Description And Operation: Description

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.

[No leak] 

Fig 1: Natural Vacuum Leakage Detection Graph (No Leak)

Courtesy of KIA MOTORS AMERICA, INC.

[With 0.5mm leak] 

Fig 2: Natural Vacuum Leakage Detection Graph (With 0.5MM Leak)

Courtesy of KIA MOTORS AMERICA, INC.

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.