sub tank - main tank fuel consumption

[MRW82 Member Find all posts View Profile Close MRW82]

Hi guys,
the last few tanks of fuel i've been working out the consumption figures, the amount it's using seems to be on par with what others get so that isn't a concern to me. but I have noticed a difference in consumption between sub tank and main tank.
Filled up both tanks and ran off the main until it coughed and spluttered. flicked over to sub, refilled main and calculated consumption (using km's at time of tank change) and I got 16L/100km.
then kept running on sub until it coughed and spluttered, refilled and calculated consumption. got a reading of 13.4L/100km.
same driving conditions, same driver, carrying the same amount of rubbish on the passanger floor etc.... nothing drastic changed, except consumption. I have always thought the main tank seems to go down quicker then the sub (taking into consideration the main is 25 litres bigger) but this is the first time i've had a noticable difference in usage.

I'm thinking it could be the return line solenoid not switching when running on the main tank, so main return feeds into sub. but haven't had a good look at the solenoid setup so could be wrong about it all.
has anyone else had a similar problem? does anyone have details of how the tank switching is setup? do the return line switch tanks with the feed line or do they just feed to the sub. (I remember reading something about the main trickle feeding to the sub to promote usage of the sub tank, is this all it is?)

either way a combined tank usage of 14.7L/100 km while running 265/75r16 muddies isn't too bad really, especially with my heavy foot.

[raydes Advanced Member Find all posts View Profile Close raydes]

Re: sub tank - main tank fuel consumption

Hi use the search ... fuel return ... pages of info ..When you run the car with the main tank the excess fuel is returned to the sub tank . When you run the car on the sub tank the excess fuel is returned to the main tank . Toyota did this so neither tank is run dry and the pumps don't run dry or dry out. To work out your fuel usage you would have to fill both tanks then run the car for a while then refill both tanks because you will have no way of knowing how much fuel is going back and forth between tanks.

[infraredfotografy Junior Member Find all posts View Profile Close infraredfotografy]

Re: sub tank - main tank fuel consumption

Hi use the search ... fuel return ... pages of info ..When you run the car with the main tank the excess fuel is returned to the sub tank . When you run the car on the sub tank the excess fuel is returned to the main tank . Toyota did this so neither tank is run dry and the pumps don't run dry or dry out. To work out your fuel usage you would have to fill both tanks then run the car for a while then refill both tanks because you will have no way of knowing how much fuel is going back and forth between tanks.

:P absolutely this is the answer for my same question also.

I was concerning about the use of sub and main tanks some times when I noticed that it seems that between sub and main tanks there is a connection that fuel can be transfered. Because I noticed the strange thing happened with my fuel when I fill both of them and run on the main tank, the sub tank is a little bit higher, and the same thing happened with the main tank.

Now the concern is clear. Thanks bro. :mrgreen: :mrgreen:

[Brett1979 Avid PP Poster! Find all posts View Profile Close Brett1979]

Re: sub tank - main tank fuel consumption

The main tank spills over a little into the sub tank if it's full through the normal course of driving. It's the way it's designed. If you check in your manual you'll see the fuel system is designed to do this to encourage you to use the subtank after a period of time if you only fill up the main tank, to help keep the subtank fuel pump lubricated and in perfect working order. Once when I stayed at a 4wd park for a few days, I had both tanks full to the brink, and ended up using the subtank first. Before I switched over to the main tank, the main tank was already depleted down to just below 3/4 full. This is because of the increased camber angles the car was on allowing so much fuel to spill over from the main tank to the subtank. I can't say I've experienced fuel spill over the other way from the subtank to the main tank though. Only from the main to the sub.

[infraredfotografy Junior Member Find all posts View Profile Close infraredfotografy]

Re: sub tank - main tank fuel consumption

I can't say I've experienced fuel spill over the other way from the subtank to the main tank though. Only from the main to the sub.

I don't think that the subtank can spill back to the main tank.

I let my car run on main tank until it could not run the engine, then I switch to subtank to use the subtank. When I drive with the subtank, there was no evidence of spilling back to main tank from the sub.

Later on, when I refill both tanks, I found that the main tank only keeps 3L of petrol in it.

[RogueAussie Senior Member Find all posts View Profile Close RogueAussie]

Re: sub tank - main tank fuel consumption

I beleive it only transfers from main to sub ... regardless ... the actual reason is not to run tanks evenly or "encourage" the use of the sub, it is simply to keep the transfer pump primed / wet. This helps to maintain the pump and reduce wear.

As for the use of tanks, it is a little common sense. If you don't want stale fuel, use your main 2 or 3 times then run the sub down every 2nd or 3rd tank. To me that will "encourage" use and avoid stale fuel. It's not that hard to flick a switch.

So anyway MRW82, the difference you are getting in the consumption values is simply the difference in tanks size coupled with the fuel transfer between tanks. The only way to measure your consumption acuratly is to either a) work out the rate of transfer and take that into account in your calculations, or b) buy a computer compatable with your vehilce. I know the Scangague II is no good on the 90 series but I have been tossing up the idea of the PLX Kiwi WiFi. Seems that it is compatible and my help with some of the readings.