Dual Tanks - How Do They Work?

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

As in the title, I'm trying to work out the various ins & outs of our recently acquired '97 Prado. 3.4 GXL with the dual tanks. It didn't come with an owner's manual. When we bought it, it was showing about 1/4 on each tank. When the main showed empty & the warning light came on Wifey changed to the sub & I took over as fuel is cheaper near my work. I put 63 litres into the sub (59 litre tank?) & another 67 into the main - both gauges showing full. We then used 1/2 a tank from the main, re-filled & switched to the sub - ie: the green light in the sub-gauge is lit, rocker switch in "on" - & carried on. I've now done 190 km & the sub-gauge hasn't budged but the main has dropped to 3/4.
Inescapable conclusion is that I'm drawing from the main, OR that the main feeds into the sub & then to the engine. Any ideas?

The only other 2 twin-tank 4WDs I've had were: 1/ an old Jackaroo with the twin 50-litre tanks & that confused me for a while until I figured out that the sub on that pumped into the main when "switched on" - that way the fuel in the sub always got used & Isuzu (sorry, Holden) got away with just one gauge.

And: 2/ an old BJ40 307 Chev. SWB 'Cruiser with twin 60-litre tanks under the front seats. That one was a simple suction/siphon system whereby the 2nd tank was always being siphoned into the main - same reasons as above.

So - how does the Prado work? Oh, I've done a search already but couldn't find anything specific on this setup. Thanks in advance guys.

[K & M McF Senior Member Find all posts View Profile Close K & M McF]

The sub-tank is actually 69 litres, hence why you could put 63 litres into a "59 litre" tank.

The fuel should be taken as follows (at least it does on mine):

• if main and sub are full and you use main - only takes from main
• if main is full and sub is less than full and you use main - seems to only take from main
• if you use sub and main has more fuel than sub - it seems to mostly take from sub but also transfers some from main to sub (ie the main goes down)
• if you use sub and main is very low - does not seem to take much, if any, from main

Therefore, from what you posted, yours does not seem to work correctly.

Hope this helps a little.

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

Thanks for that. Seems a pretty screwy system. I'll have to check it out some more.

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

• if main and sub are full and you use main - only takes from main
• if main is full and sub is less than full and you use main - seems to only take from main
• if you use sub and main has more fuel than sub - it seems to mostly take from sub but also transfers some from main to sub (ie the main goes down)
• if you use sub and main is very low - does not seem to take much, if any, from main

I don't believe this is correct. Fuel is supposed to be drawn from which ever tank is selected. The system doesn't make its own mind up based on the levels in each tank.

There seems to be a lot of confusion surrounding the twin tank setup on the 90 series. It is far simpler than K & M McF's explanation.

When the sub tank is selected via the rocker switch, fuel is being drawn from the sub tank and circulated through the fuel rail and then returned from to the sub tank.

When the main tank is selected, fuel is drawn from the main tank and circulated through the fuel rail and returned to the main tank.

The owners manual does state however that if you are running on the main tank when the sub tank is low, fuel returning from the injector rail is directed to sub tank to fill it up. I don't how this is achieved though. As far as I know there is no ECU controlling the fuel change over solenoid, so it could be an electromechanical setup.

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

oh how something so simple can get all complicated

Declan has most of it correct except for the final bit about main feeding the sub.
My owners manual and experience has shown the when running off the main tank, a small portion of the returning fuel is diverted to the sub tank all the time. This is so the fuel pump in the sub tank is kept lubricated and cool by a minimum of fuel.
Re the original post ..I have seen instances where the solenoids/valves have failed and the tank supply does not change. Also another fault is that the return supply solenoid fails and the fuel return stays to the same tank (sub or main) irrespecitive of which tank is being drawn from.
Esses how far did you run on the sub tank before filling up ? My fuel light comes on at about the 60 litre fill point (main tank) so perhaps you have never run on the sub tank, that is the tank switch system is not functioning at all.

Lee

[gphcald Member Find all posts View Profile Close gphcald]

oh how something so simple can get all complicated

Declan has most of it correct except for the final bit about main feeding the sub.
My owners manual and experience has shown the when running off the main tank, a small portion of the returning fuel is diverted to the sub tank all the time. This is so the fuel pump in the sub tank is kept lubricated and cool by a minimum of fuel.
Re the original post ..I have seen instances where the solenoids/valves have failed and the tank supply does not change. Also another fault is that the return supply solenoid fails and the fuel return stays to the same tank (sub or main) irrespecitive of which tank is being drawn from.
Esses how far did you run on the sub tank before filling up ? My fuel light comes on at about the 60 litre fill point (main tank) so perhaps you have never run on the sub tank, that is the tank switch system is not functioning at all.

Lee

My understanding is that the fuel return is always to the sub tank, this is to keep the sub tank fuel pump lubricated through use.
on my diesel 90 2001 if i only use the main tank and keep on filling only the main tank the sub tank slowly (very slowly) starts filling up, but it takes a fair few tank fulls.
i did try it reversed using the sub tank and the main didn't appear to fill up, but it wasn't very scientific
just my 2.2c (inc GST) worth

Rgds

Gerry

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

oh how something so simple can get all complicated

Declan has most of it correct except for the final bit about main feeding the sub.
My owners manual and experience has shown the when running off the main tank, a small portion of the returning fuel is diverted to the sub tank all the time. This is so the fuel pump in the sub tank is kept lubricated and cool by a minimum of fuel.
Re the original post ..I have seen instances where the solenoids/valves have failed and the tank supply does not change. Also another fault is that the return supply solenoid fails and the fuel return stays to the same tank (sub or main) irrespecitive of which tank is being drawn from.
Esses how far did you run on the sub tank before filling up ? My fuel light comes on at about the 60 litre fill point (main tank) so perhaps you have never run on the sub tank, that is the tank switch system is not functioning at all.

Lee

I think that's probably right. When the main tank warning-light came on we switched to sub (we thought) but obviously were still drawing from the main & were probably lucky in that case not to run out. I probably drove 40 km before filling up. We then went for a drive in the bush on the main, filled it up & again switched to sub (we thought) but are obviously still only drawing from the main. I'll get underneath next weekend & tinker some.

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

My owners manual and experience has shown the when running off the main tank, a small portion of the returning fuel is diverted to the sub tank all the time. This is so the fuel pump in the sub tank is kept lubricated and cool by a minimum of fuel.

That's probably the part I read and partially forgot. Close enough

If running on the main tank is constantly drip feeding the sub tank, when the sub tank is full it would start overflowing through the filler neck back into the main tank.

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

Esses,

From memory all the solenoids/valves are located on the l/h side just forward of the sub tank.

As Declan said if you never use the sub tank eventually it will overflow.
And if your really lucky all the return will stay flowing to the sub tank all the time and you can watch the sub tank guage rising/filling as you drive on the main! In 2009 at the cape a Prado travelling with me had that happen, he started leaving a petrol trail while he drove as the sub tank over filled and pressurised.
This forced fuel up to the carbon cannister which in turn bleed from its overflow into the chassis rail, yes thats standard design ...
The fuel then drains along the chassis rail and leaks out underneath, supposedly away from any heat source !

Lee

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

When OFF road, I prefer to drain the main tank first, so that the weight on the rear axle is reduced (because the rear tank is at the back of the car).

When ON road I prefer to drain the sub tank first so that when the fuel light comes on I know that I actually need fuel and I then proceed to fill up with ~150L or so. I always drain both tanks completely and then fill them both completely.

From the comments above, it would appear that best practice would be to drain the sub first.

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

From the comments above, it would appear that best practice would be to drain the sub first.

It wont matter either way. The sub tank can over flow back into the main tank through the filler neck any way.

[Steve M Addicted PP Member Find all posts View Profile Close Steve M]

When ever you start the vehicle it will always transfer some to the reserve tank, both my 95's did this. When I first bought the 95 petrol grande I thought holy shit this thing sucks the juice but then worked out my reserve was empty so every time I was starting the vehicle which was alot of short runs around town/daycare/shops/work all in a 9 klm vicinty it was transfering from the main to the reserve tank. 9 times out of 10 I would run off the reserve around town and keep enough in the main to keep fuel light off but eventually due to it transfering on start up the light would come on for low fuel in the main tank. When off road I aways used the main first to reduce the rear over hang which basically is 90kg hanging off the rear end, some times would run it till it started to splutter then flick over to reserve, might have been lucky but never had any issues with shitty fuel or blocked filters, Worst thing I ever did was convert to LPG big mistake as far as Im concerned, and the dodgy set up which is on every 95 with LPG that I have seen are really bad for petrol fumes. This is another story going LPG. Cheers Steve

[Steve M Addicted PP Member Find all posts View Profile Close Steve M]

Doesnt matter which you drain from first. As above I only used the main first to reduce overhang in the rear, I have had where our little one has flicked the switch over without me noticing or the wife had knocked it what ever. you have the little green light to advise you when the reserve is in use. No big deal which you choose to use. Must say thats one thing I odnt like about the 120, you have no choice on which tank to use, and you think the way it moves around between tanks in the 95 is weird, the fuel guage in the 120 will have you scrtching your head till you work it all out. Two full 90L tanks guage is on full, use all of the main tank and the guage will show around half/just under then it flicks back to full but then little green light coms on also indicating 90 reserve, then the guage will work its self all the way down to empty then you are empty no reserve left !!!!!!! Did you get all that LOL.....Cheers Steve

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

So for the last month I have try to observe the fuel transfer between the tanks. I ran the sub tank till it was as close to empty as I was comfortable with (I didn't want the pump to run dry) and then ran on only the main tank for a couple of weeks and the sub tank needle rose a steady pace till it was completely full. So it definitely transfers to the sub tank when running on the main tank.

I have not been able to observe any fuel transfer from the sub tank to the main tank. That doesn't mean it doesn't happen though, maybe I haven't ran my test for long enough to properly measure it.

[Steve M Addicted PP Member Find all posts View Profile Close Steve M]

It wont transfer from the reserve to the main only the other way round. If you have a look in the actual handbook it will tell you in there that it will transfer from main to reserve on start up, its to keep the pump lubricated ( I think some goes across just in general driving as well although they dont mention that). I remember reading it in my book as not realizing how it worked I thought I had just bought the biggest fuel guzzling vehicle ever produced as the main fuel guage dropped that quickly over two weeks for the small amount of klms I had done but wasnt till I saw the needle on reserve tank going up bit by bit over a couple of weeks did I realize some thing was going on, then I read the book.. Sounds like everything is working fine on yours nothing to worry about. Hey did you look at that patrol ?? Cheers Steve