I know these are part of the vacuum system but not sure of the name or function. The hoses come to and from the carb; one goes to the distributor vacuum advance. My guess is they act as a valve of sorts opening or closing with the engine temp.

Trying to do a bit of diagnostic work on a rough cold idle but figure I need to know my way around a bit first. (I had been previously led to believe this was a choke adjustment issue but the more I fiddle with that the less convinced I am) More on this later I suppose.

75 f100 390

First component- Installed onto the thermostat housing



Second component - Installed onto the intake manifold



thanks!

 

They're called "ported vacuum switches" and they do exactly what you imagine: they switch the vacuum based on temperature. The 2-port one is an on/off valve. The 3-port one switches whether the center port is connected to the top or to the bottom.

 

A 2-port PVS will often be used to control something like EGR: disable it until the engine warms up a little, then let it operate as normal.

A 3-port PVS is often used for something like overheating protection: if the engine overheats, switch the spark advance from ported to manifold vacuum so that the idle speed will bump up a bit and spin the water pump & fan a little faster, and advancing the combustion phasing, cooling things off.

Other applications exist too - those are just examples of some ways ported vacuum switches are commonly used. What yours are doing depends on what they're hooked up to.

 

Thanks... I was able to quickly find these once I knew the name.

I found this diagram for the three port switch... Gonna check later that my lines are in the right order on the tree. Does the position matter?

I can't for the life of me find the diagram for the 2 port switch although this one probably doesn't have any issues of the ports are flipped.

 

Position does matter on a 3-port PVS. Not on a 2-port (just on-off).

On the 3-port version, the middle port is normally connected to the top port, until it gets up to a certain temperature, and then it'll switch and the middle will be connected to the bottom port instead. The middle is always the distributor side in that application, while the top & bottom are the vacuum sources.

On the 2-port version, they're normally not connected, until it reaches the setpoint temperature, at which time the valve opens and they are connected.

(Technically, since these work on a phase-change basis of wax melting, there's a range of temperature over which they open/close, rather than a sudden switch at exactly the setpoint temperature, but for practical purposes, we can ignore that.)