1996 960 Vacuum issues

[dmg4]

I replaced the vacuum booster on my wife's 1996 960 about 1 yr ago. Lately, the dash vents have been closing when the engine is under load. Air flow is high one moment, and then nil as I climb a hill. Let off on the gas, and air flow returns. It's a bit like the wipers on my 1949 Chevy truck.

There is now a moaning sound when accelerating as well, coming from the area of the vacuum booster.

Engine power and acceleration seem normal, as does gas mileage. No check engine light or performance issues are evident.

Any ideas on the root cause and a cure?

[jimmy57]

If it only does it when recirc is selected then recirc vacuum servo is leaking.
If that isn't the case then remove under dash panel on driver's side and remove and plug the yellow hose from the vac servo on the HVAC case above and to the right of accel pedal and see if it stops. IF that fixes it then you choose whether to repair it or leave it plugged and have reduced floor air when in heat settings.

Booster is very unlikely to be related.
The check valves located by the a/c refrigerant lines on passenger side can fail but that is very rare.

[dmg4]

Thank you Jimmy57! I plugged both the detached vacuum line and the fitting from which it was detached, and now the airflow from the dash vents seems unaffected by load on the engine. Next question: which of the plastic whatchacalits under there is at fault? Is this something that might be leaking at a seam and could be epoxy-fixed, or has it failed internally? How difficult would remove/replacement be? If this is likely to consume an entire day or more, I can probably live with reduced airflow to the foot vents.

[dmg4]

Uh oh... this does not look like it would be a fun part to replace. Is it necessary to remove the entire HVAC box from the dash to remove/replace this vacuum servo? It looks like it is attached from the inside of the box, and can not be removed (except perhaps with a hammer) from the outside.