So I just purchased a 1974 XJ6 series 2 4.2 litre. She needs some work which I was aware of but I’m already butting heads with the twin tank system and wonder if someone could short-circuit my investigations please? …
So, opening the boot, I was confronted with this. The 2 tiny things appear to be the fuel pumps which look very different to the SU pumps I was expecting to see.
When I collected the car, the owner demonstrated the twin tanks switchover and the fuel gauge registered 2 different fill levels in each of the switch positions.
Switch “In” – left hand pump can be heard to be operating/whirring away – release the fuel switch to “Out” and the left pump stops and the right pump likewise starts whirring away.
However, with the ignition on but the engine not started, the pumps seem to operate continuously.
I’m used to, in my MK 2, when the fuel pump has pressurised the fuel line and presumably filled the carb float tanks, the fuel pump would slow its ticking and then stop – but on the XJ6, I hear the pumps just continuing to operate (one at a time, depending on the switch position) – does that sound right?
(There’s no petrol leak anywhere – the system appears to be completely sound.)
And then any recommendation on where to look at why the fuel gauge no longer registers different tank fill amounts, when the fuel switch is moved from “In” to “Out” – It worked on Friday but now seems to only reflect the amount that’s in the left tank – whereas the right tank is much fuller – it’s been on a very bumpy gravel road to get her home but I checked the fuses and they seem sound - any suggestions on where to look for the source/s of this problem? …
Many thanks in advance – much appreciated – Dave J
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The ‘left/right’ pump selection seems to work as they should, Dave. Pump running continuously with ignition ‘on’ implies that the fuel safety circuitry has malfunctioned, or is bypassed. Check at the fuel safety relay. As Doug says; it is a safety issue - without the engine running, the pumps should only run in ‘crank’…If both pumps are running with ign ‘on’ it is plain wrong - power feed should only be according to changeover switch setting…so…?
Are you absolutely sure of the fill level in the tanks - ie, that the gauge is incorrect? If gauge registers the same level when switching; it may be that the tank levels are indeed equal…
The changeover switch has two entirely separate functions; one to switch power between pumps - the other to switch gauge ground provided by tank units. There are no fuses involved - but the switch’s ‘tank’ function itself might have failed.
If the tank levels are at variance with expected readings, some further checks are required. One-way valves, either separate or incorporated in the pumps, are used to avoid cross leaking. Failure of one or both sides may cause abnormal tank level readings - typically, while standing, fuel from one tank may migrate to the other. This may take some time, but will eventually lead to both tanks reading the same level - and so being indicated by the gauge. While driving; fuel may be pumped from one tank to another by the ‘active’ pump through the ‘passive’ pump…
The aftermarket pump may or may not have a one-way valve - or that function may have failed on either pump. And as Doug also says; continuous running may be a feature of either pump - but no pump should run with ignition ‘on’ and the engine stopped…
I am not sure your last statement is correct Frank - on my 1973 car pumps are on when Ignition is on. I think it was only later with EFI that the interlock of pumps on when cranking or running came in.
I agree. On cars fitted with the original SU pumps, when you switch on the ignition you can hear the pump clicking until it reaches pressure. If you change to a different pump it will run continuously if there is no pressure cutoff of it doesn’t create enough pressure to turn off. Some pumps run continuously with a built in pressure relief valve that opens at a predetermined pressure.
My preference would be to go back to the original SUs and return the plumbing to its original state. I would go with the solid state pumps that I use on my '72 Series 1.
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You are right, Mike - the fuel safety relay was introduced on carbed Series II. On the EFI the same function, no continuous fuel pump action unless the engine is turning, is performed in a different way.
Dave, i’ve used the fuel pump on the left, as mentioned they are continuous & will burn out (start smoking) the one on the right appears to be a pulse pump which i have but don’t use…i’ve opted for the su pumps, typical of the original switch to lose contact now & then but can be cleaned if you’re careful not to lose the springs & points… i opted to replace the original with a toggle but a 6 point rocker will do it as well, i can send you a colour schematic extract of the fuel system electrics if you want it, email>>> jagschemes@live.com.au …i have the same model & ended up not just replacing the original switch but using a toggle to switch power on/off to pumps as a type of kill system
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The pumps are designed to run continuously, Tom - but only with the engine running. One expects the driver to start the car and drive off - not having the pumps running in vain. A manual pump switch should not do anything more than the original switch does…?
The SU pumps do not run continuously. They get power when the ignition is turned on. You can here the selected pump clicking when you first turn the key. Once the float bowl is full and the valve closes, the back pressure in the fuel line is sensed and pump stops. When the float valve in the carb opens and more fuel is required the pump runs again, and so forth.
