Audi Q7: On Board Diagnostic Systems
On Board Diagnostics, or OBD, is an automotive term referring to a vehicle's self-diagnostic and reporting capability. OBD systems give the vehicle owner or repair technician access to the status of the various vehicle sub-systems. Modern OBD implementations use a standardized digital communications port to provide real-time data in addition to a standardized series of Diagnostic Trouble Codes (DTCs) which allow one to rapidly identify and remedy malfunctions within the vehicle. Legislation mandates a vehicle equipped with OBD-II to light up the fault indicator lamp if its emissions exceed the prevailing limit due to system malfunction.
All cars built since January 1st, 1996 (MY 1996) are equipped OBD-II systems. Manufacturers started incorporating OBD-II in various models as early as 1994; however, some early OBD-II cars (MY 1994 and MY 1995) were not 100% compliant.
Evaporative Emission System
The evaporative emission system has been designed to minimize the release of hydrocarbons from the fuel system into the atmosphere. The evaporative emission system components all work together with the ECM to prevent fuel vapor from escaping and route it to the intake manifold to be burned during normal combustion.
The leak detection system checks the integrity of the evaporative emission system by pressurizing the system.
- There are 3 different types of evaporative emission systems used. These systems are explained below.
Leak Detection Pump (LDP) EVAP System
The leak detection pump (LDP) is integrated into the EVAP system and can have two functions. The LDP can:
- Pressurize the EVAP system and detect a drop in pressure that would indicate a leak.
- Function as the EVAP Canister Vent on vehicles that do not have a separate EVAP Canister Vent.
The LDP is a vacuum-driven, ECM controlled, diaphragm pump. In order to operate, the engine must be running and vacuum applied to the Vacuum Switch.
Tank Leak Diagnostic Module (DM - TL) EVAP System
The canister purge valve can be actively checked using the Tank Leak Diagnostic Module (DM - TL). For this purpose the electric pump is shortly activated while the combustion engine is running, to build up a minor pressure in the fuel tank and monitor the pressure decay after opening the canister purge valve. Optionally as a quick pass method, the monitoring can be carried out by passively monitoring the fuel mixture deviation when the canister purge valve is opened. If a significant fuel mixture deviation is detected, the purge valve monitor passes. The Tank Leak Diagnostic Module (DM - TL) consists of an electrically operated air pump, an orifice with a defined diameter serving as a reference leak, and a change-over valve switching the air flow between the reference leak and the tank. If neither the pump nor the change-over valve is activated, the tank is ventilated through a bypass in the module.
Natural Vacuum Leak Detection (NVLD) EVAP System
The system utilizes an engine-off natural vacuum evaporative system integrity check that tests for leaks with a diameter of 0.020 inch while the engine is off and the ignition is off. The natural vacuum leak detection (NVLD) evaporative system integrity check uses a pressure switch to detect evaporative system leaks. The correlation between the pressure and the temperature in a sealed system is used to generate a vacuum in the tank when the temperature drops. If a sufficient temperature drop is detected for a minimum time period, the vacuum level in a sealed system will exceed the threshold to close the NVLD pressure switch. Therefore, if the switch does not close under these conditions, a leak is detected. If the switch closes, the system is considered to be leak-free.
EVAP System, Checking for Leaks
The following procedure is used to diagnose EVAP System leaks.
Special tools and workshop equipment required
- Smoke tester.
- EVAP and Fuel Supply System Vacuum hose and line routing diagram.
Leak checking
- Using a Smoke tester, check the Evaporative Emission (EVAP) canister system for leaks.
- Always follow the manufacturers directions for the proper installation and operation of the smoke tester being used.
If a leak is detected:
- Check the fuel filler cap seal for damage and for proper installation. Replace if necessary.
- Check all hose connections of the fuel supply system and replace or repair any leaking lines.
- Check all hose connections of the EVAP system and replace or repair any leaking lines.
- Check that the seal under the locking flange is properly tightened on the fuel tank.
- Secure all hose connections using appropriate fittings for the model type.
- Replace seals and gaskets when performing repair work.
- Repair or replace any damaged component.
If no leaks are found in the EVAP system:
- Erase the DTC memory if a DTC was set. Refer to → Chapter "Diagnostic Mode 04 - Erase DTC Memory".
- Perform a road test to verify repair.
If a DTC was set and does not return:
Diagnosis complete. Generate readiness code. Refer to → Chapter "Readiness Code".
If the same DTC does return and no leaks are found in the EVAP system:
- Check for any related TSB's.
- Perform the diagnostic test procedure for the suspected component.
