dual fuel tanks

Is it like the Patrol, which has an auxiliary tank with electronic switch to empty it into main tank, which is always drawn on? Better than the switching mechanism which used to stuff up and leave me stranded on a Cruiser we once had.

Volkan
the reason for this is that ULP goes off

so the return from the injectors when on sub returns to the main tank,

If you continue to use the sub tank the main tank will full up

always run out both tank on a rotation bases

its called idiot proofing (sorry)

Richard

On a previous vehicle, I had a dual tank system which used solenoid valves to switch tanks. Two valves were used in a single tandem assembly; one for delivery and one for the return line. I found that cr@p caught in the valve sometimes caused the return line to not switch to reserve. This caused the return fuel to go back into the main tank when drawing off the reserve, causing the effect as you described above.
Gerry

Joc is on the money. Some of the responses above are incorrect guesses. The 90series Prados have two solenoids, so the fuel coming from the main gets returned to the main, and the fuel coming from the sub is returned to the sub. Sometimes the fuel return solenoid sticks and fuel from the sub will continue to be returned to the main. Often this is a one-off, and doesn't bother you again - sometimes you need to fix the solenoid.

Second possibility: Have a read of your manual and it will tell you that some fuel transfer will occur between tanks. This is deliberate because each tank on a petrol V6 Prado has an in-tank pump. These pumps can't be allowed to run dry, so are deliberately kept moist via a small fuel transfer. This doesn't happen on the diesels as they don't have a pump in the tank.

From the amount you mention, I'd guess the solenoid is the cause. I wouldn't do anything about it though. If it were to occur again, you can simply switch back to the main to stop the problem.

Cheers
Phil