Jaguar Repair: 1975 Jaguar XJ4.2C Series 2, vent connection, pressure control valve

Question
Fuel Tank Vent.
Good day to you sir.
I am replacing my fuel tanks with new units, which unfortunately are not identical, as they are unavailable any more.
The old units did not have a piped vent connection, just a small 1/4" connection on the top with a rubber cap....I presume if the pressure in the tank rises the rubber expands and releases the vapours? (There were signs of this on the tank around this point)
The new tanks, made for the series 3 have a threaded connection near the top of the tank, which I presume is either a fuel return or vapour connection for the emission control system fitted to later cars?
I have heard of over pressurising fuel tanks!
Question is...Do I just plug this "vent" connection, or do I need to pipe it away and fit a rubber cap as the old tank.
Note the location means access is extremely difficult with the tank in situ.
I would appreciate your insight.

Regards

John

Answer
Hi John,

I am familiar with the series III tanks (US version) and they had one large fuel outlet at the bottom and two other connections. One at the top was a fuel return pipe and a vent pipe. The vent pipe was a plastic line that went all the way to the right front fender just behind that headlight to a charcoal canister. (I believe the ser II car has a vent hose connection on the filler neck.) The return pipe connected close to the filler neck to return excess fuel sent to the injection system. It has been years since I pulled a tank down but as I remember the vent pipe was a push on fitting and the return line was a threaded fitting.

The ser III cars had a charcoal canister behind the right front head light and the vent pipe went to a pressure control valve that maintained about 3 PSI on the tank system at the canister.

The fuel was heated by the pump pressure and the heat from the road so in the ser III cars it was sent to the AC compressor to be cooled before it was returned to which ever tank was selected. The cooling was not sufficient to contract the fuel all the way so pressure was built up.

I don't know how much Jaguar depended on the pressure in the tank to help the fuel pump push the fuel all the way to the front but you may want to keep that in mind as you adapt the tank and line system.

I have not worked on the tank system in the earlier systems and I don't have any info on what was original on the ser II so you will need to look for any vent lines and any pressure valves in those vent lines to see if they pressurized the ser II cars. I worked on a few of the ser II cars but don't remember any return lines on the Stromberg carburetors.

Howard