What's the purpose of this vacuum hose routing under this metal plate above water pump?

[azmusclecar]

Thanks for all the info you all gathered for me. I just did NOT want to break another OEM part and have to pay dearly for my curiosity,
Remember the cat..

So there are two hoses one at each end and I can see if I can get them off. 40 years at that high heat point and maybe the rubber has
become part of the valve. I just know tugging and pulling on it to a level where I feel it "SHOULD" break free and doesn;t, I stop and
go Hmmmmmmmmm

Great info on my issue.

 

[pagrunt]

Well the emissions crazy politicians put a lot more hoses and valves and other power robbing garbage on this age of cars.

AZ Emissions would only allow 1966 and before exempt from emissions. I wonder how they ever chose THAT year as their benchmark

Another thank the Peoples Repulic of Kalifornia thing. They started emission equiptment about '66.

 

[azmusclecar]

Another thank the Peoples Repulic of Kalifornia thing. They started emission equiptment about '66.

Amen brother..............preach it and teach it..............

 

[Kra5379]

I believe with all 1973 and later Oldsmobile engines of this era the EGR system uses what is called a TVS (Thermal Vacuum Switch). These emission systems use a metering valve, a vacuum line to the carb, and cast-in exhaust gas passages in the intake. The EGR is controlled by carburetor vacuum, it opens and closes to allow exhaust gases into the air/fuel mixture. The exhaust gases lower the combustion temperature, and reduce (NOx) emissions produced. The valve is closed at idle, vacuum to the EGR valve is controlled by the TVS. The switch, which is located in the engine block, shuts off vacuum to the EGR valve until the engine is hot. This prevents the stalling and bumpy idle you get if Exhaust gases are recycled when the engine is cold. When the car accelerates, the throttle plates of the carb uncover the vacuum port for the EGR valve. At 3-5 in. Hg (20-34 kPa), the EGR valve opens and then some of the exhaust gases are allowed to flow into the air/fuel mixture to lower the combustion temperature. At full-throttle the valve closes again. Some California engines are equipped with a dual diaphragm EGR valve. this style further limits the exhaust gas opening (compared to the single diaphragm EGR valve) during high manifold vacuum periods, such as at cruising speed, and provides more exhaust gas recirculation during acceleration when manifold vacuum is low. Besides the hose running to the thermal vacuum switch, a second hose is usually connected directly to the intake manifold. For 1977, all California models of cars delivered in areas above 4000 ft. are equipped with backpressure EGR valves. This valve is also used on all 1978-81 models. The EGR valve receives exhaust backpressure through its hollow shaft. This creates force on the bottom of the valve diaphragm, which is opposed by a light spring. Under low exhaust pressure (low engine load and partial throttle), the EGR signal is reduced by an air bleed. Under high exhaust pressure conditions (high engine load and large throttle opening), the air bleed is closed and the EGR valve responds to an unmodified vacuum signal. At wide open throttle, the EGR flow is reduced in proportion to the amount of vacuum signal available. The 1979 and later models have a ported signal vacuum EGR valve. The valve opening is controlled by the amount of vacuum obtained from a ported vacuum source on the carburetor and the amount of backpressure in the exhaust system.

 

[Kra5379]

After digging out one of my ancient MOTOR service manuals and looking a little more into it, surprisingly i was pretty much right on with what I remembered, but what I forgot and was reminded of is that during normal engine operation, vacuum from the carburetor passes through the TVS to the distributor. If the engine should overheat while idling, the TVS connects intake manifold vacuum to the distributor which lowers coolant temperature by advancing the ignition timing, On 1973 and later 350 and 455 4-bbl carbureted engines (except 350 cu. in manual transmission models) have a thermal check and delay valve. This valve is in the vacuum line which runs between the carburetor spark port and the TVS.
When the underhood (or engine block) temperature is below 50°F (10°C), full vacuum is supplied to the distributor. Above 50°F (10°C), the valve blocks full vacuum for up to 40 seconds. If ported vacuum drops, the valve opens, causing the distributor vacuum advance to be retarded. As vacuum increases, the valve closes, blocking full vacuum again. Cars manufactured after March 15 1973 have a cover over the valve so that it is more dependent upon engine block temperature.

 

[69hurstolds]

After digging out one of my ancient MOTOR service manuals and looking a little more into it, surprisingly i was pretty much right on with what I remembered, but what I forgot and was reminded of is that during normal engine operation, vacuum from the carburetor passes through the TVS to the distributor. If the engine should overheat while idling, the TVS connects intake manifold vacuum to the distributor which lowers coolant temperature by advancing the ignition timing, On 1973 and later 350 and 455 4-bbl carbureted engines (except 350 cu. in manual transmission models) have a thermal check and delay valve. This valve is in the vacuum line which runs between the carburetor spark port and the TVS.
When the underhood (or engine block) temperature is below 50°F (10°C), full vacuum is supplied to the distributor. Above 50°F (10°C), the valve blocks full vacuum for up to 40 seconds. If ported vacuum drops, the valve opens, causing the distributor vacuum advance to be retarded. As vacuum increases, the valve closes, blocking full vacuum again. Cars manufactured after March 15 1973 have a cover over the valve so that it is more dependent upon engine block temperature.

??? I think you're mixing a TVS description with the EGR temp control valve, which is what is being discussed here. His set up is extremely simple. Has nothing to do with the distributor. He has ported vacuum from the carb to the TCV under the "stove", then from the TCV it's connected to the EGR valve. It stays closed to keep the EGR at bay until the TCV reaches opening temp, and then allows ported vacuum to the EGR valve as normal. That's it. No more, no less. It's there to improve driveability when cold. After it warms up, the TCV's job is done.