Hydrocarbon Adsorber Catalyst System


Hydrocarbon Adsorber Catalyst System (HCAC) system is used to improve emissions when three-way catalytic converter temperature is low. HCAC system consists of HCAC Vacuum Switching Valve (VSV), actuator, bypass valve and Three-Way Catalytic Converter (TWC) with Hydrocarbon (HC) adsorber. Actuator and by-pass valve are located at front of TWC on exhaust pipe. See Fig. 64 . HCAC VSV is located on bracket bolted to timing chain end of engine. It has a Brown electrical connector which contains Black and Black/White wires. See Fig. 65 .

Before engine is started, by-pass valve is open. When engine starts, Engine Control Module (ECM) delivers an output signal to HCAC VSV. HCAC VSV opens, allowing vacuum to be applied to actuator, which causes the

2001 ENGINE PERFORMANCE Theory & Operation by-pass valve to close. When by-pass valve closes, exhaust gases pass into the HC adsorber, where the gases are stored until the temperature of front TWC increases. This prevents hydrocarbons from being delivered from vehicle when TWC temperature is low. After TWC becomes warm, the HCAC VSV closes and vacuum to bypass valve is shut off, causing by-pass valve open. When rear TWC becomes warm, exhaust emissions are cleaned by the TWC. The HCAC VSV may also be operated during a deceleration condition when HC adsorber is warm in order to scavenge the hydrocarbons that remain in the HC adsorber.

Hydrocarbon Adsorber Test
Fig. 64: Locating Actuator, By-Pass Valve & HC Adsorber (Prius) Courtesy of TOYOTA MOTOR SALES, U.S.A., INC.

2001 ENGINE PERFORMANCE Theory & Operation


2001 Sequoia Vsv


Fig. 65: Locating HCAC VSV (Prius)



2001 ENGINE PERFORMANCE Theory & Operation

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