Old Gasoline in Tank
 

This may be a dumb question but does old gasoline in your Mustang tank affect how smooth the engine runs? In my 1966 convertible, which I have owned for over 40 years and rarely drive any more, some of the gasoline in the tank is probably more than 5 years old. I use regular grade gasoline but I have never drained the tank since installing a new tank about 10 years ago, Your thoughts and advice on this would be greatly appreciated.

Time to install a new tank most likely. Modern gas isn't really good for more than a few months. I doubt that even stabilizers would keep gas good for 10 years. I'm surprised it even runs.

If there is no muck in the tank, like rust or varnish (which is what gas basically
turns into after years and years); you're good to go. Run it with fresh fuel,
to "E" then change the fuel filter if you can. I know it's hard to find, but if you can,
run it with gas that does NOT have Ethanol in it. Some places still sell it,
but it costs more.

My brother has had many old cars that sat for years, and ran them after, with
fresh fuel. At worst, drain and flush the tank, but if there is no rust in it,
then it's fine if you put gas in it, and it fires up. The filter will catch all the
muck, then change it.

Ethanol fuel is what rusts the fuel systems, it's not corrosive, it's the water
that results with using Ethanol fuel.

It will, more than likely, run like crap till the old fuel is gone. I would at least
attempt to flush the tank, if anything... That way you can check for rust and
all that...

Right about the water. In cold and humid country, like Chicago where I spent my first 30 years, we knew that each time a tank ran low on gas, it was filled with humid air which was admitted from outside. The inside tank walls allowed water to "plate out" of the air, then sit in the bottom of the tank. For that reason, a product called "Heet" was often added to the fresh tankful of gas, to absorb the water accumulated there. Heet was in reality Methanol, or wood alcohol. Gasoline will not mix AT ALL with water, but WILL mix with a solution of alcohol containing water. The Heet then got burned in the engine, mixed with the fuel.

Kinda contradicts the idea that gas containing Ethanol brings water into the mix. The imponderable is, how much water is in commercial Ethanol (likely 10%) and how does that compare to the amount of water admitted to the tank in the air which cannot be stopped from entering, even with the new "closed" systems. That may not be much, volume-wise, and the Ethanol present absorbs it. With NO alcohol present, puddling water in a tank, especially today's tanks, with a small "sump" containing the pump pick-up, where the water collects due to gravity. So, 10 gallons of 87 Octane gasoline containing 15% Ethanol has 1.5 gallons of Ethanol in it, which brings .15 gallon of water into the mix. That .15 gal. of alky would have about 19 ounces of water in it.......a little over 2 cups! And, what about E85?

Since the introduction of alky into gasoline, either manufacturers secured metal parts better-protected by plating, or the problem of alcohol-caused corrosion should be completely out of control. I've heard all the rumors, but seen few hard facts.

Anyhow, I'd rather DRINK it than BURN it, myself! Don't forget the Indy-500 was run for a mighty long time exclusively on 100% alcohol, no gasoline allowed. Those old Offenhausers screamed! imp

Ethanol attracts water and it breaks down faster than gasoline. So, atmospheric water,
rain, or snow getting in the fuel fill, just makes it worse. So, with water absorbing into
the fuel, there is the chance that rust can form anywhere in the fuel system. This is
why stainless fuel lines should be used, and an aluminum fuel tank. Even plastic
tanks can be permeated. The Ethanol itself is not the metal corrosive, it's the water
absorbed into the fuel by the Ethanol. If you have a PZEV, it all has to be stainless,
and the fuel tank has to be metal because plastic can be permeated, so it cannot qualify
as PZEV with a plastic fuel tank.

Ethanol IS corrosive to the seals all along the fuel system path. It's also bad for
smaller engines. It's like a cleaning agent, always running through the system.

Even regular unleaded (E10) begins to break down within 3 weeks. This break down creates
clumps in the gasoline mixture at some point, and may clog the filter, injectors, fuel line, etc.
To help prevent that, you would use Sta-bil, just add it as directed to their gas. Nothing is
going to last years except treated diesel.

Because I run 470 miles a day, I run E15 in my work vehicle, but it is a PZEV.

I run 93 octane in my Mustang.