I have been battling on to correct the misfiring problem on my Punto and with the help of very kind people I’ve narrowed down the issue to petrol tank evaporation purge valve that’s located in the inlet manifold on the engine. It has a two pin electrical connection and a couple of tubes that connect to it. I couldn’t find a new exact replacement so got one off a newer grande punto, they look almost identical. You have to make an electrical adapter from original harness to the new pins and connect the rubber hose from the original sensor to the new one with a hose clamp as fitting is shorter. Anyway after all that it works a treat now so ready for the grid and new Formula1 season.
The pictures below show the replacement valve and the old one taken apart. The fault was the copper springy part attached to the rubber bung had failed so prevented the valve from opening and closing. The adapter cable was made from cutting off the plug socket and soldering the new plug wires to it.