Dual Tanks - How Do They Work?

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

A ball of rubbery gloop. In the 3-way junction from the 2 fuel tanks. Blocking the one-way valve from the sub-tank. That's all it was. But that took quite a few hours by my Mechanic & a complete dismantle of the fuel system between the tanks & the filter. Everything now works as it's supposed to so now I just have to run it on the front tank & see how it goes.

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

Wow, wonder what the hell that was ?? You would think petrol would dissolve most things.........Glad you fixed the problem and now dont have to worry...Cheers Steve

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

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

Had a look in my owners manual, and it does not say that, mine only says "that the main and reserve tanks return to the reserve tank to promote the use of the reserve tank".
Seems strange that there are differences.

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

Cant check my books anymore dont have the vehicles but my petrol was also 97 vx grande (same as yours) but would say what you have said is write, although Im sure my'n mentioned some thing about pump lubrication ?? (maybe different section of the book ??) Maybe I read this some where else and thinking it come from the book, couldnt imagine the books been any different...Definately dont remember anything about ""promote use of reserve tank"" but you have it there in front of you so I obviously am quoting my comment from some where else that I have read it ??? cheers Steve

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

May have been in a repair manual like "Gregorys", ect. They are right in most cases, but when they get it wrong, they really get it wrong...lol

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

I agree with Declan, the sub tank has a separate pump, filter, fuel return system and also an overflow pipe connected to the main tank. This system as I understand it works thus;
With both tanks full and the switch on main the main pump and BOTH fuel return systems are active. Thereby drawing fuel from main and returning to both.
When the main is empty or as near as you want to let it go and the sub is full, to continue travelling you switch to the sub fuel tank.
Now the sub pump is active and fuel is returned to only the sub tank.
It is recommended by toyota to switch your fuel useage between tanks at least once a month to keep the sub pump lubricated and stop components both mechanical and electrical from the damage of non-use.
Personally i change tanks every other week, have both full then use one, refill, switch to sub, drain, refill and repeat cycle.
This keeps everything hunky dory, little sediment build up and means both filters get replaced at the same time.

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

Agree with that but it will still send fuel to reserve tank if main tank is full and reserve is not full.Cheers Steve

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

Seems like just as screwy a system as our old Jackaroo - the front 50l tank being the main & always used first - sub-tank the back 50l job which had a seperate internal pump that started transfering all 50 litres straight to the front tank as soon as you flicked over. Gave you a heart-attack watching the gauge drop like a stone 'till you switched back & realised where all that petrol had gone! S'pose it acheived it's aim - MADE you use the fuel in the sub instead of just sitting there.
The other systems I've used were a twin 60-litre setup in an old SWB 'Cruiser. The left tank siphoned into the right tank so it was effectively a 120 litre tank. Mini-Cooper's with twin-tanks just draw from both all the time via a simple T-piece. "The Right Way, The Wrong Way & the BRITISH Way". I think "British" should be replaced by "Japanese" these days!

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

Seems like just as screwy a system as our old Jackaroo - the front 50l tank being the main & always used first - sub-tank the back 50l job which had a seperate internal pump that started transfering all 50 litres straight to the front tank as soon as you flicked over. Gave you a heart-attack watching the gauge drop like a stone 'till you switched back & realised where all that petrol had gone! S'pose it acheived it's aim - MADE you use the fuel in the sub instead of just sitting there.
The other systems I've used were a twin 60-litre setup in an old SWB 'Cruiser. The left tank siphoned into the right tank so it was effectively a 120 litre tank. Mini-Cooper's with twin-tanks just draw from both all the time via a simple T-piece. "The Right Way, The Wrong Way & the BRITISH Way". I think "British" should be replaced by "Japanese" these days!

No it is a simple system, as I said before, according to the owners manual, when using the sub tank, it will return unsued fuel to the sub tank, when you use the main tank, it will return unsued fuel to the sub tank, so in short, both tanks will return unused fuel to the sub tank.

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

No it is a simple system, as I said before, according to the owners manual, when using the sub tank, it will return unsued fuel to the sub tank, when you use the main tank, it will return unsued fuel to the sub tank, so in short, both tanks will return unused fuel to the sub tank.

Hmmm, maybe simple if you've got the Owner's Manual (I don't) & the system works perfectly (it didn't). I'd say that the Jackaroo's system was much simpler - 1 pump at the motor, 1 pump in the rear tank, 1 fuel-line & filter, 1 transfer-line, 1 return-line.
The Prado by contrast has 2 in-tank pumps, 2 fuel-feed-lines, 2 return-lines, 2x3-way connectors, 1 with a solenoid, the other with 2x1-way valves! Lots there to stuff up?

The old 'Cruiser was even simpler: siphon from left tank to right. 1 extra fuel-line about 6 inches long. The only thing that could go wrong with that was a blockage in the line, which to be fair, to my knowledge did happen but not to me. Of course, the simplest thing of all is just to have as big a single long-range tank as possible, but then if you hole that you really are in trouble!

On a happy note, it looks as though my system is fixed as when I filled it up today I actually got 10 litres into the sub, which had been full to the brim until the Tuesday repair.

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

Aaagh! Now it's doing the same thing again! Worked perfectly for a few days then after I filled up both tanks it's doing the same old thing, drawing only from the main tank. I suspect that there's some contamination in the sub-tank - sludge in the bottom, maybe diesel residue - something like that. I've just poured a bottle of injector cleaner into the sub to see if that does any good but ATM back to square one!
Anyone seen the results of an accidental diesel into petrol tank fill?

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

What a bummer, nothing worse then when you have fixed some thing for it to play up again, sounds like some thing blocking the outlet eh ?? Might have to drop the tank again and try and flush it out well and proper or maybe some thing has been dislodged and blocked the pump inlet ?? Only guessing here....Hope you sort it out, nothing worse.......Cheers Steve

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

My mechanic found a "ball of rubbery gloop" (his exact words) in the 3-way junction of the fuel-feed-lines, blocking the sub-tank line, NOT the main tank. If it's the same thing again it sounds to me like some sort of residue on the bottom of the sub-tank. Short of draining, dropping & cleaning the tank somehow I'm waiting to see if the injector-cleaner does any good. I put a double-dose into the sub-tank so we'll see. The shame is that the vehicle's going really nicely & seems to be getting better the longer it gets driven. God knows how long it was sitting at the Dealer, I s'pose. Just a little frustrating. Oh well, the other way to go is to convert it to LPG & "heavy" our Installer into putting a "belly-cylinder" in there & chuck that tank. He wasn't keen when I spoke to him a few days ago but he's got our Territory in there right now doing a $4500-00 Vapour Injection conversion on it so I'll have to "chat" to him.

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

My bet is you have perished fuel lines, has something other than petrol been in the tank??????
The rubber fuel line is breaking down on the inside, and the bits that come off of it are clogging the junction.

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

We've only had the Prado for a couple of months so god knows what's happened to it in the past. No service record with it beyond the fact that it had just been fully serviced, something that I checked on. My mechanic had basically all the fuel-lines, junctions, solenoids etc out of the car & on his bench, so I'd think that he'd see perished fuel lines. He literally couldn't identify the "gloop", hence the description. My bet is diesel residue or similar.
On the bright side, it LOOKS like it's working again since I dropped the cleaner into it - the main gauge hasn't moved for 90km now & the sub-gauge has moved from above the full mark to just below - there's hope!
On a related note, it occurred to me that when I switch over from main to sub, the main-tank fuel-pump should stop, yes? So when it's not been drawing from the sub (despite being switched that way) how is the fuel getting from the main to the injection? Beyond me ATM.