What is this plastic cylinder for

Vapor cainster.

great, thanks - what does it do? ie: vapour from what?

For fuel vapor from the fuel tank.

The Evap canister is part of the emissions control system: all cars have them. If your near thirty year old truck has a defective one, get it fixed before your next smog check.

You won’t find it in the Haynes manual cause those manuals are designed on tear down and reassembly of the power train. This canister is designed to store fuel vapors and then when the engine is running feed them into the engine via a vacuum port. There is usually a sticker under the hood showing the vacuum hose routing.

thank you everyone for the answers. I shall now look to replace it ASAP.

Some of the earlier ones, like yours, had a filter on the bottom of the canister that needed to be replaced periodically. Hope that helps! Jim

that's all great, but now I find that all the evap canisters on parts sites are rectangular rather than round... Does anyone have any ideas? Also, the filters alone only seem to be available on sites that don't ship to France. Normally I get parts from rockAuto, but they don't have one that looks like mine :O