Let me preface this by saying what I am going to describe is how the system
is set up on early US models. The fuel filler cap is non-venting. The car draws
gas via the fuel pump, runs it to the fuel rail where rail pressure is
maintained by a vacuum modulated pressure regulator, and excess fuel is sent
back to the tank. The engine bay temperature, along with air temperature, raises
the temperature of the fuel and a positive pressure is created in the tank.
There is a vent system attached to the top of the tank which leads to a rather
fancy vapor recovery set-up which includes a charcoal canister and several
In the vent line between the tank and the vapor recovery system is a 2-way valve
to prevent excess pressure in the fuel tank (vapor is vented to the canister
and later burned in the engine or sucked back into the tank) and to allow air to
get back into the tank when the car is stopped and the fuel cools down (prevents
the tank from collapsing due to the resulting vacuum; air is drawn back through
the charcoal canister). As I recall, more pressure is required to vent from the
tank then to suck air back in.
Were your car a US model, I would say it definitely should make a whoosh
when you open the gas cap, especially when the tank is near empty (larger
volume of vapor in the tank).
Brett and others, with somewhat later cars, have stated they have never had
any pressure in their tanks. Maybe Jaguar did away with the bi-directional
valve or the valve on later cars operates at such low pressures that escaping
vapor when the cap is opened is not noticeable? I have no idea. I do know that
on the '87-'89 cars a certain amount of positive pressure in the tank is
As for your problem, how often was it occurring? If it was a regular thing, the
only work done was eliminating the vent system, and the problem has not
recurred then I would say the problem may have been the valve (left side of the
fuel tank, hose to top of tank and another through boot floor), or the charcoal
canister is clogged, or possibly the line from the valve to the canister is
clogged or collapsed. If it were my car, I would do some more troubleshooting
and try to fix it correctly.
There is some info in the archives. Try searching using “fuel tank pressure”.-
Pete Crosby (@Pete_Crosby)
In the sunny southland (Marietta, Georgia, USA)
'88 BRG XJ40
'60 BRG MKII
Date: Sun, 02 Jul 2000 14:15:56 +0200
From: Daniel Nilsson email@example.com
Subject: [modern] Tank woooosh…
I have had some problems with my car stalling like if their was no gas.
My garage told me it could be that gas “boiled” and/or the tank ventilation.
Before my garage did their trick, when opening the gascap it created
this wooshing thing.
Now it doesnt do that anymore, so I guess I do not have any excessive
pressure in the tank anymore.
But, I remember some listmember said that it SHOULD make the woosh-thing.
I search the archive but I can not find anything on it.
Should it wooosh or not ?
ï¿½87 Jaguar Sovereign 3.6