The attached photo is of the charcoal canister on a 3.0i Z3, Euro model. Its located inwards from the left rear wheel, in front of the rear sub-frame. The sub-frame was removed when this photo was taken.
There is a vent pipe that goes from the canister, under the sub-frame and up to a clip on the side of the tank. The vent pipe or the canister may be blocked. Start by removing the vent pipe and inspecting it, it pulls out. The vent pipe would attach to the open hole on the bottom right of the canister.
Hope this helps.