4.3 Evaporative Emissions Control (EVAP) Systems
Key Takeaways
- Evaporative Emissions Control (EVAP) systems trap raw fuel vapors (hydrocarbons) evaporating from the fuel tank and store them in an activated charcoal canister until purging occurs.
- The Purge Solenoid Valve is normally closed and pulse-width modulated (PWM) by the PCM to draw stored vapors into the intake manifold during warm, closed-loop cruise.
- The Vent Solenoid Valve is normally open to atmosphere, allowing fresh air entering the canister during purge and venting air during refueling; it closes only during automated EVAP monitor leak testing.
- Standard OBD-II leak detection thresholds monitor for Gross Leaks (>0.090 inch), Medium Leaks (0.040 inch), and Fine/Small Leaks (0.020 inch).
- A stuck-open purge valve causes hard starting after refueling and rich idle codes (P0441/P0496), whereas missing fuel caps or dry-rotted vapor lines trigger gross/small leak codes (P0455/P0456).
4.3 Evaporative Emissions Control (EVAP) Systems
Gasoline volatile organic compounds evaporate readily at ambient temperatures. Without containment, an idle vehicle can release up to 20% of its total unburned hydrocarbon emissions directly into the atmosphere through fuel tank breathing. The Evaporative Emissions Control (EVAP) system traps these vapors and meters them safely into the engine combustion process.
Purpose and Chemistry of Evaporative Emissions
Gasoline is a complex blend of volatile hydrocarbons ranging from butane to octane. Vaporization occurs under three primary conditions:
- Diurnal Emissions: Fuel tank heating during daytime ambient temperature rises expands vapor space, forcing HC vapors outward.
- Running Losses: Heat from the exhaust system, fuel pump friction, and engine radiation warms fuel while driving.
- Hot Soak Emissions: Heat radiating from a hot engine after shutdown vaporizes fuel remaining in fuel rails and lines.
EVAP System Components & Operational Flow
Fuel Tank --> [FTP Sensor] --> Charcoal Canister --> [Purge Solenoid] --> Intake Manifold
|
[Vent Solenoid (Fresh Air)]
1. Fuel Tank & Sealed Fill Cap
The fuel tank is designed with expansion space (approx. 10% volume) to accommodate thermal expansion. The filler cap features a dual-action relief valve that seals tight under normal pressure but opens under extreme positive pressure (> 2.0 PSI) or negative vacuum (< -0.5 PSI) for structural safety.
2. Activated Charcoal Canister
The core storage vessel contains granules of highly porous activated carbon (charcoal). A single gram of activated charcoal possesses a surface area exceeding 1,000 square meters. As HC vapors enter the canister from the tank, hydrocarbon molecules adhere to the carbon surfaces via physical adsorption while clean air passes through to the atmosphere.
3. Canister Purge Solenoid Valve
- State: Normally Closed (NC) when de-energized.
- Operation: Located in the engine compartment between the canister and intake manifold. When engine coolant reaches operating temperature and closed-loop fuel control is achieved, the PCM pulse-width modulates (PWM) the purge valve at variable duty cycles (0% to 100%). Intake manifold vacuum pulls stored HC vapors out of the charcoal granules and into the combustion chamber. The PCM adjusts fuel injector pulse width down to compensate for the added fuel vapors.
4. Canister Vent Solenoid Valve
- State: Normally Open (NO) when de-energized.
- Operation: Located near the charcoal canister under the vehicle. During normal vehicle operation and purging, the vent valve remains wide open, allowing fresh air to sweep through the canister to strip adsorbed HC vapors off the carbon granules. The vent valve is closed ONLY when the PCM performs an automated EVAP system leak test.
5. Fuel Tank Pressure (FTP) Sensor
Mounted on top of the fuel tank assembly or filler neck, the FTP sensor is a three-wire piezoresistive pressure transducer. It measures tiny pressure and vacuum changes inside the EVAP system relative to atmospheric pressure.
- Measurement Unit: Inches of Water Column (in. H2O), where 1 PSI ≈ 27.7 in. H2O.
- Voltage Signal: 0.5V to 4.5V output, centered at 2.5V under zero gauge pressure (atmospheric equilibrium).
+-------------------------------------------------------------------------------------+
| Pressure / Vacuum State | Tank Gauge Pressure (in. H2O) | FTP Sensor Voltage |
+----------------------------+-------------------------------+------------------------+
| Deep Vacuum (Purging Test) | -8.0 to -10.0 in. H2O | 0.5V - 1.2V |
| Zero Gauge Pressure | 0.0 in. H2O (Atmospheric) | 2.5V (Baseline) |
| Slight Tank Pressure | +4.0 to +8.0 in. H2O | 3.2V - 4.2V |
+-------------------------------------------------------------------------------------+
Leak Detection Strategies & Monitor Operations
OBD-II regulations mandate automated testing of the EVAP system for structural leaks.
Standard Vacuum Decay Monitor (Intrusive Testing)
When specific engine run criteria are met (cold start, intake air temp between 40°F and 90°F, fuel level between 15% and 85%), the PCM executes a multi-stage vacuum decay test:
1. Seal System: Close Vent Solenoid (NO -> CLOSED)
2. Pull Vacuum: Open Purge Solenoid (NC -> OPEN PWM) until FTP reads -5 to -8 in. H2O
3. Trap Vacuum: Close Purge Solenoid (NC -> CLOSED)
4. Monitor Decay: Measure FTP sensor voltage stability over a 10 to 45 second dwell time
FTP Vacuum (in. H2O)
0 ------------------------------------------------------------ (Atmospheric)
-2 /-------------\ (Vacuum Loss = LEAK!)
-4 / \........ Moderate Decay (0.040" Leak)
-6 / \__________ Stable Hold (No Leak)
-8 --------------/ (Vacuum Pulled)
[Purge Open] [Purge Closed] [Dwell Phase]
- Pass Criteria: FTP vacuum remains stable with negligible loss over the dwell period.
- Fail Criteria: Vacuum decays rapidly back toward zero atmospheric pressure.
Leak Diameter Threshold Standards
OBD-II defines three leak sizes by the diameter of the equivalent orifice the monitor can detect. There is no separate "medium" category — the three thresholds below are the complete set, and each maps to its own DTC.
| Classification | Equivalent orifice | DTC |
|---|---|---|
| Gross leak | ≥ 0.090 inch (or a missing/loose fuel cap) | P0455 |
| Small leak | ≥ 0.040 inch | P0442 |
| Very small leak | ≥ 0.020 inch | P0456 |
A smaller number means a tighter detection threshold, so a P0456 monitor is the most sensitive of the three. A system that passes the 0.040 inch test can still fail the 0.020 inch test, which is why a vehicle can set P0456 with no P0442 present.
Engine-Off Natural Vacuum (EONV) & Pump-Based Monitors
Many Chrysler, BMW, and Toyota systems utilize EONV or electric Leak Detection Pumps (LDP). EONV monitors thermal changes after key-off: as fuel cools, it creates a natural vacuum inside a sealed tank. If no vacuum develops, a leak is flagged.
EVAP Diagnostic Procedures & Testing Tools
1. Low-Pressure Smoke Machine Testing
Smoke testing is the standard method for locating physical EVAP leaks.
SAFETY WARNING: EVAP smoke testing MUST be performed using inert nitrogen gas or an approved non-combustible machine pressure regulator set to a maximum of 0.5 PSI (14 in. H2O). Compressed air introduced into fuel vapors creates an explosive atmosphere!
- Connect the smoke machine adapter to the EVAP service port (green cap) or canister vent hose.
- Use a scan tool to command the Canister Vent Solenoid CLOSED.
- Supply smoke at 0.5 PSI. Watch the machine flow meter ball:
- Ball drops to bottom (0.000 GPM): System is 100% sealed.
- Ball floats at or above 0.020" line: Physical leak present.
- Use a high-intensity UV flashlight to trace thick white smoke and fluorescent dye escaping from dry-rotted hoses, cracked canister bodies, or filler neck seals.
2. Bench Testing Purge & Vent Solenoids
- Purge Valve Test: Remove the de-energized purge valve. Apply hand vacuum to the inlet port. It MUST hold vacuum indefinitely. If vacuum drops immediately without power applied, the valve is stuck open or leaking across its seat.
- Vent Valve Test: Apply hand vacuum to the de-energized vent valve; air should flow freely (normally open). Apply 12V power and ground; the valve must snap shut and hold vacuum.
EVAP Diagnostic Trouble Codes (DTCs)
+-------------------------------------------------------------------------------------+
| DTC | Description | Primary Failure Cause |
+-------+--------------------------------+--------------------------------------------+
| P0440 | EVAP System Malfunction | General functional fault / purge failure |
| P0441 | Incorrect Purge Flow | Stuck-open/closed purge valve, blocked hose|
| P0442 | EVAP Small Leak (0.040") | Cracked hose, O-ring leak, filler cap seal |
| P0446 | Vent Control Circuit Fault | Open/shorted vent solenoid coil or driver |
| P0455 | EVAP Gross Leak (0.090") | Missing/loose gas cap, disconnected hose |
| P0456 | EVAP Very Small Leak (0.020") | Hairline canister crack, micro hose tear |
| P0496 | High High Purge Flow | Purge valve leaking while de-energized |
+-------------------------------------------------------------------------------------+
A vehicle arrives with DTC P0496 (High Evaporative Emission Control System Purge Flow). The customer reports that the engine cranks for a long time and sputters immediately after refueling at gas stations. Which component is the most likely cause of this condition?
When performing a leak detection test on an EVAP system using a specialized low-pressure smoke machine, what is the maximum allowable pressure setting to prevent damaging the fuel tank and internal bladder valves?
During an automated OBD-II vacuum decay EVAP leak test, what actions does the Powertrain Control Module (PCM) take to seal and test the system?