GM 1997126 Valve Assembly, Egr & Carb Idle Speed & Vacuum

The 2-barrel engines have EGA with a metering valve, vacuum line to the carburetor, and exhaust gas passages cast into the intake manifold. Called the EGA valve, this one is operated by the vacuum from the carburetor and opens and closes to enable the mixture of the exhaust gases with the fuel and air to provide a non-misfire or pinging at part throttle. This system reduces combustion temperature and hence decrease nitrogen oxides (NOx) emission, with the valve closed at idle and at full throttle. In many configurations, it is a thermal vacuum switch that regulates vacuum to the EGR valve in order to eliminate stalling and rough idle when the engine is cold. As the vehicle accelerates, the valve opens at specific vacuum with direct aim of admitting exhaust gases into the mixture. Some of the California engines have a provision that includes a dual diaphragm EGR valve to help in operation during non-constant vacuum conditions. Starting from models of the year 1977, manufacturers have employed back pressure EGR valves which are triggered by exhaust back pressure in order to control the EGR flow with respect to the load on the engine. The later models incorporate ported signal vacuum EGR valves and the particular systems employ pulse width modulation in order to control EGR under some circumstances. Measures that are taken to test for EGA issues include checking for EGA that is either too large or small, examining the vacuum hoses and testing the functionality of the EGA valve. An EGR valve and the thermal vacuum switch are removed and reinstalled in the system; details should be paid to vacuum lines, and sealing should be appropriate.

To replace the thermal vacuum switch, the engine coolant should be drained, the vacuum hoses should be disconnected, the switch should be removed using a wrench, thread sealer should be applied to the threads, and the new switch should be installed.

To replace the EGR valve, the vacuum hose should be disconnected, the nuts or bolts securing the valve should be removed, the mounting surfaces should be cleaned, and the new valve should be installed with a new gasket.

If you are in doubt about the quality of the valve, you can go to your local parts store and compare the free movement of your EGR valve with a new valve.

The purpose of the EGR system is to reduce the emissions of oxides of nitrogen (NOx) from the exhaust.

The thermal vacuum switch can be tested with a vacuum gauge. If it is found to be defective, it should be replaced.

If there is uncertainty about the valve's quality, comparing its movement with that of a new valve at a local parts store can be helpful.

Common issues include rough idling, stalling, and poor performance under light throttle.

Most EGR valves are open underneath to check the diaphragm. Using moderate pressure, you should be able to press the diaphragm up and down within the housing.