EVAP system — what it is and what it does

I just had my check engine light fixed at a cost of $273. The adviser said it was an “EVAP solenoid” that had gone bad. I’m curious what this part does and why I have one. — Dallas M.

Cars and light trucks built since the early 1970s have employed an evaporative emissions system (EVAP) to reduce/eliminate the release of fuel vapors (hydrocarbons) from the fuel tank, and in the old days, the carburetor, to the atmosphere. Fuel vapors contain quite a few nasty chemicals and are a component of air pollution. Sealing the fuel system is said to reduce the total hydrocarbon emissions of a vehicle by about 20 percent.

Here’s how a typical EVAP system works: Metal or plastic lines and/or rubber hoses connect between the top of the fuel tank and a plastic canister containing charcoal granules. Charcoal has an amazing ability to capture and suspend a large quantity of fuel vapors. As the vehicle is parked or driven, the fuel tank breathes to the atmosphere via the canister, trapping vapors that would have otherwise escaped to the atmosphere. Solenoids (electric valves), along with additional hoses, are used to control the flow of vapors to the engine (purge valve) for consumption, and to temporarily seal the system for leakage testing (vent valve). Each time the vehicle is driven the PCM (powertrain control module), when conditions are appropriate, instructs the purge valve to meter stored fuel vapors into the engine’s intake manifold, cleaning out the canister. Adjustments are made to fuel injector delivery to compensate for this added dose of fuel and air via predetermined calculations and information from the oxygen or air-fuel sensor(s). Purging is important or the canister will eventually fill up with so much vapor it becomes saturated with liquid fuel.

Since 1996, government regulations require that vehicles test themselves to insure proper sealing of the fuel system and purge flow is occurring. The OBD-II (on-board diagnostics 2) system orders up a test (EVAP monitor) when/if conditions are appropriate, each time the vehicle is driven, typically with a fuel tank level between 25-75 percent. A fuel tank pressure sensor observes pressure and/ or vacuum before, and as the vent valve is closed, and the purge valve is cycled. When the vapor pressure falls (vacuum rises) due to being connected to engine vacuum, to a selected level, the purge valve is ordered closed. If the vapor pressure falls fairly rapidly and then holds steady during these two test stages, the PC Massumes the purge valve flows adequately and there are no system leaks. The vent valve is then opened to restore normal operation. Depending on the vehicle, there are variations in how this test and others may be done.

The OBD-II system is capable of observing electrical faults, inferring purge or vent valve problems, purge flow, and system vapor leakage as the EVAP monitor runs. Typical faults are a loose or defective fuel cap, hose or fitting leaks, a leaky canister or fuel tank neck, and sticky or leaky solenoids. Troubleshooting an EVAP system, verifying purge flow, finding leaks, and insuring repair success requires higher-end technical skills. It certainly doesn’t help that there are many system variations vehicle-to-vehicle, andmost of the components are difficult to access for testing/repair.

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