Twin Tanks

I’ve just been cleaning out the underseat tank getting all the crud out of bottom. I’ve never really used it properly as it stalled soon after switching over and was never sure if it was a vacuum problem or blocked intake tube, so it’s piped up again, new intake pipe and gauze , wired switch for both tanks on fuel gauge and going to fill up tomorrow and go for a run, the return spill pipe just goes to main tank, question is when running from lh tank any ideas how much diesel will be returning to main tank via spill pipe , easy enough to test just wondered if anyone looked at this before.

steve2286w said:

I’ve just been cleaning out the underseat tank getting all the crud out of bottom. I’ve never really used it properly as it stalled soon after switching over and was never sure if it was a vacuum problem or blocked intake tube, so it’s piped up again, new intake pipe and gauze , wired switch for both tanks on fuel gauge and going to fill up tomorrow and go for a run, the return spill pipe just goes to main tank, question is when running from lh tank any ideas how much diesel will be returning to main tank via spill pipe , easy enough to test just wondered if anyone looked at this before.

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Normally a diesel will be putting a fair amount of fuel back through the return pipe so I would expect it to drain into the main tank at a steady rate, not instant, but quick enough to be something to be aware of. I do not have a figure for a series as have never needed it but i have had leak off pipes fail on later models and they pump a lot of fuel out!

tanks have to be linked or have 2 separate return pipes

Agreed, I have my return to one tank which I use a the "main" tank and the feed from the other goes to a fuel pump then to the main tank. I have a switch thet changes the gauge over and a switch to run the transfer pump. Its works well as a system. My change over switch was missing and the ransfer pump was aleary fitted so I went down that route. The other option is to use a Pollak valve, these are designed for twin tank marine deisel set ups and switch the feed, return and gauge and are sized for feed and return. I used one on a veg oil conversion with twin tanks, worked a treat but not cheap.
https://www.oilybits.com/pollak-6-port-fuel-selector-valve-with-switch.html

Thanks for tips, I hadn’t considered spill pipe leaking which I need to check, a cheap 3 way ball valve should do , I think I’ll check the flow out of curiosity, if it’s too much , Is the CAV pump easily adjustable to reduce the flow slightly to injectors

steve2286w said:

Thanks for tips, I hadn’t considered spill pipe leaking which I need to check, a cheap 3 way ball valve should do , I think I’ll check the flow out of curiosity, if it’s too much , Is the CAV pump easily adjustable to reduce the flow slightly to injectors

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no ,theres allways a return,so you need to return to the tank your drawing from

Understood thanks , the valve should divert return to either tank

a sorce of trouble I got rid of mine to much pipe work

The original changeover valve is 3 way and is only availble for petrol, to change over a diesel you need two 3 way valves linked to work together preferably linked to an electrical switch to make sure the gauge is reading the tank you are using. The Pollak does that and despite being £70 it hard to do it cheaper once you take account of all the time and messing about. The alternative is the "main" and "researve" tank option where the engine is always fed off the same tank and fuel pumped across when needed. This is simple and works fine, it just costs a couple of switches and fuel pump (one switch for the pump and one to swap the gauge reading over)

I'm going for the Pollack changeover solenoid on my twin tanker petrol. I plan to use a DPDT switch on the dash to changeover both the fuel solenoid and the fuel gauge senders so that the sender on the 'live' tank is the one that the fuel gauge reads from.

The simple three-port solenoid fuel valves are not that expensive, but of course, being a petrol, I don't have to worry about a return line.

The big advantage of the Pollak is that it is motorised not solenoid so it drives from one position to the other then stays there without drawing power or moving when the ignition is turned off. This also means it doesn't get hot.