Petrol expansion tank connections

[Doddsy333]

I am restoring my Series 3 Roadster and would appreciate help with advice on how the fuel expansion tank in the near side rear wing connects to the main petrol tank. There are four connections on the expansion tank but only three on the main tank. Where does the fourth connect?

[vacbag]

If it is the same as a series 2 and I think it is then the fourth connection goes to the inlet manifold. All to do with US emission laws.
Steve.

[Doddsy333]

Steve, many thanks for the prompt reply. My series 3 does not have exhaust emission equipment fitted . I cannot find reference to how this should be connected in the manual or parts book. Can anyone please advise how this should be routed.

[vee12eman]

Hi Doddsy,

Sorry for a slow answer, I had to do a bit of checking of my own car, but as I?m sure you are aware, it is difficult to see anything once installed. You don?t mention the year of your car, but as far as I can see, there were two designs of tank used on the Series III, one which has three connections, another with only two. My car has only two (it is an early UK spec car). However, I had the same problem as my tank, indeed the whole boot and its? contents were not fitted when I bought the car (although I know the parts are original). No-one was able to offer help to me, so I had to reason it out for myself, so I hope this helps:

If petrol gets in to the expansion tank, through heat changes, hard cornering or over filling, the expansion tank is designed to retain this, but, as in a proper vacuum tank cooling system, return the fuel to the tank as the fuel is drawn from the main tank, creating a vacuum which will draw fuel back in. What you will have to do is get a piece of wire and investigate the expansion tank, unfortunately I did not record my fittings for posterity, so you will have to repeat my reasoning! The tubes going in and out of the top of expansion tank differ. From memory, two go all the way to the bottom of the tank, two end at the top of the tank immediately after entering.

Now as my main tank appears slightly different to yours, you will have to check all the pipes exiting both the main and expansion tank. I think there is one pipe which goes to the bottom of the main tank. This feeds the expansion tank as the fuel expands or sloshes about and connects to the pipe ending immediately after entering the expansion tank; petrol falls to the bottom of the expansion tank from this pipe. Another pipe feeds from the top of the main tank (i.e. ends immediately after entering the tank). This is connected to a pipe going all the way to the bottom of the expansion tank, and as the fuel drains from the main tank, it sucks fuel back from the expansion tank. This is where our systems differ, as I have only two pipes on the main tank, so you will have to work out where the final pipe goes, but my guess is that it will finish high in the main tank and low in the expansion tank, acting as a vent for the main tank, but maintaining a vacuum if there is fuel in the expansion tank, to allow it to be drawn back into the main tank.

This leaves one connection on the expansion tank, which I believe is the one you still have disconnected and will be the same regardless of how many connections there are on the main tank and is designed to vent the system. If I am right, this connection should end high in the expansion tank. As Steve commented, this would go back to the Emission Control system of cars so equipped, so the vacuum at the manifolds draws the vapour into the carburettors. If no emission control is fitted, this connection will vent to atmosphere and obviously you don?t want this inside the car, so the connection goes down to the vent hole just behind the left rear wheel.

I hope this helps, if not please post again and I will see if I can make it clearer, but if you do, let me know where all the pipes end in either tank, as this is what determines operation and, not being able to see your system, I may be making incorrect assumptions and so far it is all from memory anyway!

The general idea, whatever the design, is a vacuum between main and expansion tank, a suction from the expansion tank lower region to the main tank upper region and a vent, either to atmosphere (outside the car) or to the emission control system as appropriate.

Regards,

[Doddsy333]

Simon thank you for this excellent explanation which really helped. I investigated both tanks to try and identify which seemed to end at the top and bottom of the respective tanks. I also still had the old hoses so with reference to the attached picture, I believe this is how it is supposed to be:

The fourth connection D must therefore be the vent. My body shell is at the rust doctors right now so it will be a while before I can see where the external vent is but it does appear to be somewhere in the rear lefthand wheel arch as you mention, I persume through a grommet of sorts. :D

[vee12eman]

Hi Dodsy,

Glad I could help, although as I said, this was only my reasoning of the system, but it made sense to me (eventually).

I noticed an error in my post in that I said the connections enter the expansion tank at the top, whereas they actually enter at the bottom, however, the principle remains the same in that the pipes actually terminate in either the upper (dry or vent region) area of the tank, or the lower (potentially wet or overflow region, i.e. the area which will contain any fuel which does overflow into the expansion tank) area. Judging by the orientation of the components in your photo, you have realised this, but it is worth mentioning all the same.

Regards