The Engine Management System (EMS) uses several different pressure sensors to gather and control various emissions and driveability aspects of the vehicle. These may include but are not limited to:
These sensors provide critical information to the EMS such as, barometric pressure, atmospheric pressure, intake manifold/engine vacuum, fuel tank pressures and changes as the vehicle is operated.
The EMS uses the MAP sensor on systems that have a Mass Airflow Sensor (MAF) as a backup. The EMS also uses the MAP sensor as an EGR diagnostic device and as an engine load verification device. The EMS system uses this information to calculate engine load and EGR flow rates. The EMS uses speed-density calculations (non-mass airflow sensor systems) to determine the required amount of fuel delivery.
The EMS uses a barometric (BARO) sensor on some vehicles to help compensate for barometric changes at different altitudes. Barometric pressure changes with the weather and with altitude. Essentially the density and humidity associated with the air around the vehicle is monitored so the EMS can adjust for varying conditions. Since barometric pressure affects the density of the air entering the engine and ultimately the air/fuel ratio, some engine management systems use a barometric pressure sensor so that the spark advance and EGR flow can be regulated to control emissions more precisely.
The Fuel Tank Pressure (FTP) sensor is part of the evaporative emissions system. The Engine Management System (EMS) monitors gasoline vapor pressures in the fuel tank to determine fuel tank sealing. The FTP sensor can be used in conjunction with an EVAP vent valve and an EVAP canister purge valve.
When ambient temperatures are above 68° Fahrenheit the gasoline in the fuel tank vaporizes increasing fuel tank pressure. The FTP sensor is used to determine how much evaporative pressure is being maintained in fuel tank. This is a test for gross evaporative emissions leaks.
Some vehicles will route a small amount of engine vacuum through the canister purge valve to the fuel tank. When both the canister purge valve and vent solenoid valve are closed the vacuum is trapped between the fuel tank and canister purge valve. The fuel tank pressure sensor signals the EMS that the vacuum is present. The vacuum in the system should reach a certain level and remain for a pre-determined amount of time. This is used to test for minute system pressure leaks.
The following symptoms can be caused by a loose connection, bad grounds, opens or shorts in the circuit, high resistance in the circuit, or a damaged sensor
MIL Lamp and DTC Set - If there is a leak in the system it will fail the evaporative emissions diagnostic monitor test. Verify that all vapor lines are properly routed and connected and that the correct fuel cap is properly installed before doing any other diagnostics.