Circuit Description
This diagnostic tests the evaporative emission (EVAP) system for a small leak. The diagnostic runs when the ignition is in the OFF position, and the correct conditions are met. While the engine is operating, there are sources of heat transferred into the fuel tank from the following:
- Exhaust heat
- Engine heat
- Ambient temperature
When the engine is stopped and the ignition is in the OFF position, a change in the fuel tank vapor temperature occurs. This results in a change in the pressure of the fuel tank vapor space. This change in pressure is monitored by the control module, using the fuel tank pressure (FTP) sensor input. The EVAP diagnostic detects leaks as small as 0.51 mm (0.02 in).
FTP SENSOR SIGNAL VOLTAGE AND FUEL TANK PRESSURE REFERENCE
| FTP Sensor Signal Voltage | Fuel Tank Pressure |
|---|---|
| High, Approximately 1.5 Volts or More | Negative Pressure/Vacuum |
| Low, Approximately 1.5 Volts or Less | Positive Pressure |