Two different systems are used in 1979-80 models, but both are given the same name. Basically, both systems are designed to control distributor vacuum advance.1979 Models
All U.S. models with a catalytic converter and all models sold in Canada have this system. It is designed to control distributor vacuum advance during acceleration to limit emissions of HC and NOx.
The system simply consists of a vacuum delay valve spliced into the distributor vacuum advance hose. During acceleration, the valve restricts the amount of air flow in the vacuum hose. Inside the valve are a metering disc and a one-way umbrella valve. Air flows freely past the umbrella valve, from the throttle to the distributor. The metering valve restricts air flow in the opposite direction, from the distributor to the throttle.1980-88 Models
This system is slightly more complicated than the 1979 system. Spark timing is controlled by a single Thermal Vacuum Valve (TVV) on the California and Canadian models. The 1980 (California) models and all 1981 and later (USA) models have a TVV installed in the engine thermostat housing. The valve has two vacuum connections: one for fresh air from the vacuum connector, the other to the distributor vacuum advance. The 1980 and later (Canadian) models have a TVV installed in the intake manifold heater housing. The valve has three vacuum connections: one for fresh air, one to the EGR valve and one to the distributor vacuum advance.
The USA models have two TVVs and a one-way valve. One TVV is installed in the intake manifold in the same manner as the Canadian TVV; it has the same connections as the Canadian TVV. The other TVV is installed in the thermostat housing and also has three vacuum connections: one to the one-way valve, one to the distributor vacuum advance unit and one to a vacuum source on the throttle chamber. The one-way valve is installed in the line between the vacuum connector and the TVV installed in the thermostat housing.
- Remove the valve from the distributor vacuum hose.
- Blow through the valve from the throttle side. This is the black side of the valve. Air should flow through freely.
- Blow through the distributor side of the valve. This is the brown side. There should be resistance to the air flow.
- If the valve does not perform correctly, replace it. When installing the valve, be sure the brown side is connected to the distributor hose and the black side is connected to the throttle hose.
This inspection procedure applies to the entire system, regardless of the specific components used.
- Check all vacuum hoses for leaks, kinks, breaks or improper connections, and correct, as necessary.
- Check that the distributor vacuum advance unit is working properly.
- Connect a timing light to the engine.
Check the TVV as follows:
- Start the engine; it must be cold. Check and record the ignition timing.
- As the engine warms up, check and record the ignition timing. The timing should be retarded from its cold setting.
- Allow the engine to warm up to normal operating temperature. The timing should advance to its normal setting.
- If the timing does not change as specified, replace the TVV (both TVVs on USA models).
The one-way valve installed on USA models can be inspected as follows:
- Remove the one-way valve.
- Blow air through the vacuum connector side of the valve (black side). Air should flow through freely.
- Blow air through the TVV side of the valve (white side); there should be resistance to the air flow.
- If the valve does not perform correctly, replace it. When installing the valve, be sure that the black side is connected to the vacuum connector hose and the white side is connected to the TVV hose.