Evaporative Emission Control System Operation

Fig. 28a Evaporative emission control system. 1983 4-135 engine:

4 CYLINDER ENGINES
In this system, Fig. 28a, when fuel evaporates in the carburetor float chamber or fuel tank, the vapors pass through the vent hoses to the charcoal canister where they are temporarily held until they can be drawn into the intake manifold during engine operation. On fuel injected and turbocharged engines, the vapors are drawn into the engine at idle as well as off idle. This system is called bi-level purge system where there is a dual source of vacuum to remove fuel vapor from canister. On fuel injected and turbocharged engines, the source of vacuum at idle is a tee in the PCV system. On carbureted engines, it may have a vacuum source at the carburetor isolator.