Evaporative Emissions System: Testing and Inspection
LEAK DIAGNOSTICS
Vapor which evaporates from the fuel in the fuel tank is routed to and stored in the EVAP canister from where it is introduced into the combustion process by way of the canister purge (CP) valve and negative pressure in the intake manifold later.A leakage diagnostic has been introduced in certain markets to ensure that the fuel tank system is not leaking. This diagnostic test has been designed in such a way that the system will detect a leak/hole greater than or equal to 1 mm. The tank system consists of the fuel tank, filler pipe, roll-over valve, EVAP canister, canister purge (CP) valve and all lines between these. It now also has a fuel tank pressure sensor and EVAP canister shut-off valve to allow the diagnostic test of the system.
The diagnostic test is divided into phases in which the various components are tested, if a fault is detected the diagnostic testis interrupted and a diagnostic trouble code (DTC) is posted for that component. The diagnostic test is as follows:
1. EVAP canister shut-off valve (B) shuts off the EVAP canister, the fuel tank system is entirely closed. Pressure (A) in the fuel tank should be fairly stable, if the pressure in the fuel tank drops this indicates a leaking canister purge (CP) valve (C), and diagnostic trouble code (DTC) EFI-315 canister purge (CP) valve, leakage is posted.
2. EVAP canister shut-off valve (B) is opened, the tank system is open.
3. Canister purge (CP) valve (C) is synchronized and because of the negative pressure in the intake manifold the engine starts to suck air through the EVAP canister. Because the EVAP canister shut-off valve (B) is open, pressure (A) in the tank should drop slowly. If pressure in the tank drops rapidly, this indicates that the EVAP canister shut-off valve is sealed, diagnostic trouble code (DTC) EFI-614 EVAP canister shut-off valve, flow fault is posted.
4. EVAP canister shut-off valve (B) is closed, the canister purge (CP) valve (C) is still synchronized, pressure (A) in the fuel tank should now drop fairly quickly. If the pressure does not drop enough, this indicates a serious leak somewhere in the fuel tank system and diagnostic trouble code (DTC) EFI-611 Fuel tank system, serious leakage, is posted.
5. Canister purge (CP) valve (C) is closed, EVAP canister shut-off valve (B) remains closed, there is now a negative pressure (A) in the fuel tank.
6. The negative pressure (A) in the fuel tank has stabilized, if pressure starts to increase too quickly, this indicates a small leakage in the fuel tank system and diagnostic trouble code (DTC) EFI-612 fuel tank system, small leakage, is posted.
7. EVAP canister shut-off valve (6) is opened, the diagnostic test is concluded.
There are several factors that are taken into account while the system is determining whether tank system pressure is OK or not, for example:
- The amount of fuel in the tank
- Height above sea level
- Fuel temperature and evaporation.
The system can calculate these variables. It is therefore hard to say how quickly or much pressure can rise or fall in the system.
To carry out the diagnostic test:
- Engine temperature must be above -10°C when the engine is started
- temperature must be below +107°C
- There must be no permanent faults posted for the throttle position (TP) sensor, vehicle speed signal (VSS), canister purge (CP) valve, idle air control (IAC) valve, mass air flow (MAF) sensor, battery voltage, fuel tank pressure sensor, EVAP canister shut-off valve, engine coolant temperature (ECT) sensor and the engine must not be misfiring
- The engine is idling and load (TI) must not exceed 2.7 ms (automatic) and 2.3 ms (manual)
- Speed must be 0 km/h
- The concentration of fuel vapor in the canister must not be too high
- There must be more than 11 liters of fuel in the tank
- The pressure in the tank must not be too high.
- Fuel trim must be active.
- The car must be below 2,700 meters above sea level.
The diagnostic test starts at the earliest 17 minutes after the engine has started when all conditions have been fulfilled, and takes approx. 30 seconds. If the diagnostic test is interrupted it will be started again when all conditions are fulfilled. The engine control module (ECM) makes a maximum of 3 attempts in one journey, if the diagnostic test is interrupted all three times it will not try again until the engine is started again.