Fuel cross flow

[Tintin]

I have the standard arrangement of two tanks and one (new) SU pump for each tank in the boot of my S1. The pump switch on the dashboard simply runs the LHS or RHS pump as selected and the relevant pump sends fuel from the appropriate tank to a "T" piece on the delivery pipe and thence via a common pipe to the engine bay.

Annoyingly, there is a cross flow of fuel between tanks over time and eventually the levels in each even up. Earlier this summer I filled both tanks to the brim and ran on the LH pump exclusively only to find yesterday there was little fuel remaining in the RH tank when I changed over (nearly ran out!).

The series 2 and 3 cars have a fuel valve solenoid in the pipe work - presumably to stop this happening. Can I fit that valve (CBC 4269) to my system to cure the problem and can anyone advise me of the correct electrical and fuel line connections to make please?

Thanks in advance.

Robbie

[oldtimer]

Hi Tintin
The early model S type with two petrol tanks has exactly the same fuel system layout as you describe for your carburettored XJ6. As my '66 S type was my daily commute to West London the fuel did not stay in either tank long enough to experience the problem you describe. By virtue of the SU fuel pump design there is inevitably some leak through the valves based on the longevity of the fuel load imbalance. I guess it is a problem you will have to live with.

You mention that Series 2 and 3 motors have a fuel control valve in the system but it should noted that both of theses went over to fuel tank immersed pumps. I'm not sure of the detail but it might co inside with the introduction of fuel injection.

My detailed interest has been with the V12 version with carbs where the layout is a little different in that it has return fuel lines requiring control valves.

I have a couple of the valves to which you refer in the workshop, I acquired these in connection with an investigation as per the sentence above.

You are welcome to see one of the valves should you consider a bit of plumbing to introduce a control vale.

Hope that helps alittle

[piman]

Hello Robbie,

as both tanks seem to be emptying together it seems that the possibility is that both pumps are live at the same time? I know that is not how it is supposed to work but there is little likelyhood of a back feed to the other tank of that magnitude due to the valves in the S.U. Pump, nor would that empty the spare either? The proof of the S.U. pump valves is that the pump stops ticking when the floats are full.
The other possibility is a leak from the RH tank?

[Tintin]

Thank you both for your comments. I put a meter across the pump live terminals today and got 12v from each when switched "on" and less than 0.1v when "off", so I am as sure as I can be that they are not running concurrently.

The volume of fuel "lost" from the second tank is such that I am sure I would have noticed if it was due to a leak. In addition, I have now checked the mileage covered since the last fill up and assuming all the fuel from both tanks was burned in the engine I am returning about 13 mpg.

Both tanks self pressurise to a considerable degree due to ambient temperature and/or heat rising from the rear silencers (when the filler caps are released there is a "whoosh" of escaping vapour and the tanks creak as they take up a more relaxed shape). I have replaced the filler cap seals which may have accentuated the issue. As fuel from one tank is used the pressure difference between the two will become considerable and I think it may be this difference that may overcome the pump valves.

To confirm the position I may use a hose clamp on one of the tank feeds, as much as dislike using those gadgets long-term due to the risk of hose distortion.

Robbie