If you've spent any time under the back end of a classic pony car, you know that a leaky 70 mustang fuel tank is basically a rite of passage for owners. It's one of those parts that you don't really think about until you start smelling that sharp, varnish-like scent of old gasoline every time you walk into the garage. Or worse, you're driving down the road and your engine starts sputtering because fifty years of sediment and rust flakes finally decided to take a trip into your fuel pump.
Replacing the tank in a 1970 Mustang isn't exactly the most glamorous job in the world—it's not like bolting on a shiny new intake manifold or a set of headers—but it's one of the most satisfying "peace of mind" projects you can do. There is something incredibly reassuring about knowing your fuel system is clean and that you aren't leaving a trail of high-octane breadcrumbs behind you on the pavement.
Why the 1970 Tank is a Bit Different
If you're new to the Mustang world, you might think a tank is a tank, but the 70 mustang fuel tank actually has a couple of quirks compared to the earlier models. For one, Ford bumped the capacity up to 22 gallons for the 1970 model year. If you try to shove an earlier 16-gallon or 20-gallon tank in there, it'll technically fit the hole, but your filler neck alignment might be a nightmare and you'll be stopping at the gas station a lot more often.
The 1970 design is also unique because it essentially acts as a portion of the trunk floor. This was a common design for Fords of that era, and while it makes the tank incredibly easy to access compared to modern cars where you have to drop the rear axle, it also means that if your tank is rusting from the top down (which happens if your trunk seal is shot), you're going to see it every time you open the trunk.
Spotting the Signs of Trouble
Usually, your car will tell you when it's time for a new 70 mustang fuel tank long before it actually leaves you stranded. The most obvious sign is the smell. If your garage smells like a refinery, you've probably got a pinhole leak or a seeping seam. These tanks are made of Terne-plated steel, which was great for the 70s, but once that coating wears thin, moisture in the fuel starts eating away at the metal from the inside out.
Another big red flag is a clogged fuel filter that looks like it's full of red pepper flakes. That's rust, plain and simple. You can try to clean out an old tank with a kit, and sometimes that works for a year or two, but honestly, with how affordable replacement tanks are these days, it's usually better to just start fresh. Why risk ruining a $500 carburetor or an expensive fuel pump to save a few bucks on a patch job?
Choosing the Right Replacement
When you start shopping for a new 70 mustang fuel tank, you're going to see a few different options. Most people go with the standard Ni-Terne steel replacement. It looks original, it's durable, and it'll probably outlast most of us if the car is kept in a dry garage.
However, if you live in a humid climate or if you're planning on keeping the car forever, you might want to look at a stainless steel version. They cost more, but they're virtually immune to the internal corrosion caused by the ethanol found in modern pump gas. Speaking of modern upgrades, if you're planning on swapping to EFI (Electronic Fuel Injection) down the road, you might want to grab a tank that's already baffled and set up for an internal high-pressure pump. It'll save you a massive headache later on.
The "While You're In There" Parts
Don't make the mistake of ordering just the tank. There are a few small bits that you absolutely should replace at the same time to ensure you don't have to pull the whole thing back out two weeks later.
First off, get a new fuel sending unit. The old one is likely covered in the same gunk as the tank, and the float might be half-full of gas anyway. Getting a new unit ensures your gas gauge actually tells the truth for once. You'll also need a new lock ring and a fresh gasket.
Secondly, check your filler neck hose and the trunk floor seal. If that rubber hose is cracked or stiff, it's going to leak when you're at the gas station. And that foam seal that sits between the tank and the trunk floor? It's probably disintegrated. A fresh seal kit will keep exhaust fumes and moisture out of your interior, which makes the driving experience a lot more pleasant.
Getting Down to Business: The Install
Replacing a 70 mustang fuel tank is actually a pretty straightforward DIY job. You don't need a lift, though it certainly helps. A couple of jack stands and a floor jack are usually enough to get the job done in an afternoon.
Step 1: Drain the gas. This sounds obvious, but you'd be surprised how many people try to drop a tank with five gallons still sloshing around inside. Gas is heavy, and it's dangerous. Most 70 Mustang tanks have a drain plug, which makes this part easy. Just make sure you're in a well-ventilated area and away from any pilot lights or stray sparks.
Step 2: Disconnect the plumbing. You'll need to unhook the fuel line from the sending unit and pull the wires for the gauge. Then, head into the trunk and unscrew the filler neck from the tail panel.
Step 3: The J-bolts. The tank is held in place by a series of bolts around the perimeter of the flange. These can be a bit of a pain if they're rusted, so hit them with some penetrating oil the night before. Once those are out, the tank should drop right out of the bottom. If it's stuck, it's probably just the old sealant or gasket acting like glue. A gentle pry bar should do the trick.
Step 4: Prep and Install. Clean the flange area on the car before you put the new tank in. This is the best time to throw some paint or undercoating on the metal that's usually hidden. Drop the new sending unit into the tank, lock it down, and then lift the new 70 mustang fuel tank into place. Having a buddy hold it while you start the bolts makes this ten times easier.
Common Headaches to Avoid
One thing that trips people up is the "anti-squeak" kit. These are basically strips of rubber or felt that go between the tank and the floor. Don't skip these! Without them, the metal-on-metal contact will drive you crazy with squeaks and rattles every time you hit a bump.
Also, be careful with the filler neck alignment. The 1970 Mustang has a specific angle for that neck. If you tighten all the tank bolts before you have the filler neck lined up with the hole in the rear বীর-panel, you might find yourself fighting to get the gas cap on later. Keep everything slightly loose until you're sure the neck is centered, then torque it all down.
Wrapping Things Up
Once everything is buttoned up, pour in a few gallons of fresh 91 or 93 octane, check for leaks around the sending unit, and fire it up. It might take a minute for the fuel pump to prime and pull gas from the back of the car, so don't freak out if it doesn't start on the first flick of the key.
There's a real sense of accomplishment in finishing a job like this. Knowing that your 70 mustang fuel tank is clean and secure means you can finally take those long backroad cruises without constantly eyeing the fuel filter or wondering if that smell is "normal classic car stuff" or a looming disaster. It's a dirty job, sure, but it's one of the best investments you can make in your Mustang's longevity. Plus, your garage will finally stop smelling like an old lawnmower, and your spouse will probably thank you for that.