About loose gas caps

One of the more common causes of the Check Engine Light coming on is a failure of the evaporative emissions system. This system is important to the environment because it prevents gasoline vapors from being released into the air. Gasoline vapors combine with sun light to form low-level smog. According to the Environmental Protection Agency unburned gasoline evaporating from the gas tank can be four to ten times the amount exiting the tail pipe. To combat pollution Federal law requires all cars to have a system that captures evaporated gasoline fumes and burn them in the normal engine operation.

The evaporative emission system is a very innocuous system that operates in the background - until it develops a leak. On most vehicles the system will run without a problem for five to seven years. Eventually something small begins to leak. All of this time your car has been testing the air-tightness of the system every day that you drive. When the test fails, the Check Engine Light will come on.

The simplest failure of the system occurs when someone forgets to put the gas cap back on after refueling, or does not tighten the cap adequately. You should turn the cap until it clicks three times. The computer does not test the evaporative system when the vehicle is not moving, so the Check Engine Light does not come on when refueling even if the engine is running. Federal law requires that the test detect a leak that is as small as half the diameter of a human hair.

The Check Engine Light does not pinpoint where the leak has occurred, only that there is a leak or other malfunction. The technician must first use a diagnostic scanner to check the electrical operation of the system. Very often the vehicle will pass this portion of the test. There are three valves in the system that must open in one mode of operation and close in another mode. Often times one of these three valves has developed a small defect in its seal. The valve functions correctly electrically, so the computer does not know that is where the problem is.

The next step is to perform a smoke test on the evaporative emissions system. This test results in an additional diagnostic charge. The diagnostic computer is used to command the evaporative system into a sealed mode. A special machine is attached to the evaporative system that fills the system with smoke and pressurizes the system to a very specific pressure. Then the technician has to visually inspect the system to see where the smoke is leaking from.

Sometimes the leak is coming from a small crack in a rubber hose anywhere along the bottom of the vehicle or under the hood. Other times the leak is found coming from a valve that the computer believes to be shut. The least common place for the leak to occur is from the gas cap itself. Gas caps are often replaced in hopes that this will correct the problem, but it rarely does.

Diagnosing evaporative system leaks is one of the most time consuming diagnostic procedures. The very small size of the leak that will turn the Check Engine Light on also means that there is often more than one leak present. The entire system is the same age. If there is one leak there are usually several others present. This is most common on cars where the leak is the result of brittle and degraded rubber hoses.

Brian Klaubert has written for the Neighborhood News for five years, and owns Christian Brothers Automotive – Hamilton Mill on Braselton Hwy in Dacula. They have won “Best Mechanic” in the 2007 “Best of Gwinnett” award in Gwinnett Magazine.