Introduction. This article discusses the diagnosis and treatment of a smell of raw gas in an ATV. While written about my 2010 Polaris Sportsman 550 XP, it is generally applicable to all ATV’s and hopefully will be helpful to anyone experiencing a gas smell. The bottom line, provided for those who doesn’t want to read further, is that the gas smell on my ATV comes from the gas tank venting system (described below) and has been virtually eliminated by using non- ethanol gas.
Background. I have been smelling gas for about a year, off and on, although generally only in hot weather and always only after the ATV was running for a couple of hours and most noticeably when moving slowly or stopped. After a ride near Featherville in April, the smell got so strong that other riders noticed it and it became obvious that something had to be done to make sure there was no danger of a fire. I conducted a series of examinations, did Internet searches, re-routed my gas tank vent line so I could observe it and tried different gasolines.
Separating the Gas Smell Causes. Gas smells can be separated into those caused by a leak and those caused by the gas tank vent system. Leaks can come from the carburetor bowl vent on carbureted ATVs, a fuel line, the gas tank fill line, leaking gas cap, the gas tank drain line if there is one, or the fuel pump gas tank gasket. Most leaks show up as wet spots somewhere. In general, if the gas smell occurs all the time, regardless of whether or not the machine is warm, it is most likely to be caused by a leak. Gas smell related to tank venting generally will not appear until the ATV has been running for some time because it takes time for the fuel to heat enough to generate a significant smell.
Gas Tank Hardware. All ATV’s have vented gas tank with a variety of different designs. Polaris, for example, uses a non-ventilated gas cap with a clear plastic tank vent hose that comes from a “roll-over valve” on the top of the tank and terminates with a small, red-tipped cylindrical filter located under the headlight console and no check valve. The filter is to prevent dust from entering the gas tank as air comes in to replace the used gas. Other machines, such as Kawasaki, have vented gas caps, sometimes with a small vertical stem on the cap and other times with a vent line that terminates into the ATV frame. Some machines have pressurized gas tanks. Others use a check valve that lets gas vapor escape but allows air to come in as the gasoline is burned. No ATV has a vapor control system like those mandated by the EPA on automobiles with filters and capture canisters, although such a system may be required in the future.
Gasoline Characteristics. All liquids create a vapor at the liquid surface. The pressure of this vapor, which determines how much vapor there is, depends on the characteristics of the liquid, the outside pressure the liquid/vapor mixture is under and the temperature of the liquid. Increasing the liquid temperature can increase the vapor significantly. Gasoline, for example, has a vapor pressure of about 8.5 psi (pounds per square inch) at atmospheric pressure and room temperature, which is more than enough to smell it. Gasoline vapor is lighter than air, so it tends to rise. The addition of 10% ethanol to regular, non-ethanol gasoline, increases the vapor pressure of the mixture to about 9.5 psi, a 12% increase, at 130 degrees F, according to a study conducted by the American Petroleum Institute (API). If the fuel is at 32 degrees F, there is almost no increase in vapor pressure from the ethanol. There is a slightly smaller vapor pressure increase when ethanol is added to premium gasoline.
Fuel Heating. There are 3 general sources of fuel heating on an ATV: (1) Air flow from the radiator and/or engine heating the gas tank; the amount of heating depends on where the tank is located; (2) Radiant heat from the engine, absorbed by the black gas tank and transferred into the fuel; and (3) Fuel returned to the tank from the fuel injection system. In the case of my ATV, Polaris has chosen the worst possible location for the gas tank by putting it inside the frame under the headlight console, about 5” ahead of the engine and about 14” behind the radiator. It is shielded by plastic panels on each side but not on the front or back. This arrangement works because the ATV is fuel-injected. The fuel pump increases the fuel pressure to 40-60 psi, which prevents it from vaporizing in the fuel line and causing vapor lock. However, fuel-injection systems return as much as 95% of the fuel pumped out back to the tank, depending on the throttle setting; when the fuel is returned to the tank, it is hotter than when it left.
Diagnosis. To test for the source of the gas smell, I moved the clear plastic gas tank vent line from under the headlight console, extended it about 8” with a ¼” ID clear plastic line and zip-tied the line to the left side of the handlebars. This arrangement allowed me to observe the line. I fueled the ATV with regular E-10 gasoline, as I have been doing for about a year. It was immediately obvious what was going on. After about 2 hours of running on a warm day, I started seeing bubbles in the vent line, sometimes to the extent that small droplets of liquid gas actually came out of the extended vent line and splashed onto my leg or the side of the ATV. This occurred on several rides under about the same conditions with ethanol gas in the ATV. It is no wonder I was smelling gas and somewhat amazing that I didn’t have a more serious problem with this spillage when the vent line terminated under the headlight console. For the next 3-day ride, I refueled with premium non-ethanol gas but the tank was only about half empty. During the first day, the bubbling behavior was unchanged with about 50% E-10 gas. The next day’s ride and refueling with non-ethanol gas, and with about 10% E-10 gas in the tank, there was a noticeable decrease in bubbling. The following day’s refueling with non-ethanol and almost no E-10 left, there was no bubbling at all. Therefore, with no other changes other than gas, it is pretty clear that ethanol is the cause of the gas smell cause in my ATV.
Discussion. With an open vented tank, there is always going to be some vapor that escapes from the tank due to fuel heating, although it is normally not enough to notice when riding. The amount of vapor is significantly increased by ethanol gas, as confirmed by API testing. The 2-hour delay in bubbling on warm days and no bubbling in cold weather is explained by the time it takes for the fuel to heat. I do not believe that ATV’s are designed to use ethanol fuel and the Polaris fuel tank location and vent system contributes to the problem. Discussion of this problem with Grizzly, where I bought my ATV, and their subsequent discussion with Polaris technical folks resulted in a recommendation to re-route the gas tank vent line along the frame and out the back of my ATV. I was reluctant to do this until I understood the source of the problem, but I have now done so in a way that I can continue to observe the tank vent line if there is bubbling in the future.
A second Polaris recommendation was not to over-fill the gas tank. As with most ATVs, there is an additional half-gallon that can be put in the tank if I fill it to the bottom of the plastic fuel line under the gas cap – this adds over 10 miles to my ATV’s range. In my testing, I filled the gas tank completely full and it made no difference in the bubbling and gas smell. This is explained by the fact that it takes a couple of hours of running for the gas to heat and by that time the gas tank was down to about 80% full.
Recommendations. I strongly recommend using non-ethanol fuel but do not believe it is necessary to use premium. Because different manufacturers use a variety of approaches for their gas tank venting, my recommendation is that you examine your machine and see what vent design is used. Grizzly maintenance folks did say they have had a hard time finding a gas smell problem on some Kawasaki machines because Kawasaki vents the tank into the frame and any gasoline bubbling puts liquid inside the frame, where it will run out at the closest weld joint where there is a small gap. ATV owners of other machines on the Internet have reported a gas tank rupture due to over-pressurization and a gas collapse due to a check valve failure, although these seem to be isolated instances. A periodic check on the gas tank itself as well as the operation of any check valves in well worthwhile.
Any input or comments would be appreciated.
Brian Sack, Member of BATRC