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

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.

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:

(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:

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

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. 

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. 

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.

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. 

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.

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.

[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:

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

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:

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.

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.”

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. 

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.

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.