Home » Why BMW i3 Owners Have Been Building Potentially-Dangerous Gas Tanks Into Their Front Trunks

Why BMW i3 Owners Have Been Building Potentially-Dangerous Gas Tanks Into Their Front Trunks

P90129191 Highres Bmw I3 07 2013
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The BMW i3 is an electric vehicle that can be optioned with a nifty range-extender engine which runs on gasoline — a series hybrid set up exactly like the new Ram Ramcharger. It’s a great way to avoid the problem of slow EV chargers on long trips, save for one problem—it has a tiny fuel tank that lasts maybe an hour or two at best on the highway. That’s led several owners to develop their own frunk-mounted auxiliary tanks to give their cars longer legs out on the open road. Yes, a gas tank in the front trunk. Let’s dive into how it all works. [Ed Note: We do not condone this mod; we think it’s a bad idea. -DT]

For some owners, the i3’s gas tank is too small, at just 1.9 gallons in 2014 to 2016 models and 2.4 gallons thereafter (note that the small tank could be “coded” to go from 1.9 to 2.4 gallons, as it’s artificially reduced via software for regulatory reasons). At best, an i3 with the 120 Ah battery and the range extender will get around 126 miles of EV range and a further 80 miles or so from gasoline. Thus, those taking the i3 on longer trips often desire a much larger tank that would allow them to drive for hours without refueling or recharging the car.

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It’s A Tiny Gas Tank But A Reasonably Sizeable Frunk

If you’re wondering just how small it is, well… it’s about as big as a shoebox. A tiny one:

If you’re driving at highway speed on the range extender, you’ll need to refill the i3’s tank on an hourly basis. Credit: eBay 

(That filler is on the front passenger’s side fender, and the tank is in the passenger’s front wheel-well area).

The i3 is a subcompact without a lot of space to spare. By virtue of not having a traditional engine under the hood, though, the frunk presents a nifty storage space that’s outside the cabin:

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It’s just roomy enough to stash a small additional fuel tank without the hassle of gas fumes permeating the passenger cabin. Some enterprising owners have taken advantage of this by plumbing in an additional tank to the i3’s fuel system to net hundreds of miles of extra range. It might sound difficult and maybe a bit absurd to plumb in an additional fuel tank, but i3 owners have gone for it, and even found a few shortcuts that make the job easier.

How People Have Turned That Frunk Into A Gas Tank

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The i3 has enough space to carry an 8-gallon fuel cell. With a little plumbing and a small transfer pump, this can be used to refill the i3’s main tank on the go. Credit: Paul Housley

Jason Sharp is an i3 owner from Idaho, and his car has an 8 gallon tank from Summit Racing stashed in the frunk. He found that the overflow line running alongside the i3’s fuel filler pipe was the perfect place to tap in. He cut the i3’s overflow line, plumbing the tank end into the output from a transfer pump hooked up to the auxiliary tank. The auxiliary tank’s vent is then hooked up to the other end of the cut overflow line.

As the i3’s main tank empties, it naturally draws fuel from the auxiliary tank, even with the pump off. “It will pull half a tank into the [i3’s] small tank, but only when both tanks are full,” Sharp told The Autopian, noting that he uses a switch to activate the electric pump to transfer fuel when the vacuum from the main tank isn’t enough to do the job. He states that this setup keeps the fuel system sealed, which is key to avoiding problems. “Otherwise it will trigger a check engine light,” says Sharp. 

 

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Jason Sharp has a tank in the frunk of his i3 with a fuel pump that feeds fuel into the main tank at the back of the car. Credit: Jason Sharp
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Image sourced from eBay listing.

Yes, if you’ve ever had a leaky gas cap, you’ve experienced the annoyance of it triggering a check engine light. Thankfully, that’s avoidable by keeping the fuel system properly sealed. Sharp’s setup achieves this neatly by the way it’s plumbed into the fuel system. The external tank’s fuel feed is fed directly to the main tank, and its vent runs up to the gas cap. The system remains sealed, and no check engine lights are thrown.

Paul Housley has a similar setup on his own i3, which he uses for long trips a few times a year. Housley flicks a switch mounted at the base of the dash to turn on a pump which fills the i3’s tank from the auxiliary one in the frunk. Like Sharp, his pump runs into the same line next to the fuel filler. “You just turn on the pump as needed while driving and then it slowly fills the tank while you are driving,” says Sharp.“Usually it is best to turn the pump off when the fuel gauge says three-quarters full because it does not respond very rapidly and you don’t want to overfill it,” he adds. He finds the system most useful on country drives where there are no gas stations or EV chargers for hundreds of miles. In those situations, the i3’s short range can be a real liability. 

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Housley used a tee fitting to tap into the overflow line alongside the i3’s main filler tube. The external tank’s vent is run separately.

Sharp notes that the additional tank nets him a range of 350 miles or more on gasoline, a huge jump over what’s possible with the stock car’s standard tank. Similarly, Housley claims a total range of over 400 miles with both tanks topped off and a full charge in the i3’s battery. That’s a huge boost over the stock i3, which is capable of 200 miles in its longest-range trim, according to EPA figures.

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Plus, it enables the car to go hundreds of miles further with a simple refill of the auxiliary and main tanks, without having to wait for the main battery to charge. Try that with the standard tank only, and a 5-minute stop for gas will only net you another 80 miles or so. 

 

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Not everyone goes the pre-made tank route. These photos from Jorge Montes de Oca show his custom tank under construction, built especially to fit the dimensions of the i3’s frunk.

Some may find the modification unduly invasive, but other owners have found solutions that don’t involve cutting into the i3’s fuel system. A guide shared on BMW forum Bimmerfest explains how to plumb an external fuel line into a hole drilled into the i3’s fuel cap, sealing it with RTV and epoxy. It functionally achieves the same thing, but it’s easier to replace a fuel cap if you want to put your car back to stock.

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The cheapest version of the hack uses a simple plastic gas can as a tank, with a pickup hose from a small transfer pump running through the gas can’s plastic cap. The cap also has a tiny hole drilled into it for venting purposes. Without this, the gas can would deform and get sucked in by the vacuum created as the fuel was drawn out of the tank. This works, in that it allows the gas can to effectively serve as an auxiliary fuel tank. However, this cheap vent system creates a risk of fuel vapors building up in the frunk.

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A simple switch engages the auxiliary tank’s fuel pump to top off the i3’s main tank. It’s important not to leave it running to avoid overfilling the main tank. Credit: Paul Housley

[Editor’s Note: Here’s a look at the pdf Lewin mentioned; it shows the entire installation process for a jerry can into the i3’s frunk. Here are some screengrabs. Note how the installer drilled a hole into the fuel cap, and just slathered some RTV to seal a tube that a fuel line from the jerry can plugs into:

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Notice how the installer actually trimmed their fuel filler door to fit the fuel hose nipple on the fuel cap.

 

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Here’s the on-off switch for the silver electric fuel pump, shown above (the relay is shown just above the fuel pump image). And you can see the final product in the frunk:

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It’s all a bit wild. You can read the full installation instructions here. -DT]

It May Not Seem Safe, But It Does Seem Useful

Indeed, all of these setups do pose a certain safety risk. When BMW designed the i3, it didn’t account for a fuel tank in the frunk. Indeed, the front end must deform to dissipate energy in a crash, as with any modern car. With a fuel tank in the frunk, there is the distinct possibility of it bursting like a balloon in an impact,with any fuel inside spraying everywhere. A partially-empty tank is perhaps even more dangerous, by virtue of the easy ignition of the gasoline vapor inside. In the 20th century, automakers learned not to put fuel tanks in positions where they could be easily damaged or punctured. Putting a fuel tank in the frunk is very much contrary to safe design practices, even if it’s only a small 8-gallon tank.

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Sharp isn’t unduly concerned about the potential fire risk, though. “Around town I keep it empty,” he says, noting that it’s “not the safest, but I only travel long distance once every few months now.”

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It’s possible to modify a gas jug to use it as an auxiliary tank, but using a proper tank is safer and far less likely to leak. It’s also easier, as they’re generally ready to hook up with fittings and accessories included. Credit: Summit Racing

Housley also points out that he used a proper fuel cell for the tank, which is at least nominally designed to survive a crash without a major failure. He also he notes that removing his tank only takes a few minutes. “It is only in the car when I know I am going to need it,” he says. 

Anyone attempting such a build should consider the safety aspects involved in modifying a fuel system. Beyond the crash risk, it’s also worth noting the dangers in routing your own fuel lines. Run one too close to an exhaust pipe or other hot part of the car, and you can easily burn your ride to the ground. Tying a fuel line to any moving components of the car can also see them torn or cut open, creating a dangerous leak. Even simply zip-tying a rubber fuel hose to a metal line can cause problems if you’re unlucky, with vibration between the two slowly working a hole into the softer rubber line. 

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Whenever you’re playing with fuel lines, make sure you’re routing them carefully and using the right fittings. Leaks can quickly turn ugly. Credit: Paul Housley

If you’re attempting such a retrofit to your own i3, consider the path of any fuel hoses carefully, and make sure you’re using properly rated lines. If you’re using a low-pressure pump to transfer fuel into the fuel filler, for example, there’s no need to spring for the expensive high-pressure hoses needed for EFI installs. However, you’ll still need to get the stuff that’s properly rated to handle fuel without degrading. Wiring for any pumps, relays, and switches should also be carefully considered. You don’t want to accidentally strike a spark while you’re working on your auxiliary fuel tank, after all. It pays to remember that fuel vapor is far more flammable than the actual fuel itself. 

The mod is definitely a game changer for road trips. Regardless, even with the additional fuel, the i3 can still struggle on longer routes. “The biggest issue is just climbing hills for an extended time,” says Sharp, adding “I’ve ran the battery down from 75 percent to zero often.” He explains that when the battery has run out, the i3 can struggle to maintain 80 mph uphill using only the range extender. That’s perhaps unsurprising, given the 0.65-liter, two-cylinder range extender is only capable of putting out 33 horsepower. It’s a small fraction of the 167 horsepower available from the electric powertrain when the battery has some charge.

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A cynic would say that a BMW i3 is simply not well suited for long-range travel, and that owners should simply buy another car. As these owners demonstrate, though, it’s quite easy to extend the range of an i3 by 100% or more with just a few hundred dollars in parts. If you’ve already got the car, and you want to take the occasional long trip, it’s hard to argue with the value there. It’s theoretically a lot less fuss than trading in your car for another vehicle, and cheaper than renting one for the occasional weekend. 

Still, while I applaud the ingenuity, I’m not sure I’d want a gas tank under my hood, especially one that wasn’t originally designed to be there.

 

 

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Jim Duncalf
Jim Duncalf
1 month ago

Yes, the politicians strike again. They force you into buying an EV which is born with a carbon footprint of from 17 to 20 tons of CO2, about as much as driving a midsize car 120,000 Km, then when someone designs a car that can produce a Kw of energy with half the CO2 as the typical rich power companies, the lawmakers, in their quest for more power over the lives of citizens, force the manufacturer to put in a gas tank smaller than a motorbike. This even though many such Range Extended EV designs are half a ton lighter, handle better, stop faster, are easier on the roads and tires, and are vastly safer. They also completely eliminate the biggest problem with EVs, Range Anxiety”! Years, ago at an SAE meeting (Society of Automotive Engineers), it was forecasted that eventually EVs would be abandoned like they were over a century ago. Many there thought the future power trains would be Range Extended EVs. https://www.kamtech-sa.com/

ScottyB
ScottyB
6 months ago

This would be like driving an early production Pinto without the safety retrofit backwards. What could go wrong?

Camp Fire
Camp Fire
7 months ago

The most enlightening part of this article has been the comments. I’m surprised how many people are terrified of fuel system modifications. Custom fuel systems are common in the hot rod world, and custom/sketchy/modified vehicles are a big part of the culture promoted here (David’s Jeep FC, for example). So I expected such a practical mod would find good support here.

I’m not about to justify the use of clear dishwasher tubing for a gasoline system. That’s a hill too far for me. But the concept here, when using proper materials, isn’t any sketchier than most of David’s car fleet. I’d feel safer with a proper fuel cell in the front than I would on, say, a motorcycle. Or a beetle. Or numerous other rust buckets I see on the road regularly.

I take this article as a sign that David is considering such a mod for his i3, and I’m looking forward to that article.

Turkina
Turkina
7 months ago

Kei cars and trucks make more hp with their 660cc. Is the low horsepower a compromise to limit the charge rate for the battery? I don’t wanna math right now, but the 660cc class can put out 63 hp even with NA engine technology from the 90s.

Defenestrator
Defenestrator
7 months ago
Reply to  Turkina

Optimized for efficiency and sustained moderate power rather than shorter bursts of peak power.

Jake Baldridge
Jake Baldridge
7 months ago

Over/Under on how long it takes David to try this?

Widgetsltd
Widgetsltd
7 months ago

If you want to use a control switch with an integrated high-current relay, indicator lamp, and timer for a project like this, simply swipe a rear defroster switch with connector from a 2000-05 Dodge/Plymouth neon or 2001-05 Chrysler PT Cruiser. I used that switch on the fuel transfer pump system for the aux fuel cell on my endurance roadracing car. The cell in question was an ATL Well Cell, which is FIA certified and has the usual suppression foam inside – Always a good idea when adding a cell.

Dogisbadob
Dogisbadob
7 months ago

Now all they need to do is offer smaller wheels that take normal size tires, and a square setup 😀

Cyko9
Cyko9
7 months ago
Reply to  Dogisbadob

That’s always my take when someone lauds the i3. It’s a fun design and very handy, but the custom tires, a consumable, don’t make this vehicle feel like a good fit for long-term mods.

AMGx2
AMGx2
7 months ago

Some people sit with their chest on top of a fuel tank while driving on two wheels only with no crash zone whatsoever.

We call those motorcycles / bikes / motorbikes and strangely enough those fuel tanks seems to be no problem on the road…

Anoos
Anoos
7 months ago

That is some very questionable plumbing. I would be shocked if any of those fittings or tubing are suited for this application. It looks like someone went to the hardware store and just jammed a few things together as long as they physically fit together.

Even if that clear tubing and the plastic tee is chemically compatible with gasoline, it’s going to collapse under vacuum – which it will see by design in this setup. The picture of that plumbing setup is giving me anxiety.

Strangely, the plumbing looks much more suitable on the setup where a lunatic pumps fuel from his portable tank to his gas cap – which now stands proud of the bodywork like the intake on a hot rod only much more sad.

Turkina
Turkina
7 months ago
Reply to  Anoos

Motorcycles use similar fuel lines, although I would definitely shoot for fuel grade tubing and connectors whether they be black or clear. Or go with the rubber reinforced stuff that we are familiar with.

Jayson Elliot
Jayson Elliot
7 months ago

I’d be far more interested in a mod to put larger batteries into an i3.

If I could get an i3 with 250 miles of range, I’d probably never drive anything else.

Anoos
Anoos
7 months ago
Reply to  Jayson Elliot

Right? This isn’t much of a hack for a vehicle with electric propulsion.

Extending range by adding a larger tank for gasoline seems like a solution that would work as well on a Model T.

Scott Baysinger
Scott Baysinger
7 months ago
Reply to  Jayson Elliot

As a Spark EV driver I would be tickled-pink to have *half* that.

Anchor
Anchor
7 months ago
Reply to  Jayson Elliot

Yeah man, you can add another fuel tank or buy a few thousand worth of used battery cells, do all the very specific wiring, cobble in a charge controller somehow and find a way to make the factory stuff recognize it’s there.

MEK
MEK
7 months ago

Hi my name is Joan of Arc and today I’ll be demonstrating how I extended the range of my i3…

Duke Woolworth
Duke Woolworth
7 months ago

Our local emissions test would be very unhappy to see a modified fuel cap. Fail.

Jakob K's Garage
Jakob K's Garage
7 months ago

Porsche and VW had gas tanks under the hood for ages, without going all Ford Pinto…

It’s when the gasoline meets very hot items, or mechanical or electrical sparks, it ignites. But I think it’s hard to both rupture a soft small auxillary fuel tank and get the gas to run anywhere flammable in such a safe stiff new car as the i3. I bet it has some impact trigger shutting everything off in case of a crash. Aren’t those mandatory nowadays?

A little worse with the guy welding up his own custom tank. That will leak quite easy in a crash. And the guy using clear tubes should change them to regular fuel hoses for safety also.

Anchor
Anchor
7 months ago

Porsche and VW didn’t put them 4 inches from the front bumper though

Here4thecars
Here4thecars
7 months ago

If I recall correctly, there was a guy that revived a Jeep FC the was covered in rodent dung and went off-roading using a jug on a pole as a gas tank. I think his name was David Tracy?

JDE
JDE
7 months ago
Reply to  Here4thecars

Eh, there are plenty of Boat fuel tanks sloshing around the trunk or the passenger seat floor board all over the US thanks to the trend of “let’s see if this 20 year old barn find will make 700 miles home”

Thomas Metcalf
Thomas Metcalf
7 months ago
Reply to  Here4thecars

Rumor is that he moved to California and went all Hollywood. Started buying EV’s with no rust or feces in them.

FloridaNative
FloridaNative
7 months ago
Reply to  Thomas Metcalf

But he still imported feces into at least one Jeep via various four-legged creatures!

M0L0TOV
M0L0TOV
7 months ago
Reply to  Thomas Metcalf

I heard one of his Jeeps is the cat’s meow now.

Al Camino
Al Camino
7 months ago

Don’t do this, and oh, by the way, here are the detailed instructions on how to do this.

Camp Fire
Camp Fire
7 months ago
Reply to  Al Camino

*wink*

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