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Trouble Shooting Gas Fume Leaks on older Corvettes and other cars and trucks

Helping Your Nose Know

John Pfanstiehl Jul 21, 2017
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Nobody wants to be assaulted by the stench of gas fumes when entering a garage. Years ago, some of us truly enjoyed the smell of gasoline. It foretold of imminent fun, fast rides and freedom. Although driving still thrills us, either our noses have changed or gasoline has changed. Today, owners of older Corvettes frequently complain of gas fumes whose source they cannot locate.

In this article we will look closely from stem to stern, or more accurately from gas cap to carburetor, at the many potential sources of unwanted gas fumes. Troubleshooting is not always as simple as it might seem. A single drop of liquid gasoline that never makes it to the garage floor, or simply fumes escaping into the air, can easily affront the nose.

Our sense of smell is pretty remarkable. Although we cannot track the trail of fugitives as skillfully as bloodhounds, humans can detect as little as 0.1 parts per million (ppm) of gasoline. That sensitivity rivals the best electronic Combustible Gas Detectors. Detectors still have advantages: they are objective, they can reach into confined areas and they don’t suffer from olfactory fatigue like we do. If you feel your nose is not enough for troubleshooting, detectors claiming 5-ppm sensitivity for gasoline fumes can be purchased for just over $20.

One surprising and troublesome problem is that hoses appearing to be in good condition and not leaking can be allowing fumes to migrate through the rubber. A friend recently learned this lesson on a much larger new fiberglass toy. After a few months of use, each of his stateroom heads began smelling like a porta potty at the end of a five day BBQ festival. It turned out the offensive odors were migrating through the rubber hose connections even though the hoses were dry on the outside. Replacement with better quality rubber hoses solved the problem.

To prevent gas leaks and unwanted fumes, it is critical to use good quality hoses that were designed for this application. The rubber hoses should be marked on the outside with “FUEL” and “EMISSION” to hold up to the many blends of gasoline. It is equally critical to use this type of hose on the evaporative emissions control (EEC) system components because gas fumes are just as tough on rubber as is liquid gas. Replacement and reproduction hoses and hose sets can make the job easier. One particular example is that the proper molded “S” hose should be used on the fuel pump.

The steel lines running between the rubber hoses can present surprises, too. Things were simpler for C1 and C2 Corvettes. A single fuel line ran from the gas tank to the fuel pump. During the third generation, a second, smaller fuel line was added to return gas back to the tank from the fuel pump. This allows gas to be constantly moving through the lines, which greatly reduces the chance of gas boiling in the lines and creating a vapor lock while the car is caught in a traffic jam on a hot day.

A third line was then added to vent gas fumes from the tank when Corvettes went to sealed gas caps. This small line ran from the gas tank along the driver side of the frame to a vapor canister in the engine compartment. All of these three lines can rust and leak over time but this third vapor line should not have gas inside and therefore can leak fumes without ever showing a telltale drop of gasoline. An easy way to test this steel tubing is to block one end off and then apply suction to the other end.

Although the following focuses on early Corvettes, much of the information applies to all generations of Corvettes, with details varying from year to year. Before inspecting and replacing parts, a little troubleshooting may help identify a problem at either end of the car. For example, are the gas fumes evident only after filling the gas tank? If so the gas cap, its gasket, the filler neck gasket (or hose on C1s), or the fuel vapor separator and its hoses are prime suspects.

Are fumes only noticed when the car is shut off? Start the engine when cold and then shut it off. If fumes are noticed later, there’s a good chance the carburetor is leaking. If fumes are only noticed when the engine is shut off when hot, then look inside the carburetor. Seeing gas dripping in the throttle bores indicates that gas may be percolating in the carburetor. An insulator plate may need to be installed to reduce the conduction of engine heat to the carburetor.

If your eyes tear from gas fumes when entering your garage, it’s time to fix the problem. Otherwise, pressing your garage door opener may result in an unplanned remodel. Finding and fixing the source of gas fumes is not only good for your wallet and the environment, it frees your nose to relish the other olfactory pleasures of a well-stocked garage, including fresh leather interiors and new tires.

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1. Let’s start with the gas cap. After all, this is the beginning of the journey of gasoline through your car. Make sure it is a sealed cap if that’s what your car came with and that the evaporative emissions control (EEC) system has not been disconnected. And make sure the gas cap’s gasket is in good shape.

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2. With the current trend to offer only expensive complete part assemblies, it’s a pleasure to see that individual component parts like this gas cap gasket are available. Zip Products offers either the gas cap with gasket or the gasket by itself.

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3. Remove the gas lid and bezel for access to the rubber boot (gas tank filler neck seal) and the gas tank filler neck itself. Use extra caution to ensure that the lid and bezel do not damage the surrounding paint during removal.

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4. Look for a rubber bumper inside the gas lid spring mechanism. It cushions the lid when opened and prevents the gas lid from opening up too far.

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5. Carefully work the filler boot out and over the filler neck. Insert something inside the filler neck to keep dirt from falling into the gas tank when removing the old boot. The filler boot should fit snuggly around filler neck. Tip: to test if the filler neck gasket leaks, now take a spirited ride to slosh the gas around with the tank full.

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6. This is a good time to look into the gas tank for rust, dirt and water. Puddles of water are hardest to see. Bump the car a little to get the gas to slosh to see the outline of the water as it settles in the recess of the tank ribs. Tip: Water and dirt particles can be vacuumed out using a steel tube with a rubber siphon hose. In this case, there’s no water visible but the filter sock has deteriorated.

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7. It’s easy to access the filler neck bolts after removing the boot. Take special care not to drop the bolts down onto the tank, or worse, into the tank. After removing the filler neck, if necessary use a small hammer to tap the neck’s flange area near the boltholes until its seal surface is flat again.

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8. The filler neck gasket is cork and therefore can shrink and fail over time. It is also fragile so be careful handling it. Apply a gasket sealer to both sides of the gasket and on the bolt threads before installation.

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9. Zip Products offers the overflow hose, nipple, spring and tie as a kit. Note that this rubber hose has extrusion ribs on the outside as original. The spring goes inside to keep the thin-wall hose from collapsing. On chrome bumper cars, the cable tie holds the overflow hose inside to the bumper’s lower support rod.

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10. Installing this spring into the rubber overflow hose can be challenging. Tip: spray plenty of silicone lubricant on both the inside of the hose and on the spring. Tie a piece of wire to one end of the spring, feed the wire through the hose and then pull the spring through the hose up to near its end.

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11. Insert the plastic nipple through the filler neck boot and into the overflow hose before installing the boot over the filler neck. Use special caution again to protect the paint when installing the gas lid and bezel.

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12. The new gas cap and boot from Zip Products are much more pleasing to the eye during the all too frequent fill-ups. Plus, there are not many Corvette parts easier to change than these.

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13. Replacing the two hoses on the fuel vapor separator is more of a challenge because it’s located on the top of the gas tank and is barely visible. This view shows what the separator looks like and helps because you’ll be working more by feel than by sight. Paragon offers these two hoses with original-style clamps as a set.

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14. At the other end of the car, the fuel vapor canister has one hose connected to the steel line on the frame that runs back to the fuel vapor separator. Use extra caution removing hoses from the plastic nipples on the canister as some canisters are no longer available. Slitting the old hoses can make their removal less stressful on the canister (or the vapor separator). Paragon offers hoses for both the fuel and emissions systems.

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15. Use locking pliers to clamp shut the supply hose from the tank. Cut and remove a portion of the hose that connects to the tubing on the frame. Then install the new hose with both clamps onto the tubing. Finally, remove the remaining portion of the old hose from the tank and immediately slip the new hose on in its place. This enables installing the new hose with minimal leakage of gas without draining the tank.

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16. A molded 3/8-inch S-shaped hose delivers the gasoline to the fuel pump. Clamping the supply hose at the gas tank helps minimize gas leakage when replacing this hose. The fuel pump’s gas return hose is 1/4-inch and its bends are also molded. Corvette America offers these two hoses as a set.

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17. Corvette America also offers the molded 3/8-inch hose by itself in a replacement version and in a reproduction version with clamps. Replacing the fuel pump hoses requires access from below. Tip: the hoses slide on easier when the spring clamps are installed onto the tubing first. Also, make sure the fuel pump hoses do not contact the frame or they can wear through as the engine rocks.

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18. This is a good time to consider eliminating an aftermarket inline fuel filter if one has been installed. Because this fuel line is under pressure, the effect of a leaking or burst hose or filter can be catastrophic.

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19. The reproduction steel fuel line from Corvette America looks better and is much safer. Make sure there’s a fuel filter inside the carburetor, it may have been removed when the aftermarket inline filter was installed.

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20. It’s easy to determine if a Q-jet is leaking internally. After shutting off the engine, insert a small, lightweight rod or strip into the float bowl vent tube until it rests on the top of the float. Mark the height, and check it the next day. If the float has dropped, the bowl has drained.

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21. If gas fumes are particularly strong after shutting off the engine when it’s hot, remove the air cleaner and look down into the carburetor. If gas is dripping or if the venturies or throttle plates are wet, then gas is likely boiling in the float bowl. Now you’ve completed looking at the common causes of unwanted gas fumes from stem to stern.

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22. Be sure to check the garage for other potential sources of gas fumes. Gas can nozzles are prime suspects but also inspect anything that uses gasoline, such as a lawn mower, chain saw or weed whacker. These often have vented gas caps, which can expel fumes when the temperature rises.

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23. Combustible gas detectors have advantages over your nose: they are objective, they can reach into confined areas and they don’t suffer from olfactory fatigue. Prices vary widely but we found a couple of gas detectors available online for about $20.

Photography by John Pfanstiehl

Sources

Zip Products
Mechanicsville, VA 23111
800-962-9632
www.zip-corvette.com
Paragon Corvette Reproductions
Swartz Creek, MI
800-882-4688
https://www.paragoncorvette.com/
Corvette America
Reedsville, PA
800-458-3475
www.corvetteamerica.com

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