Q:

I have a 1960 Corvette and have a gasoline odor problem that I have not been able to repair, and I wonder if you can help me with it? My Corvette runs fine, but whenever I fill the tank, the smell of gasoline is very strong inside the car. It stays that way until the tank is at least half empty. I have checked for a gas leak all around and under the tank, and all along the fuel line that goes up to the engine. There is no leak evident any where around the fuel pump, filter or carburetor. There is no wet area that I can find anywhere under the car. I have replaced the gas tank sending unit, the gasket, and the screws with the special rubber washers, the vent hose and the gas cap, twice, and still I smell gas. My wife won't ride in the car until I fix it. What can be wrong?


A:

I know what a problem that can be. I have had that same issue with several early Corvettes myself, and it can be difficult to find the source of the raw gas odor.


The bad news is that it only takes a drop of gasoline to smell up the whole car. The good news is that it can be repaired. The best hint in your letter is the fact that the smell goes away when the gasoline level drops down below one half tank. You mention that you have checked for leaks ail around the tank. I presume that you have looked at the seam that splits the tank in two. It is located half way down the tank; and it goes all around the tank horizontally. The tank was made in two halves, and welded together with a standing seam. Some times, the seam gives way to a minor leak that will only let a tiny amount of gasoline out. It will not be enough to cause a drip to fall off, because it will evaporate before it can drip. We call that type of a leak, wicking, because it just wets the affected area, and doesn't drip.


Usually, it will cause a very dark stain to form around the affected area. This stain will be black in color, and it will be like soot. That is, it will make your fingers very black if you touch it. It will seem moist, and depending upon how long it has been leaking, and how fast it leaks, the stain will grow larger, away from the source. You can easily see evidence of a leak like that. As you look at the tank from several feet away, you will be able to see that the stain is situated in a certain area, and the rest of the tank will be dry, and without that kind of color.


You may also want to look up under the tank from below the lower body panel, near the rear axle. There are two large holes that GM left open under the tank for venting, and, because these holes are quite large, you can see up inside the lower portion of the gas tank quite well.  Look for a dark, moist stain. That is the most effective way to identify that type of leak.


There is another prime source for a gasoline leak on the early Corvette. It is the connector hose that couples the gas tank to the filler neck. This, neoprene hose is clamped on the neck and tank outlet with simple hose clamps. The hose is hidden from view when you are looking at the top of the tank with the gas tank cover off. Because it is hidden from view, it is often missed when looking for a leak. As the rubber hose ages, it hardens and shrinks, causing the clamps to become loose. I have replaced many of these hoses where they have become as brittle as a potato chip. In this condition, there is little sealing of the gasoline, especially when the tank is full.


Also note that the gas filler neck is located at the top of the tank, and lies nearly flat. When the tank is full, there is standing liquid in the filler neck. This rapidly increases the rate of leak, and therefore the gas smell. When I remove the gas filler neck to replace the connector hose, I always check the gas cap and neck for leaks before installing it back on the car. I take the neck, with the gas cap on it, invert it, and fill it with water. Holding it up in the air, I can check to see if the gas cap will seal the liquid or not. It could fail because of a bad cap, or it could be because the top of the neck is damaged, so the cap cannot seal it. Many times, an owner would lose the key to a locking gas cap, and someone would have to pry the locking cap off of the filler neck. In doing so, the top of the neck, the smooth sealing portion, would become damaged, rough and irregular, and even a new cap wouldn't seal. Filling it with water is a cheap and effective way of checking the whole filler neck assembly. It will hold water if it's in good condition. If it seals properly, empty all the water, and blow out the tube and cap to keep water out of the tank. When you replace the rubber connector hose, be sure to use a gasoline resistant hose. We commonly call this a Neoprene hose. You must never use a water hose for this purpose because the rubber in a water hose cannot stand up to gasoline. It will cause the hose to swell up and fail.


After you have repaired your leak, and tested it to be sure it's fixed, be sure to seal the gas tank cover over the tank with some kind of sealer. The cover was designed to be sealed up air tight, so that any minor odors would be vented downward, out the large vent holes in the under side of the gas tank compartment. The gas tank is just behind the seats, and if the cover isn't sealed, you are effectively driving around with the gas tank in the cockpit with you and your wife. I use a non-drying auto body putty to seal the cover, just like GM did. Don't use something that will stick the cover down so tightly that it can never be removed again.