Fuel tank Vent
The 928 has a fuel tank breather system. This system prevents fuel vapor from escaping to open air. A carbon canister is part of this system and absorbs excess vapor. These filters have to be purged from time to time. The 928 has an automatic system for this. In order to meter the additional fuel quantity correctly that is drawn off from the carbon canister after evaporation has occurred, a solenoid valve is fitted in the line.
Operation: A diaphragm valve controlled by the throttle valve is located in the line off the carbon canister. This valve is operated by the LH ecu when the engine is at operating temperature. At idle, no vacuum is applied, and the valve is closed. When the throttle valve is opened, vacuum is applied at the diaphragm valve, causing the valve to open and allowing the gases to escape to the diaphragm valve. This valve is timed by the LH control unit with a frequency of 6 Hz.
Similar to the trigger signal of the injection valves, the duration of this signal is dependent on the intake air flow rate. When the intake air flow rate is low, the tank ventilation valve receives only a short timer signal, when flow rate is high, the timer signal length increases. This ensures that the correct purge air quantity is added in accordance with the intake air quantity.

Ok so far some theory.

The Y connector on our Porsche 928s is prone to breaking. The one on my 1980 928 had broken down and was causing fuel smell coming into the cabin especially in stop and go traffic.

You can see the broken connector in the pictures below:

This is a close up with the wheel removed. Note the broken piece stuck inside the hose:

Here is the new steel Y connector getting installed:

Unlike newer cars, you don’t need to remove the wheel well cover to get to the connector.

The finished job looks like:

Here is a picture of the Y-piece in closeup:

Measurements for a replacement part: