evaporat.pdf

Section 5

Evaporative Systems Diagnosis

Vacuum Check Valve

Tank Valve Assembly

On-board Recovery

Valve (Fill Check Valve)

Vapor Pressure

Sensor

Tank Pressure Valve

To Manifold Vacuum

Purge Valve

Filtered Air

Canister Closed Valve

Canister

Air Drain Valve

Air Inlet Valve

Bypass Valve

Air Valve Assembly

Learning Objectives:

1.Determine the condition of the EVAP system operation based on

engine data and the EVAP pressure tester.

2.Determine the condition of the vapor pressure sensor.

3.Determine EVAP monitor status using Readiness Tests and

CARB modes.

Engine Control Systems II - Course 874

Section 5

Evaporative System Diagnosis

EVAP System

Service Port

EVAP (Purge) VSV

Vapor

Pressure

Sensor

Bypass

VSV

CCV

Fig.5-1

TL874f501

Overview

There are a variety of EVAP systems in use with different monitoring

strategies. It is essential that the EVAP system be correctly identified

before beginning diagnosis (the Repair Manual is the best source). The

following information covers the different systems.

Usually, the customer is unaware of an EVAP system problem until the

MIL illuminates. The customer needs to be aware that they must

correctly install the fuel cap after refueling.

Check mode does not work for EVAP codes. Use the Readiness Test

Confirmation procedure.

A common failure is a leak in the system, but restrictions will also set

EVAP related DTCs.

5-1

Engine Control Systems II - Course 874

Section 5

EVAP System

with ORVR

Vacuum Check Valve

Tank Valve Assembly

On-board Recovery

Valve (Fill Check Valve)

Vapor Pressure

Sensor

Tank Pressure Valve

To Manifold Vacuum

Purge Valve

Filtered Air

Canister Closed Valve

Canister

Air Drain Valve

Air Inlet Valve

Bypass Valve

Fig. 5-2

TL874f502

Air Valve Assembly

Condition

Purge

Port

Air Inlet

Port

Tank

Vacuum

Ball Check

Tank

Pressure

Valve

ORVR

Valve

Air Drain

CLOSED

VSV Purge Valve ON

OPEN (V)

CLOSED

CLOSED (NV) CLOSED (NV)

VSV Purge Valve OFF

CLOSED

Pressure in Tank

CLOSED

OPEN

CLOSED

OPEN

CLOSED

Vacuum in Tank

Refill

CLOSED

OPEN

Late Type

EVAP Pressure

in Fuel Tank

Fuel vapor pressure is

vented into the charcoal

canister when vapor

pressure forces the tank

pressure control valve

open.

Fig. 5-3

TL874f503

5-2

TOYOTA Technical Training

Evaporative System Diagnosis

Late Type

EVAP Vacuum

in Fuel Tank

Vacuum in the tank can

be relieved by allowing

air to enter through the

charcoal canister or fuel

tank cap.

Fig. 5-4

TL874f504

Late Type

EVAP Purge Mode

During purge, vapors are

drawn out of the canister

and burned in the

engine.

Fig. 5-5

TL874f505

5-3

Engine Control Systems II - Course 874

Section 5

Late Type EVAP

ORVR Operation

During refueling, the

differential pressure valve

and air drain valve are

open. Fuel vapors are

absorbed by the charcoal

canister and excess air is

vented to the

atmosphere.

Fig. 5-6

TL874f506

Evaporative

Systems

Monitoring

Regulations require that the EVAP system be monitored for system

performance and leak detection. Leaks, restrictions, and many

components are checked by measuring the pressure of the EVAP system

at various stages.

Early EVAP systems had to detect a 1mm (0.040 in.) hole and greater. It

is often called the early type or non-intrusive system.

Beginning with 2000 model year, a new EVAP monitor system was

implemented to meet the new, mandated standard of detecting a hole

down to .5mm (0.020 in.). This new system is referred to as the Late

Type or Intrusive Type. The intrusive system is being phased in.

In addition, the EVAP monitor has to monitor vapor purge and

component performance. All EVAP monitor DTCs require two trips.

Early-Type

(Non-Intrusive)

Pressure

Detection

The ECM relies on the VP sensor signal to accurately measure pressure

in the EVAP system and the purge side of the charcoal canister. The

changes in the pressures being measured are very small, often 15.5

mmHg (0.3 psi) or less. For the VP sensor to measure tank pressure and

canister pressure, a three-way VSV is connected to the VP sensor, fuel

tank and lines, and charcoal canister. When there is no power to the

three-way VSV VP sensor, the VP sensor measures canister purge

pressure. When the ECM turns on the VSV, the VP sensor measures

fuel tank pressure.

5-4

TOYOTA Technical Training

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