THERMO Valve
On 1974-75 models, thermo valve prevent EGR operation when coolant temperature is below 140°F (60°C). On 1976-77 models, thermo valve prevent EGR operation when coolant temperature is below 100°F (38°C) on California and high altitude models or below 113°F (45°C) on Federal models. By closing EGR valve at these temperatures, engine driveability problems at initial start up are prevented when coolant temperature reaches pre-set value, thermo valve will close thus opening the EGR valve.
On 1978 models, thermo valve is used to sense engine coolant temperature and prevent EGR operation when below preset value. When engine warms up, thermo valve opens to allow vacuum passage to EGR valve, which them determines EGR flow. Below 113°F (45°C), thermo valve is open, causing vacuum to be vented to atmosphere, thus not reaching EGR valve. Above 131°F (55°C), thermo valve closes and normal EGR operation is accomplished.