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Evaporative Emissions Issues - Technically Speaking

James Berry Aug 1, 2013
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My ’05 Corvette has a “check engine” light that refers to an evaporative leak. I had similar fault codes on my ’07 Impala recently, and that repair cost me more than $500. Worse, the light came back on a few days after the car was “fixed.” When I took it back to the shop, they stated that they couldn’t find a leak, so they simply shut off the light with a code reader. It came back on again after about a day.

I know there is a charcoal canister on both cars, but I have no idea how this system works. Since I have my own scanner and do most of my repairs at home, I’m hoping you can walk me through the process of fixing it myself.

Scott - Via email


Prior to the introduction of evaporative-emissions systems, the gas tank was vented into the atmosphere through the gas cap. This allowed air to enter the tank as the gasoline inside was consumed, thus filling the void. On warm days, when the gasoline vaporized, the fumes and pressure vented backwards through the cap into the atmosphere. This is known as an “open” system.

The Clean Air Act of 1970 required a 90 percent reduction in vehicle emissions by 1975 and established the U.S. Environmental Protection Agency (EPA) to manage and enforce the new regulations.

The EPA quickly discovered that a significant amount of hydrocarbon emissions were emanating from gas-tank fuel vapors. To combat this, the agency established evaporative-emissions standards in 1971. These new laws resulted in the implementation of “closed” or Evaporative Emissions Control systems (EVAP). Soon after, the first charcoal canisters were introduced to trap gasoline vapors, ensuring that none escaped into the atmosphere.

Starting in 1996 vehicle manufacturers were required to comply with even stricter EPA evaporative-emissions standards. The new standards required vehicles to have the ability to perform a diagnostic self-test of the evaporative fuel system, to ensure that there were no leaks and that the system was functioning properly. Since the inception of these stricture emission standards, evaporative-emissions fault codes have accounted for approximately 30 percent of all “check engine” lights.

How the System Works

A closed system must have a way to vent the vapor pressure generated by ambient heat, so car manufacturers developed a method of short-term storage that uses a plastic container filled with activated charcoal. As vapor pressures rise, the excess vapor is pushed through a vacuum line into the canister, where it can be absorbed by the charcoal. The same process occurs whenever you fill your tank with fuel.

At some point the charcoal becomes saturated with these gas vapors, so it purges them into the engine to be burned. How? The vacuum line contains a purge solenoid, while another line exposed to the atmosphere contains a vent solenoid. When the vent solenoid is open to the atmosphere, and the purge solenoid is open to engine vacuum, outside air is drawn across the charcoal inside the canister, forcing the gas vapors into the intake manifold to be burned.

OBD-II–equipped (’96-up) vehicles control the frequency and duration of these system purges; they also test for purge volume and leaks. A typical purge system only works at idle and under part throttle.

Key Components

  • Charcoal canister: While the charcoal canister itself does not commonly fail on OBD-II vehicles, such failures do occur from time to time.
  • Vent-valve solenoid: The vent-valve solenoid controls fresh airflow into the charcoal canister. This valve is normally open to the atmosphere, since a properly functioning canister and filter create a natural restriction in the system. This is the most common failure item in OBD-II vehicles, especially in dusty environments where the filter can become plugged.
  • Purge solenoid: The purge solenoid controls the flow of vapors from the charcoal canister into the intake manifold. Normally closed, this valve is opened using pulse-width modulation from the engine controller. This solenoid is normally located on or near the intake. A quick way to test this component is to check resistance across the two pins of the solenoid. If the reading is infinite or open, the solenoid has an internal failure.
  • Fuel-tank pressure sensor (FTP): Located on top of the fuel-pump module, the FTP measures the difference between the pressure/vacuum in the fuel tank and outside pressure. It operates on a 5-volt reference signal from the engine controller, which you can monitor in the sensor-data section of your scanner. Normal voltage will vary between 0 and 5 volts. Low voltage indicates high fuel-tank pressure, while high voltage indicates a low fuel-tank pressure. If this sensor fails, the voltage will typically “lock” on one reading and not fluctuate during a testdrive. The FTP is not a common failure item.
  • Other items that commonly trigger EVAP fault codes include a loose or defective gas cap, and cut, worn, or loose vacuum lines in the EVAP system.


The vent-valve solenoid controls airflow to the charcoal canister. It’s been known to fail and trigger an EVAP fault code, especially in dusty environments.

Step-by-Step Testing

Now that you know how the system works, let’s look at the step-by-step EVAP-testing procedure. Note that you’ll need a purpose-built smoke machine to perform a proper diagnosis. While these machines can be expensive when new, you may be able to locate a more-reasonably priced used one through Craigslist or eBay.

  1. Start by confirming your codes using a scan tool.
  2. Don’t disturb any components, hoses, or the gas cap.
  3. If your scan tool is so equipped, run a forced EVAP test. If the system fails, there’s definitely a leak. Even if it passes, there could still be an intermittent leak.
  4. Calibrate your smoke machine using the manufacturer’s instructions.
  5. Connect the smoke machine to the test port. (If the vehicle you’re testing is not equipped with such a port, connect near the purge valve.) You’ll need to test both sides, upstream and downstream.
  6. Fill the system with smoke until you either find the leak or smoke comes out of the vent valve. If smoke emanates from the valve, and you can’t find any other leaks, you’ll need to close the valve using your scan tool. If your scanner is not equipped with this function, you can close the valve by supplying voltage and a ground. Don’t simply block it off, as it may prove to be the leaking component.
  7. If you don’t see any smoke coming from the vehicle, check the smoke machine’s flow gauge while tapping on hoses and other components. Look for a change in the gauge reading.
  8. Cycle the vent and purge valves to isolate an intermittent leak. Remember, leakage in the purge valve will dump smoke into the intake manifold.
  9. Some components—such as the gas cap and the filler neck (above the sealing ball)—may fail only under vacuum. For this test, close the vent valve, apply light vacuum to the system with a scan tool, and watch for vacuum decay on the FTP sensor.

I hope this information helps get you pointed in the right direction. Good luck with your EVAP-system test, and let us know how it turns out.

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Just jot it down on a paper towel or a lightly soiled shop rag and send it to us at VETTE Magazine, Attn: Technically Speaking, 9036 Brittany Way, Tampa, FL 33619. Alternatively, you can submit your question via the Web, by emailing it to us at vette@sorc.com. Be sure to put “Technically Speaking” in the subject line.



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