My plan is to have a large SVO tank in the boot of the car and a smaller heated tank under the bonnet. SVO will be pumped from the boot tank to the heated tank with a windscreen washer pump. Any ideas how to regulate the level of fuel in the heated tank? I want a system that cuts in when the tank is low then allows the tank to fill and then when full cuts off the power to the pump ... ie so the pump isn't running continually. Any ideas would be appreciated.

Cheers gavin

Are you sure a windscreen washer pump will be up to coping with the additional viscosity of the SVO?

You might be able to salvage suitable level switches out of an old washing machine. Alternatively, when I was in the UK, an outfit called K. R. Whiston used to sell all sorts of useful, surplus, engineering bits and pieces and their catalogue often contained things like level switches. If they're still around their catalogue is worth a look for anyone tinkering with things mechanical.

I wrote several other options before thinking of this one: many high-end cars now have washer fluid sensors - to tell the driver when the reservoir needs refilled. Check wrecking yards. Use one sensor for "full", and another for "empty". It then takes a simple latching relay circuit, or you could use a timer and eliminate the second sensor. Ask me directly for details if you need help figuring it out.
Have you checked the washer pump's compatibility with the oil? What do you do when the weather gets cold and the rear tank solidifies?

I don't know what you call them, but I just ordered a couple submersible 12V oil and fuel pumps - I'll see if I can repair the pump I already messed up, since it handled cold veg oil fine (but I think I twisted an internal electrical lead), but these new ones may be a good alternative (I'd offer them to the forum if someone needs one).

Hey Gavin,
A standard fuel level sensor (that sit in ones tank) changes voltage with fuel level.
An op amp can be set to turn on or off at what ever voltage you chose. If you don't know electronics this could be over your head.
Master cylinders on brakes and antifreze resevoirs often have level sensors too. Some of the floats are styrofoam so you might have to make a float that doesn't desolve in vegy/diesel.