I recently had the dubious pleasure of replacing the fuel tank selector valve on my 1989 Ford F-150. It's a kind of Rube Goldberg contraption that uses pressure from a low-pressure pump located in the fuel tank to operate valves that direct gas to the high pressure pump, filter, fuel injector rail and back to the tank. I really don't expect parts containing rubber diaphragms to last much more than 20 years, so I wasn't that perturbed, although the availability of this Ford-only part left much to be desired. Compared to the caucus race of locating the part (for "only" $80) installation was uneventful.
Unfortunately, the substitution didn't fix my problem. To my surprise, the valve was actually good except for being stuck in one position by the accumulation of gunk from the gas tank. (I should have checked!) In this day of skyrocketing gasoline prices, you might well want to check your tank(s), because a car will still run with a disturbing accumulation of sludge in the tank, but it will perform a lot better with a good supply of clean gas!
Siphon a little gas from your tank into a mason jar and compare its color to some clean gas from an approved container. Is it nice and light, almost clear -- or is your gas an amber yellow tending toward brown? If so, you probably have an alarming accretion of tarry goo in your tank. Gasoline, kerosene, and Diesel fuel are all petroleum distillates -- refineries simply isolate a particular class of hydrocarbons from crude oil using an elaborate "still" called a fractionating tower. They also alter their product mix by "cracking" and polymerizing, but we won't go into that -- it's enough to know that the long-chain hydrocarbons (like gear oil) are dark and viscous, while short-chain fuels are light and fluid. Gas is really just very thin oil.
Of course everybody knows you can't make 200-proof moonshine without distilling your mash more than once, and your fuel isn't all one type of hydrocarbon molecule, either -- there are lighter and heavier molecules mixed in with those with average-length chains. Over time, the shorter-chain molecules are burned or evaporate, and the longer-chain ones accumulate as a tarry sludge in the bottom of your tank. As you add gas some of this residue dissolves, adulterating your fuel -- eventually to the point where your electronic fuel injection can no longer cope with the garbage you're feeding it.
Removing your fuel tank is relatively straightforward. Generally speaking you will find a fill tube, fuel delivery and return lines, an electrical connector and a vent line to be detatched. You then only need to empty the tank and remove the bolts that secure it to the frame. If you don't already have one, a shop manual for your particular vehicle is recommended for more detailed instructions. A floor jack is helpful in this procedure, but not absolutely required.
The traditional method of cleaning a fuel tank is steam cleaning, but I was unable to find anyone who still offers this service locally. I have no idea what automotive repair shop hours are going for these days since they priced me out of the market years ago, but I suspect that if you do find someone to clean your tank, they will charge you nearly the cost of a new tank. That would account for the disappearance of the service. The prospect of taking a perfectly good tank to the landfill didn't appeal to me, so I devised the following cleaning procedures:
If the design of your tank is such that you can get your hand inside it, begin by scraping as much goo and rust from the inner surfaces as you can. This is a thankless task, but its fast and productive, so keep at it using whatever paint scrapers, putty knives, pry bars, and wire brushes you have on hand. After removing all the debris you can reach, the following procedures will be much more effective on the relatively small amounts of contaminants that remain.
Old-fashioned soap-making involved mixing grease (usually lard) with caustic soda or lye (NaOH). You can remove the oily residues from your tank with crystal drain cleaner. I used Crystal Drano, but any brand should be fine. You might also use any suitable oven cleaner. These products are hard on skin and eyes, so the use of rubber gloves and safety goggles are recommended, if not expected. You don't have to saponify all the tar in your tank, but just enough to dislodge it from the metal surfaces, so go easy on the lye and add more as required. Be sure that all the surfaces of the tank get a good soaking.
Once the tar is removed, you're likely to find hydrolyzed aluminum silicates in the tank -- better known as dirt. You can remove this by sloshing around a strong solution of laundry detergent or trisodium phosphate. Lather, rinse, repeat. When the soapy water comes out clean, you've done the best you can.
If your tank is polypropylene, count yourself lucky. If it's steel, you're going to want to do something about rust. Due to the nature of metallic bonding and the accumulation of electrical charges, "rust never sleeps," but you can slow it down a little. The "right" way to do this is by lightly sand-blasting or bead-blasting the interior of the tank. You could do the same thing with sandpaper if you could reach all the surfaces. Of course, the paper is just a convenience -- the essential abrasive is sand.
Sponge out the inside of your tank and let it dry thoroughly. Wet sand will clump together and won't expose as many jagged little surfaces as dry sand. Put a generous amount in the tank and tilt it back and forth. If you were using sandpaper, you know that the harder you pushed down the faster this would go. Unfortunately, you can't exert much pressure here, so just be patient and think of what a great work-out you're getting. Peek into the tank with a flashlight to see how you're doing. It is not necessary to sand the tank down to bright metal, you just need to remove the loose rust scale. If you like, you can use phosphoric acid (better known as naval jelly) at this point.
Don't try to remove all the rust. Your tank started out as galvinized steel, but now that the zinc coating has eroded away, the phosphoric acid can convert the thin coating of rust that remains to metalophosphate, providing at least some corrosion resistance through passivation. You can ensure that all the acid is neutralized by treating the tank with baking soda. Be sure to rinse and dry the tank thoroughly, and you're done!
If you tend to be a picky, don't let the perfect become the enemy of the good. Your tank will probably never pass a white glove test, but think of the pounds of gunk you've removed rather than the ounces or grams that remain. On the other hand, if you're a lick and a promise sort, perservere. A little elbow grease now will save you a lot of grief down the road.
If all this seems like a lot of trouble to go through, it is. I'm sure many more people will simply replace their tanks than will follow this procedure. Hopefully, a few will buy their new tanks here
. On the other hand, there are some who are as stubborn as I am, and quite a few who are offended by the cultural notion that everything is disposable. If you do rehabilitate your tank, at some point it will occur to you what a poor choice petroleum fuels are. You might even wonder why gasoline is the "mainstream" choice, and gaseous fuels like methane are termed "alternative" fuels.
Consider the fact that many fleet operators converted to compressed natural gas (CNG) decades ago, because it was more cost-effective, yet there is only one production CNG car on the market today, and practically no CNG filling stations, despite the fact that anybody with natural gas and electicity can "Phill" up at home. Consider that GM built and leased an electric car, the EV1 (to comply with California's stringent emissions requirements) that everybody seemed to love, but killed it because it didn't break down and contribute to the lucrative repair side of their business.
That goo isn't just in your tank. It's in your fuel lines, fuel gauge sender, injectors, valves, cylinders -- throughout your engine really. All completely unnecessary, but very lucrative for the special interests that continue to maintain that alternative energy technologies of twenty years ago "will take decades to develop." Don't let the fact that T. Boone Pickens' plan is to make a pile of money for T. Boone Pickens distract you from the fact that cost-effective alternative energy is available now.
Further resources
Internal Combustion: How Corporations and Governments Addicted the World to Oil and Derailed the Alternatives Book
Who Killed the Electric Car? DVD
21st Century Complete Guide to Natural Gas Vehicles CD
Build Your Own Electric Vehicle Book
or the gift guides 
