Description
Evaporative Emissions
Evaporative Emissions Component Layout:
GENERAL
The Evaporative emission (EVAP) control system reduces the level of hydrocarbons released into the atmosphere by fuel vapor venting from the fuel tank. The system comprises a charcoal canister, purge valve and interconnecting vent pipes and hoses. The vent pipes are connected to the system components using quick release connectors.
Fuel vapor is generated by the fuel in the tank and the amount of vapor produced increases as the fuel heats up. Fuel vapor can flow freely to the charcoal canister via the tank venting system. The venting system consists of roll over valves and a liquid vapor separator mounted internally in the tank and then externally via a breather line. The breather line allows the fuel vapor to flow to the charcoal canister via a 'Y' piece mounted on the filler head.
On NAS vehicles the vapor generated during fuel filling and under ambient conditions flows without restriction to the charcoal canister.
On all vehicles except NAS, the same vapor is restricted in its path to the charcoal canister but can flow freely during the fuel filling operation to atmosphere, via the filler opening.
The vapor passes into the charcoal canister where it is absorbed and stored by the charcoal. Because there is a limit to the amount of vapor the canister can contain, the fuel vapor is purged from the canister when the engine is running and burned in the engine.
PURGE VALVE AND HOSES
Purge Valve And Hoses:
The purge valve is located at the rear of the engine, on a bracket which is attached to the transmission bell housing. The purge hose is routed from the purge valve, along the left hand side of the air intake manifold, to the elbow assembly which locates the electric throttle.
The purge hose is connected, at the right hand rear of the engine, with a quick release coupling to the purge line which runs parallel with the fuel feed line along the top of the fuel tank to the charcoal canister.
The purge hose continues from the purge valve and is routed to a connection on the air intake elbow assembly. The hose is connected to the elbow with a quick release connector.
The purge valve is located on a bracket on the bell housing and is secured with a single bolt. The purge valve is a solenoid operated valve which is closed when de-energized. The valve is controlled by the Engine Control Module (ECM) and is operated when engine operating conditions are correct to allow purging of the charcoal canister.
The ECM keeps the purge valve closed (de-energized) below a predetermined engine coolant temperature and engine speed to protect the engine tune and catalytic converter performance. If the purge valve is opened during cold running conditions or at engine idle speed, the additional fuel vapor can cause the engine to have erratic idle speed or even stall. When engine operating conditions are correct, the ECM opens the purge valve (energized) and the depression at the inlet manifold draws a fuel vapor and fresh air mix from the charcoal canister. When the purging process is active, fresh air is drawn into the charcoal canister via the DMTL pump atmospheric vent connection and its filter on NAS vehicles and via the atmospheric vent hose connection and the spider trap on non NAS vehicles.
On NAS vehicles the system does not include a pressure test point. Pressure testing of the purge valve hose is achieved by disconnecting the purge valve joint on the underside of the vehicle, forward of the fuel tank and connecting a special tool to allow the system to be pressure tested. The test performs a pressure test on the purge hose connection forward of the fuel tank back to the charcoal canister. The special tool is then connected to the purge hose connection forward of the fuel tank to perform a pressure test on the purge hose to the purge valve.
CHARCOAL CANISTER
Charcoal Canister - All Except NAS:
Charcoal Canister - NAS:
The charcoal canister is located in a central position, forward of the spare wheel. It is attached at the rear with two bolts which screw into the spare wheel carrier. At the front, the canister has two lugs which locate in the parking brake module support bracket.
The canister on ROW vehicles has a capacity of 1400 cc (85.4 cu. in).
The canister on NAS vehicles has a capacity of 3000 cc (183 cu. in).
The canister has three ports which allow for the attachment of the atmospheric vent hose, the purge hose and the tank vent hose. On NAS vehicles the atmospheric vent hose connection allows for the attachment of the DMTL pump.
The canister contains a bed of activated charcoal or carbon. The charcoal is produced using special manufacturing techniques to treat the charcoal with oxygen. The oxygen treatment opens up millions of pores between the carbon atoms resulting in a highly porous charcoal with a very large effective surface area which is capable of absorbing large quantities of fuel vapor. Once treated the charcoal is known as 'activated' carbon or charcoal. The charcoal canister on NAS vehicles uses a higher grade charcoal which the fuel system to meet the requirements of LEV2 emission regulations.
DIAGNOSTIC MONITORING OF TANK LEAKAGE (DMTL) - NAS ONLY
The DMTL system is a legislative requirement for NAS vehicles. The DMTL system periodically checks the EVAP system and the fuel tank for leaks when the ignition is switched off.
The DMTL system comprises the previously described components of the EVAP system with the following additional components; a DMTL pump and a DMTL filter.
The DMTL pump is connected to the atmospheric vent of the charcoal canister and incorporates a Positive Temperature Co-efficient (PTC) heating element, a normally open valve and a reference orifice. The DMTL pump is only operated when the ignition is switched off and is controlled by the ECM. The ECM also monitors the electric air pump operation and the normally open valve for faults.
The DMTL filter protects the pump from dust being drawn into the system when the pump is being operated. The filter is located on the fuel filler head and is connected to the DMTL pump by a hose.