In which case, disconnect the valve to tank hose, plug the end temporarily with a rag and then try it. Or extend the hose and re-route top below the car as always used to be the case.
All we're trying to do at this stage is make sure that the ECU doesnt react with an EML, and the fuelling isnt messed up. If as I suspect there is no problem then proper arrangements can be made long term to vent the tank in a safe way and to block up the feed from valve to manifold.
