P80 EVAP SEQUENCE by User lexia

[abscate]

Worthy of its own reference thread

The EVAP system consists of:

Fuel tank
To avoid the fuel tank deforming due to negative pressure it has a reinforcement plate screwed to its underside.

Roll-over valve
The roll-over valve is a safety valve which closes if the car tilts sideways by more than 45°, preventing fuel leaking in the event of an accident.

EVAP canister
Fuel vapor from the fuel tank passes a bed of activated charcoal in the EVAP canister. The activated charcoal binds the fuel and pure air flows out into the atmosphere through the vent at the top of the EVAP canister. 80-140 g fuel can be absorbed by the EVAP canister, depending on the volume of charcoal.

Canister purge (CP) valve
The canister purge (CP) valve is located between the EVAP canister and the engine intake manifold, the valve is used to control the flow of fuel vapor from the EVAP canister to the engine. It is connected to a 12 V supply from the main relay and to the engine control module (ECM). The engine control module (ECM) opens the valve by grounding the cable.

EVAP canister shut-off valve
Certain markets require the engine control module (ECM) to detect leakages in the tank system, in these markets a leakage diagnostic has been introduced. In order to check the fuel tank system, cars in these markets have an EVAP canister shut-off valve. This is mounted in front of the left A-pillar inside the front fender or close to the rear right tire, depending on the model, and is connected to the EVAP canister vent. The valve is connected to a 12 V supply from the main relay and can be grounded by the engine control module (ECM). The engine control module (ECM) can close the valve by grounding the cable. The EVAP canister shut-off valve valve is normally open (turned off), it is only closed (turned on) when the engine control module (ECM) is checking the system.

Control
Normally the canister purge (CP) valve is closed and only opens when the engine control module (ECM) is emptying the canister. When the EVAP canister is emptied the valve is first pulsed, this pulsing increases to bring the mixture from the canister up to a value which varies according to engine load and speed. Because of the negative pressure in the inlet manifold, fresh air is sucked into the engine through the EVAP canister vent or through the EVAP canister shut-off valve. As the air passes through the EVAP canister, fuel stored in the activated charcoal is drawn into the engine and burned.

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 via the canister purge (CP) valve and negative pressure in the intake manifold. A leakage diagnostic has been introduced in certain markets to ensure that the fuel tank system is not leaking. This diagnostic is designed so that the system will detect a leak/hole greater than 1 mm. The fuel tank system consists of the fuel tank, fuel filler pipe, roll-over valve, EVAP canister, canister purge (CP) valve and all hoses 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.


LEAK TEST
The diagnostic test is divided into phases in which the various components are tested, if a fault is detected the diagnostic test is interrupted and a diagnostic trouble code (DTC) is stored for that component. The diagnostic test works as follows:

-EVAP canister shut-off valve shuts off the EVAP canister (valve turned on), the fuel tank system is entirely closed. Pressure in the fuel tank should be fairly stable, if the pressure in the fuel tank drops this indicates a leaking canister purge (CP) valve, and diagnostic trouble code (DTC) EFI-315 (Canister purge (CP) valve, leakage) is stored.

-EVAP canister shut-off valve is opened (valve turned off), the tank system is open.

-Canister purge (CP) valve is energized, 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 is open, pressure 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 stored.

-EVAP canister shut-off valve is closed (valve turned on), the canister purge (CP) valve is still energized, pressure 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, OBD P0455) is stored.

-Canister purge (CP) valve is closed (turned off), EVAP canister shut-off valve remains closed, there is now a negative pressure in the fuel tank. The negative pressure 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 stored.

-EVAP canister shut-off valve is opened (turned off), the diagnostic test is concluded.

The pressure in the fuel tank system is determined for its various phases by the following factors:
-the volume of fuel in the fuel tank.
-height above sea level
-fuel temperature and evaporation.
The system can calculate this information. Because of this, it is difficult to say how fast or much the pressure can rise or fall in the various phases. To carry out the diagnostic:
-engine temperature must be above -10° C when the engine is started
-engine coolant temperature (ECT) must be below +107 °C
-there must be no permanent faults stored 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 (Tl) 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 2700 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 approximately 30 seconds. If the diagnostic is interrupted it will try again when all the conditions have been fulfilled again. 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.

[scot850]

Agreed. This is a very useful document for people like me who know what it is for, but not necessarily the way it works.

Neil.

[sleddriver]

Agreed.
I thought I'd read everything on MVS re: P0455. Somehow missed this one. Excellent information.

[abscate]

Comment. Note the condition of V speed = 0 mph

This means if you get on the highway before 17 minutes and don’t have stop and go congestion, the Evap test never occurs. If you don’t stop for more than 30seconds the test will stop, and after three attempts, give up until the next drive cycle.

[wizechatmgr]

Comment. Note the condition of V speed = 0 mph

This means if you get on the highway before 17 minutes and don’t have stop and go congestion, the Evap test never occurs. If you don’t stop for more than 30seconds the test will stop, and after three attempts, give up until the next drive cycle.

This explains why us highway dwellers almost never get the P0455 set.