In 1974, the Distributor Vacuum Valve (DVV) was introduced in 350 and 455 cu. in. engines sold in California and on some 455 cu. in. engines sold in the other 49 states.
The DVV switches the distributor vacuum advance unit's vacuum source from the carburetor spark port to the EGR port. Below 7 in. Hg (48 kPa) the vacuum unit operates from the spark port. Above 7 in. Hg (48 kPa) the vacuum supply is switched by the SVV from the spark port to the EGR port.
Distributor Vacuum Test
- Start the engine and allow it to reach normal operating temperature. Remove the air cleaner and plug the manifold vacuum fitting.
- Remove the hose from the "D" port of the TVS and attach a vacuum gauge to the port. Using a "T" fitting, attach the second vacuum gauge to the hose at the carburetor EGR port.
- Gradually increase the engine speed. The first gauge should rise to 8 in. Hg (55 kPa) and remain there, and then the second gauge should rise to 8 in. Hg (55 kPa) and remain.
If the engine is a California version, the readings will be 7 in. Hg. (48 kPa)
- If the conditions are not met, replace the DVV valve.