able to determine which has failed.
Open the boot or bonnet to expose the gas ram that you want to remove.
On the end is a sprung metal clip that reaches inside and cups a ball, (no laughing at the back).
This sprung clip needs to be levered up slightly and slid forwards to free the fixing inside.
Be careful not to slide the clip too far forward, they do have a habit of putting themselves into orbit.
As soon as the clip is slid forwards, the end of the ram can be pulled away.
To refit, just offer the end up against the ball fitment and push the clip back in place.
No. These are filled to a precise level with pressurised gas using industrial equipment.
They can't be taken apart so there are no user serviceable parts inside.