Vacuum in fuel tank

[alloydog]

Over past year, I have had a couple of problems with our 307SW. One, I commented in Sluurp! Gurglwgurglegurle..., the other more recent in Dodgy CPU (or whatever it's called...)?.

I think the two are linked - I remembered back to when the we had the noisy fuel cap, once, the car refused to start, but as soon as I opened the fuel cap, it started OK. We've had the starting problem again. It happened for the third time in a about a month. This time, I remembered the fuel-cap business. When I open the fuel-cap, there was the sound of in-rushing air and a bit of gurgle. The started OK after that.

The fuel-cap doesn't have any obvious air hole in it. Anyone else come across something like this?

[jester]

You may find you have a breather pipe fitted on your tank which maybe blocked or kinked could be on the filler neck itself the air has to get into the tank somewhere to let the fuel out have a hunt about would be my advise.

[alloydog]

Yeah, I figured it something like that - I just wondered if it was a common problem, as 307SW seem to have more than their fair share of design faults or features.

[nige65]

This is a post off a jaguar site so may not be the same system but would have thought it would be very similar.

A bit of pressure release after the car has been standing on a warm day is normal. Any pressure release, or vacuum when the cap is opened just after the engine has been running is not normal and indicates a problem with the tank breather system.
If I remember correctly, the tank is connected to the engine intake manifold via the evaporative flange on the tank, a pressure vacuum valve, a carbon cannister, and an electric purge valve.
With engine off, the pressure vacuum valve should open at about 2 psi. The released gas passes through the carbon cannister, the theory being that petrol vapours are absorbed by the carbon, and the pressure is released to atmosphere. If that valve fails, there is another pressure valve in the evaporative flange that oens at about 3psi, and vents direct to atmosphere, and if that fails, a pressure relief valve in the cap (certainly on the convertible) will open.
With the engine running, vacuum keeps the 2 psi valve open all the time. Every so often, the ECU signals the electric purge valve to open, and when this opens, air is sucked into the manifold via the carbon cannister, and is designed to purge the cannister of the petrol aborbed.

Both the 2psi valve and the purge valve can get gunked up. The PV valve is on the side of the cannister (mounted in LH side front wheel arch). If you find the cannister, follow the small hosés, and one will lead to the purge valve. The Jaguar factory manual available on CD has a good description of the system, and, as mentioned earlier, so oes Kirby Palms book, but dont not get mixed up between the earlier and later types of evaporative control systems