I have a small problem with the running on my XJ6 Sport that someone may be able to help with. I recently lost the fuel cap and not sure weather this has something to do with it, up to now its been great and ran with no problems at all, now its very lumpy, it always starts first time but feels like its missing. It has just been serviced so shouldnt be any problems plug wise. So I thought that without the fuel cap maybe the pressure in the tank and the fuel lines isnt where it should be and therefore it wouldn't run properly?? Any help would be greatfully recieved.

sounds like you have condensation buildup in the fuel tank, also on new models of jag if you dont click the petrol cap more than 3 times you can get a MIL light as this helps protect against condensation and helps create a slight vacum.... you could also have all sorts of bugs and crud in the tank as dust and grime always finds its way into an opening..

best thing to do is get a new fuel cap and drain the tank