Description
The Evaporative Emission Control System prevents fuel vapor stored in fuel tank from vaporizing into the atmosphere. When the fuel evaporates in the fuel tank, the vapor passes through vent hoses or tubes to a canister filled with charcoal. The canister temporarily holds the vapor in the charcoal. The ECM will control the system in order to draw the gathered vapor into the combustion chambers during certain operating conditions. Engine manifold vacuum is used to draw the vapor into intake manifold.
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Process overview
If a leak is detected by NVLD, inspect the vehicle as the flow chart below to distinguish whether the error was caused by leakage of fuel system or NVLD itself.
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A ratchet tightening device on the threaded fuel filler cap reduces the chances of incorrect installation, which seals the fuel filler. After the gasket on the fuel filler cap and the filler neck flange contact each other, the ratchet produces a loud clicking noise indicating the seal has been set.
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- Turn the ignition switch OFF and disconnect the battery negative (-) cable.
- Lift the vehicle.
- Disconnect the canister close valve connector (A).
- Disconnect the ventilation hose (B) from the fuel tank air filter and canister close valve.
- Remove the fuel tank air filter assembly after removing bolts (C).
- Release the lever (A), and then separate the canister close valve (B) from the fuel tank air filter (C) after rotating it in the direction of the arrow in the illustrations below.
NVLD (Natural Vacuum Leakage Detection) is located between the canister and the fuel tank air filter. NVLD is to detect leaks equivalent to a 0.5 mm hole in the fuel tank, canister and hoses. When the engine has been running, the temperature of the fuel in the fuel tank is raised slightly. When the engine is shut off, the temperature of the fuel should drop. The ideal gas law states that in a sealed system the pressure drops proportionately to a drop in temperature.
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While the engine is shut off, ECM monitors a pressure switch that is built into the NVLD for detecting slight drop in pressure. If the switch change is not detected after an engine shut down, it assumes there is a leak in the sealed system.