2. With the engine warm and idling, remove the valve stem and hose from the cylinder head cover or intake manifold.
3. Plug the valve with your finger or pinch the hose. If there is no vacuum, check the cleanliness of the hose, the hole in the manifold, or the valve itself. Replace the dirty or faulty part.
4. Stop the engine and shake the valve. If no knocking is heard, replace the valve. To do this, disconnect the hose and put it on the new valve. When purchasing a valve, take into account that the new valve exactly matches the make of the car and engine (compare the new valve with the one being replaced).
5. Check the condition of all hoses and seals. Replace if there are any tears or hardening.
6. Insert the valve core back into place.