10.6 EVAP Monitors and Leak-Test Procedures

OBD-II EVAP Leak Monitors, DTCs & Low-Pressure Smoke Testing (Tasks G.11, G.12)

EVAP Monitor Enable Criteria & Operating Modes (Mode \$06)

Before the PCM runs its OBD-II EVAP leak monitor, strict entry conditions must be satisfied:

• Fuel Tank Level: Must be between 15% and 85% (~1/8 to 7/8 tank). Below 15%, fuel slosh and large vapor volume skew decay rates; above 85%, liquid fuel is too close to the vapor dome.
• Cold Soak / Temperature Window: Requires a verified cold start where ECT and IAT are within 10°F (5.5°C) of each other, ambient temperature is between 40°F and 95°F (4°C–35°C), and barometric pressure is >75 kPa (altitude <8,000 ft).

The following are two common OBD-II EVAP leak-detection strategies; other designs use leak-detection pumps or natural-vacuum switches:

1. Engine-Running Vacuum-Decay Monitor: The PCM commands the Normally Open vent solenoid CLOSED, pulses the Normally Closed purge solenoid OPEN until the FTP sensor registers a calibrated light vacuum (~8 to 12 in. H2O), commands the purge valve CLOSED to seal the system, and measures the rate at which FTP vacuum decays over 15 to 30 seconds.
2. Engine-Off Natural Vacuum (EONV) Monitor: Used on late-model GM and Ford medium-duty trucks to test down to a 0.020 in. (0.5 mm) orifice standard after key-off. Following a warm drive cycle, the PCM wakes up after shutdown, closes the EVAP vent valve, and monitors the FTP sensor as the sealed fuel and vapor naturally warm (building slight positive pressure) and then cool (creating natural vacuum) without any fuel slosh.

Worked Example: Diagnostic Scenario: Class 4 Box Truck with Hard Start After Refueling & P0496

• Concern: A delivery driver reports that a Class 4 box truck cranks for 8 to 10 seconds and stumbles roughly every time it is restarted immediately after filling the fuel tank, accompanied by DTCs P0496 (EVAP System Flow During Non-Purge) and P0172 (System Too Rich Bank 1).
• Scan Tool & Bay Testing: At warm curb idle, the scan tool shows EVAP Purge Command at 0%, yet STFT Bank 1 is at -16% and the FTP sensor reads -11.5 in. H2O (vacuum) when the technician momentarily commands the EVAP Vent Valve closed. Unplugging the 2-wire electrical connector and removing the canister-side vapor hose from the intake-mounted EVAP purge solenoid reveals strong, continuous manifold vacuum (18 in. Hg) sucking straight through the de-energized purge valve.
• Root Cause & Fix: Tapping the removed purge solenoid onto a clean shop towel shakes loose several hard black activated-charcoal pellets that had migrated from a ruptured charcoal canister baffle and wedged the NC purge pintle open. The technician replaces both the ruptured charcoal canister and the contaminated EVAP purge solenoid, services the isolated vapor lines by the approved cleaning or replacement method, and verifies that the new purge valve holds 15 in. Hg vacuum with 0.0 in. H2O tank pressure at idle.

Applied Diagnostic Check

An EVAP flowmeter reports flow under the tool's test pressure and setup. Its calibrated hole-size mark is an equivalent leak reference, not a measurement of a crack's physical width. A long crack, several small paths and a round reference hole can produce similar flow. Use smoke location or section isolation to find the actual defect. If no smoke appears, inspect the tool connections and valve state before increasing pressure. A seal can open only under a particular temperature or vacuum condition; follow the specified alternate test rather than declaring the system good from one static observation.

Testing the Canister and Vapor Lines Without Creating a New Fault

Inspect the fuel cap seal and filler neck, canister case, vapor tubes, connectors and vent filter before applying smoke. Check that the tool connection itself is sealed. A leak at an adapter can look like a tank fault, and a vent valve left open can produce heavy flow without any broken pipe. Command and verify the required valve state, then compare the flow reading with the tool's calibrated leak standard.

Isolate sections only using fittings or flexible sections the manufacturer permits. Do not pinch hard nylon lines or pressurize the system with unregulated shop air. If a damaged canister has released charcoal into a valve, inspect the affected line and follow the cleaning or replacement instructions. After reassembly, repeat the seal test and verify purge and vent operation. Monitor completion is an additional verification step, not a substitute for checking that the replaced components were installed without leaks.