FUEL EVAPORATION SYSTEMS
1986 FUEL EVAPORATION SYSTEMS Mercedes-Benz
DESCRIPTION

The fuel evaporation system is designed to prevent fuel vapor from escaping from the fuel tank and entering the atmosphere. The system includes a fuel tank with vapor separator, thermo valve, purge valve, charcoal canister and vent valve.
OPERATION
EVAPORATION SYSTEM - ENGINE NOT RUNNING

Fuel tank pressure is maintained by the vent valve, which is a pressure/relief valve. When engine is off and vapor expands (heated by sun), the pressure relief valve in vent valve opens, allowing fuel vapor to escape to charcoal canister.

As fuel cools down, volume is reduced, creating a vacuum in fuel tank. Below a preset value, the vacuum portion of the vent valve opens, allowing air and/or fuel vapor from canister to travel to fuel tank, reducing vacuum.

If system malfunction causes fuel tank pressure to increase above 1.5-4.5 psi (.1-.3 kg/cm 2 ) relief valve portion of filler cap opens to vent excess pressure.
PURGE SYSTEM - ENGINE RUNNING

The purge valve is installed in vacuum line between charcoal canister and throttle valve housing. A vacuum control line is routed from throttle valve housing through a thermo valve and on to the purge valve.

When the engine is running, throttle valve position is above idle, and engine coolant is above 122°F (50°C), canister purging takes place. As the throttle is opened further, more vacuum is applied to the canister and purge rate continues.

Fig. 1: Fuel Evaporation System
TESTING
SYSTEM CHECK

1. Disconnect hose between purge valve and canister at canister. Place finger over hose end. Slowly increase engine speed to 3000 RPM. No vacuum should be present at idle; vacuum should gradually increase with engine speed.
2. If no vacuum was present at hose, trace hose back to throttle valve housing. Blow through connection on housing and ensure hose is clear.
3. If vacuum is still not present, remove White/Purple/Black hose from purge valve. If vacuum is now present, replace purge valve. If vacuum is not present, replace thermo valve.

MAINTENANCE

Check fuel tank, lines, and hoses for leaks, damage, and deterioration at first 1000 mile service and every 15,000 miles thereafter.