Checking the vacuum limiter (Pulldown) (VW Passat B3)

The vacuum limiter in the carburetor serves to overcome the force of the bimetallic spring and slightly open the air damper when the gas pedal is pressed at the moment when the air damper is closed (when the automatic device for enriching the combustible mixture is operating at start-up). The valve opens to one of two positions. The opening is controlled by a vacuum created in a separate container. When the vacuum limiter is faulty, an over-enriched mixture enters the engine. Difficulties arise when warming up the engine.



The opening of the air damper is controlled by the membrane mechanism "1" depending on the temperature and time by the thermal valve with a time delay "2".

When the thermal valve "2" is open, the vacuum decreases due to the ventilation flap "3" and the air flap is held in the first position (small gap, 1.7 ±0.2 mm).

When the thermal valve is closed, a full vacuum acts on the membrane and the air damper moves to the second position (large gap, 3.0 ±0.2 mm).

Examination

Remove the air intake guide shroud, see below.

Close the vacuum hose from the bottom of the carburetor housing to the temperature regulator in the air filter housing, for example, plug it with a clean bolt of the appropriate diameter.



Start the engine and let it idle.

Air damper (carburetor upper flap) close by hand.

If the air damper closes easily to a gap of approximately 3 mm and then exhibits increased resistance, then the vacuum limiter is OK.

If the valve can be closed completely without resistance on its part, then the malfunction is caused by either a crack in the membrane or a leak in the vacuum line. Check the vacuum limiter for leaks.