fuel tank and the carbon canister

**prelude** · 6th April 2025, 08:43 PM

During my build I come across a lot of tiny little challenges that I had not thought of up front or purposefully pushed back to deal with when we cross that bridge. Hooking up an auxiliary fuel tank is one of those things that sound rather easy but bring a small set of extra challenges.

In short: I have mounted an extra fuel tank in the place where the spare wheel used to fit and I want to hook this into my (GEMS) P38 petrol. I have a filler neck, a breather pipe and a suction pipe in/on the fuel tank. I am considering two possible options to use this tank:

use it as a big jerry can, the suction pipe will go to a small fuel pump that can fill my main tank as and when needed
hook it up as an independent system

hooking it up as an independent system allows me to deal with potential problems like a leaking tank, broken pump, etc. which would be nice.

The first is of course quite easy to do. Simply hook the fuel suction line (through a pump) into any of the existing fuel filler or even breather lines of the main tank and at least hook both breather lines together, the pressure will equalize nicely and the entire system will regulate itself through the evap canister up front etc.

The second one can go either of two options:

two pumps with non return valves. Switch when needed
one pump with a valve to switch between suction hoses (bypassing the in tank pump)

The one headache I have is that there is also a fuel return line. If I do not alter that it will simply dump excess fuel back into the main tank and I would be running of that quite soon again

also in case of a damaged tank/pump I would be dumping fuel where I do not want it. I don't even know how much fuel gets returned? Adding valves to switch between supply could be augmented with valves to switch between return lines as well, though my design treats the fuel return line as a breather hose as well if need be I could add a third connection to the second tank to accommodate. What bothers me somewhat there is that I would need to have physical valves place somewhere, not being inside the car, so I would have to get out of the car to switch over. There are electric valves available but they have very tiny ports and seem to restrict flow a bit, not sure if that would cause trouble.

This brings us nicely to the breather setup and EVAP canister. I have been reading up on how they work. It seems a certain amount of pressure is allowed to build up in the fuel tank, but beyond that fuel vapours get vented to charcoal inside the canister where they are "stored" and released into the engine under certain conditions. The vapours mainly being created by the fuel (and tank) heating up. The extra pressure buildup allows for the entire thing to cool down and not suck in air that contains moist and what not. All in all a nice system and probably mostly only for environmental reasons. But... it's in the way

I have been shuffling most of the components on the (for me as a LHD car) left hand side of the engine bay. The air filter has been removed and an electric AC compressor now lives there, the EAS valve body has also been moved into that location and a bypass system has been fitted and the compressor has been deleted, it will live some place else (not sure where yet). The cruise control can't move too far away but should be manageable. The spot where the EAS and CC systems lived will need to house a custom air box. I plan to mount a raised air intake on that side and need to bring a decent sized pipe out the side, exactly where that bloody black box lives.

So, can I delete it? What would one need to do? And how does one integrate an auxiliary fuel tank?

Mind you, I have replaced the fuel filler neck and caps of the range rover with two defender style units and they are vented fuel caps (one for each tank).

Thank you all in advance for thinking along

Cheers,
-P

**POD** · 8th April 2025, 08:55 AM

I would be going for the 'big jerry can' system. Whilst having redundancy in case of a damaged fuel tank sounds nice, if you were to chart a risk-control matrix I think you would find that the frequency and severity of a fuel tank failure would not justify the complexity; in any case, you would still have a reserve of fuel in your aux tank so could deal with a problem should one occur.
A transfer pump to top up the main tank as required works very well. I did many outback miles with a '88 Rangie with two 40-litre sill tanks transferring to a 25 litre running tank (vehicle had an LPG conversion with LPG tanks in place of the original petrol tank) and the system worked very well (unless I forgot the transfer pump was on and tried to pump 40 litres into the 25 litre tank!).
I'm not conversant with the P38 system, but on the earlier EFI models, the pipe that suspended the in-tank pump was prone to failure in corrugated conditions, this problem is avoided by an externally-mounted pump, which, with minor cobbling, could be made to draw from the auxilliary if the main tank was holed.
I think that tapping the vent line from the aux tank into the main tank would be sufficient connection to the evaporative emissions system.

**prelude** · 8th April 2025, 05:47 PM

You make a good point. When I use the second tank as a "jerry-can" and route the fuel line with the transfer pump in the most efficient way, ie. so that I can get to it -g- I would be able to bypass the main fuel tank in case of emergency.

I also need to keep in mind that the transfer pump is running due to size differences and the fuel meter on a car is usually very slow, unless you reset the ignition (ie turn the car off and on). Ever tried filling up with a running engine? the needle is soooooooooo slow. I reckon I could fit an aftermarket fuel gauge to both tanks, they are not expensive and it saves me from having to calculate and time everything in my head.

The only thing I wonder is if I hook both vents up, in the worst case scenario there will be a lot more fuel vapour being produced by two half empty tanks than the canister is designed for. On the other hand, the system has an overpressure release valve built-in so it will eventually vent to the outside air, like all older cars would.

Cheers,
-P

**POD** · 9th April 2025, 08:35 AM

After over-filling my running tank a few times by forgetting that the transfer pump was on, pumping fuel overboard via the tank vent (and filling the charcoal canister system with fuel!), I developed a low-tech warning system; when I switched the pump on, I would pick up a box of matches from the centre console and place it in the binnacle. Every time I looked at the instruments, the matchbox sitting there reminded me that the pump was running. This warning system cost very little to install and never broke down!
It's not necessary to run the pump until the tank is full, just run it for a few minutes when the low-fuel warning comes on, you'll get the hang of how long it can be left on to nearly fill the tank.

**TonyC** · 9th April 2025, 08:47 AM

Hi Prelude,

Is your main tank bigger than your auxiliary?

If so, just run the main tank till there is enough space for the whole auxiliary to transfer without overflowing.

Tony

**V8Ian** · 9th April 2025, 12:26 PM

Just put a timer in the pump circuit. Many of you could build one for a few dollars with bits and pieces from Jaycar, but for those who could't be bothered, a turbo timer would do the job.

Pardon our interruption...

**Blknight.aus** · 9th April 2025, 12:58 PM

set the aux tank up as a sealed tank.

connect the fill vents together on both fuel tanks

plumb a facet pump from the aux Tank into a t piece on the return line to the main tank. The facet pump is a pair of one way valves so fuel wont flow from the return into the AUX tank and if you happen to leave it on too long or turn it on too early the overflow from the main tank will simply back fill into the aux tank via the breather.

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