Enhanced EVAP System - Diagnosis And Repair: Overview
Group: FUEL SYSTEMNumber: 97-30-004
Date: AUGUST, 1997
Model: 1996-97 ELANTRA
1997 DOHC ACCENT & TIBURON
Subject:
ENHANCED EVAPORATIVE EMISSION CONTROL SYSTEM DIAGNOSIS AND REPAIR
This TSB provides information about system operation, diagnosis and repair methods for enhanced evaporative emission control systems.
DESCRIPTION:
OBD II regulations are very demanding of the evaporative system, and require that leaks as small as 1 mm (0.040 in.) be detected. In order for the system to detect any leaks, a pressure monitor located near the fuel tank and a solenoid valve used to seal the system have been added. The new evaporative system that can monitor system malfunctions is referred to as an "enhanced evaporative emission control system". The system measures small pressure changes (on the order of 0.002 psi) to detect if gasoline vapor is leaking into the atmosphere. If a leak is detected, the "Check Engine" light or Malfunction Indicator Light (MIL) will illuminate and set a diagnostic code.
The enhanced evaporative emission system consists of the following major components (see schematic):
Note:
All values are measured using the HDS.
Purge Control Valve:
Also referred to as the canister purge valve (CPV). An electrically operated solenoid valve that applies engine vacuum to the canister to remove fuel vapors stored in the canister. If the canister purge valve is removed from the vehicle, the valve should be closed, and no air should pass through the valve. When the valve is in the car and its control signal viewed on HDS, the valve is closed if HDS reads battery voltage. The valve opens (air passes through the valve) when the Powertrain Control Module (PCM) grounds the valve.
Differential Pressure Sensor (DPS) or Fuel Tank Pressure Sensor (FTP):
Measures pressure in the fuel tank relative to atmospheric pressure. The PCM uses the DPS values to calculate pass/fail conditions. With the gas cap removed, the DPS voltage should read 2.5 +/- 0.2 volts (this DPS voltage value will be referred to as "atmospheric pressure"). Vacuum in the fuel tank will read lower DPS voltage than atmospheric pressure, and a pressurized fuel tank will read higher DPS voltage than atmospheric pressure.
Charcoal Canister:
Absorbs fuel vapors from the fuel tank and stores them until they are purged into the intake manifold. Fresh air drawn in by engine vacuum replaces the fuel vapors and recharges the charcoal granules. The canister has three ports creating three circuits; a "Fresh Air" circuit, a "Purge - Engine Vacuum" circuit, and a "Fuel Tank" circuit.
Canister Close Valve (CCV):
An electrically operated solenoid valve that closes the fresh air supply to the canister; thus sealing the system. If the canister close valve is removed from the vehicle, the valve should be in the open position, and air should pass through the valve. When the valve is in the car and its control signal viewed on HDS, the valve is open if HDS reads battery voltage. The valve is closed (no air passes through the valve) when the PCM grounds the valve. To avoid excessive vacuum build up in the system, the canister close valve is normally open to allow fresh air to replace the purged fuel vapors in the canister. The valve is only used for system self check purposes.
Two Way Valve:
A directional valve that allows fresh air into the canister. The valve also acts as a pressure relief valve if the system pressure gets too high (approximately 3 psi). The arrow on the valve points toward the nipple that connects to the canister close valve (toward fresh air source).
Fuel Tank:
Must be completely sealed from outside air to prevent unwanted fuel vapors from being vented into the atmosphere. The fuel filler cap uses a rubber 0-ring to seal the cap, and a high pressure vent in case of a venting malfunction.
Fuel/Vapor Separator:
Returns liquid fuel to the tank and allows only vapors to flow to the canister. Liquid fuel may quickly saturate the charcoal in the canister and reduce its storage capacity.
EVAPORATIVE EMISSION CONTROL SYSTEM OPERATION:
Gasoline vapors produced in the fuel tank are stored in the charcoal canister. During normal engine operation, the purge control valve opens and closes, depending on throttle angle and intake manifold vacuum. The PCM cycles the purge control valve, allowing engine vacuum to draw the stored fuel vapors from the canister into the engine. The vacuum draws in fresh air into the canister where it mixes with stored fuel vapors. The mixture of fresh air and fuel vapors is then passed through the purge control valve, into the intake manifold, and burned in the normal combustion cycle.