Just repaired yet another vacuum leak at my 1988 928s4. The culprit was the EVAP (evaporative emissions) valve, which leaked badly. The EVAP emission control system prevents the escape of fuel vapors from a vehicle's fuel system. Fuel vapors are routed by hoses to a charcoal canister for storage. Later, when the engine is running a purge control valve opens, allowing intake vacuum to siphon the fuel vapors into the engine. That valve was leaking pretty badly (as in: not holding any vacuum) on my 928s4.
Tony's website is very helpful, its item #11 on the left side of the engine just below the water tank. Here's a link:
'88 928s4 cherry red
The tank evaporation works like this:
1. Vacuum valve tank breather shutoff. The fuel vapors should not get into the
ambient air and a coal filter was placed to absorb these particles. If there is
no vacuum (engine off) the valve is closed, and the breather function for the
tank is open, causing the tank o compensate any pressures and vapors. When the
engine runs a vacuum exists and the valve opens, and the connection between the
intake and the charcoal canister is made. A second valve, electrical and located
in front of the engine a bit to the passenger side, is in series and still closed
so no airflow is allowed.
2. At some point the coal filter will get saturated and become inefficient. To
prevent this the LH will open he electrical valve at rpm's above idle by sending
pulses of 6 Hz to the valve. It sucks the captured fuel vapors into the intake
and cleans the filter (purge mode). At idle the valve closes again to prevent
any false air disturbing the idle rpm.
'92 928GTS Midnight Blue
So, I ran my car up to temperature and then vacuum tested the evap line at tick over and with throttle; it holds vacuum with throttle applied not at tick over; just as it should be!