Twin Fuel Tanks and how they work

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Growlerbearnz
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Twin Fuel Tanks and how they work

Postby Growlerbearnz » Sat Nov 21, 2015 12:35 am

Automatic Diesel 4WD L300 Delicas* have a dual fuel tank. Not a lot is known about how these work, the English language workshop manuals ignore them entirely, and the parts books are typically vague. Well, I've just refurbished mine and I figured out how it all works- and I took pictures. Here we go:

*and some others, but I'm not entirely sure which.

——Filling and fill vents——

Fill.JPG
Fill.JPG (127.2 KiB) Viewed 1501 times


Filling is straightforward- a large 50mm pipe goes from the filler neck to the front tank, another 50mm pipe connects the front tank to the rear tank.

The air in the tanks needs to vent while filling. The tanks vent through 15mm pipes coming out of the tops of the tanks. The front tank vents into the rear tank, and then the rear tank vent goes to the vent port visible just inside the filler neck.

——Fuel Supply to engine——
Supply.JPG
Supply.JPG (162.84 KiB) Viewed 1501 times


Here’s where it gets tricky. The fuel supply to the engine is primarily drawn from the rear tank, but it’s a bit more complicated than that. The rear tank’s pickup draws fuel through the strainer and into the pickup chamber.

RearTankPickupTogether.JPG
RearTankPickupTogether.JPG (152.98 KiB) Viewed 1501 times


The pickup chamber is also connected to the front tank- so the fuel is also drawn from the front tank. Normally the pipe connecting the front tank to the rear tank would be full of fuel and acting as a syphon, keeping the tank levels equal.

RearTankPickup.JPG
RearTankPickup.JPG (93.89 KiB) Viewed 1501 times


If the syphon loses its prime the fuel level in the rear tank can get too low, and the float inside the strainer blocks the inlet into the pickup chamber. Fuel is now only drawn from the front tank.

Excess fuel from the engine is returned into the front tank.


——Emissions vents——

Emissions vapour/ fuel expansion is vented through the small tubes. The front tank’s vapour vent is connected to the rear tank, and the rear tank’s vent goes to the purge valve on the chassis. (On a gasoline car I imagine this vent line would go to the evaporative control canister).

——Fuel gauge and low fuel light——

gauges.JPG
gauges.JPG (138.88 KiB) Viewed 1501 times


There are two fuel level senders, connected in series so the fuel gauge shows the sum of their levels.

The low fuel warning light sensors are likewise connected in series, but work in different ways. The front tank’s sensor is a float switch which turns on when fuel level gets too low.

FrontTankSenderGuts.JPG
FrontTankSenderGuts.JPG (170.48 KiB) Viewed 1501 times


FrontSenderWires.JPG
FrontSenderWires.JPG (164.73 KiB) Viewed 1501 times


The rear tank’s sensor is a NTC Thermistor- a small component that has high resistance when cool (like when immersed in fuel), but rapidly heats up when not immersed, flowing more and more current until there’s enough to make the light in the dashboard glow.

sensor.JPG
sensor.JPG (54.21 KiB) Viewed 1501 times

Not actually a Delica level sender- but pretty close. The NTC Thermistor is inside the metal can (circled).

For the light to be on, both the front tank switch and the rear tank sensor need to be on.

Here's what all those pipes and stuff look like on the tanks- they're pretty much impossible to see while the tanks are in place, and they're going to be covered in dirt and rocks. Gross.

Front tank pipes:

FrontTankPipes.JPG
FrontTankPipes.JPG (157.11 KiB) Viewed 1501 times


Rear tank pipes:

RearTankPipes.JPG
RearTankPipes.JPG (231.2 KiB) Viewed 1501 times


Finally, I really don't recommend removing your fuel tanks unless you have to. Dirt and rocks fall on your head constantly, diesel will spill out of everywhere, and half the bolts will be rusted solid. Not fun.
Nothing says "poor workmanship" more than wrinkles in the duct tape.

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Mr. Flibble
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Re: Dual Fuel Tanks and how they work

Postby Mr. Flibble » Sat Nov 21, 2015 9:35 am

Wow! What a great write up!

I was thinking about putting s Montero tank in the back of my L400 and linking them like the L300 tanks - but now looking at your writeup, this is too complex and I will stick with basic dual tanks.
Canadian living in Washington USA

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Growlerbearnz
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Re: Dual Fuel Tanks and how they work

Postby Growlerbearnz » Sat Nov 21, 2015 2:04 pm

The L300 setup is bizarrely complex because it had to behave like a regular fuel tank to suit your average car buyer, but if you're adding your own auxiliary tank it could be a bit less complicated.

The simplest setup would be to give the auxiliary tank its own filler, tee the fuel outlet into the main tank's fuel return line, then use a pump to move auxiliary fuel into the main tank when needed. That saves modifying the main tank and figuring out how to get a syphon to work.

There are a few neat neat setups here:
http://dinoevo.de/auxiliary-diesel-tank/
http://www.delicaclub.com/viewtopic.php?t=13078
and the one I've just finished here: viewtopic.php?f=10&t=17205 (which is definitely not simple)
Nothing says "poor workmanship" more than wrinkles in the duct tape.

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Re: Dual Fuel Tanks and how they work

Postby FalcoColumbarius » Mon Nov 23, 2015 8:09 pm

I've just moved this thread to the Delica FAQ Section so it doesn't get buried. Fabulous article, GBNZ!

Falco.
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