Question about heat rise valve in manifold?

[earnhardtgm3]

Truck is a 77 with a stock 350 with all emissions still intact and the sensor broke off in thermostat that goes to the vacuum canister for the valve to open/close the valve for the heat riser to work.If no vacuum is the valve open or closed?Or do I need to put vacuum on it all the time till I can get headers installed.

[ray_mcavoy]

I'm pretty sure you want to leave it no vacuum to keep that valve open.

As I understand it, that thermal vacuum switch (broken piece) in the thermostat housing was designed to apply vacuum to the exhust valve actuator canister to close the valve when the engine is cold. Once up to operating temp, the thermal switch would then switch off vacuum to the actuator and vent it to the atmosphere so it would spring-return the exhaust valve to the open position.

[Duster]

Correct.

That heat riser is probably inop anyways. Its designed fail in the open position.

[motornut]

up here they can fail/seize closed tho
do you have a nib left?
my 79 removed

i've seen holes drilled and coat hangers pushed in to check

[Duster]

You live in "Camaro County" Georgia? Awesome... Never knew there was a place.

[chengny]

That's an EFE (early fuel evaporation ) valve. Here is the diagnostic chart.



The way I read it (the right hand side of the chart);

When coolant is cold, the TVS is open. This allows manifold vacuum to act on the diaphram of the EFE valve actuator, which causes the butterfly to close. When operating temp is reached the TVS closes off manifold vacuum and opens to atmoshere. Loss of vacuum allows the EFE valve to fully open.