Fuel injection?

Test drove a 2nd hand s40. Put 25 euro worth of petrol in and it ran out 20 mins later - then wouldn't start - got it going after a couple of minutes - put ten more euro petrol in - then all the fuel kicked in and I had loads of petrol! Brought it back to the dealer who told me it happened because two of the leads were loose. Is this possible or is he trying to con me? Also when I picked up speed to 120kmph the whole car started to shake and vibrate.