I have a Jaguar XJ6 4.0 X300 on a 1997 plate. Im getting a smell of petrol in the boot of the car which you can smell from inside the car. You only notice the smell inside the car when you stop for a short while. It also seems to be doing really bad on fuel. The problem occurs most of the time. But on some occasions the smell goes and the car does good on fuel. As soon as you put fuel in the car problem sometimes comes back. Its really weird and hard to work out what is causing the problem. My old Jaguar XJ6 4.0 sport on a 1996 plate had the same problem which is weird. I have only had 2 Jags and the problem was the same on both. Any suggestions would be greatly appreciated ?.

 

Check the charcoal canister under car on drivers side.

 

Also as fuel leaks do not fix themselves , which is what it sounds like and the fact that it comes and goes I would check the fuel tank first .I am wondering if the smell is there and goes when the fuel is below a fitting (fuel sender etc) Fill her up and then check.

 

The fuel level sender is about half way up the tank, so if the gasket is leaking that could explain where the fuel smell is coming from. Not difficult to change, just make sure the fuel level is low before you start.

 

the sender gasket is definitely a good call, and not difficult to check, but if the leak is at the stage that you are noticing it in your mpg, that must be a reasonable amount of fuel being lost, which would be sloshing around in the boot by now if it was coming from the sender.
Has the car been serviced recently? The reason I ask is that if the fuel filter has been changed, it is the easiest thing to dislodge one of the little rubber o rings on the ends of the metal pipes. Everything bolts back together as normal, but it doesn't seal. The leak is only visible with the ignition on, and the system pressurised.
The other thought is corrosion of the fuel supply or return pipes. These run more or less parallel from the back to the front of the car, and are a known point of failure, usually in the area sort of under the rear seat, where they seem to catch road spray etc. A pinhole in one of these would get rid of a decent amount of fuel, but again, only while the system is pressurised ( and remember, both the supply and return pipe have pressure in them when the ignition is on).