Can somebody tell me the correct terminology for the two plastic fittings that screw into the thermostat housing on a '79 351M? One comes right out of the top of the housing and has two ports on it. The other comes out of the side/front and has 4 ports on it. Both fittings on my housing are blue.

Also, what do these parts do in general? The fitting with 4 ports on my thermostat housing has one of the ports broken off. Therefore whatever cicuit that is supposed be hooked up to it is getting no air I guess. How bad is this situation? The local auto store says the part in question is about $80 to replace. Ouch!

Thanks for any explanation/advice you can give me.

 

I am not sure what Ford calls them, I have always called them thermal vacuum switches.

 

Ford literature refers to that part as a "ported vacuum switch" (emission label abbreviation: PVS). The housing color indicates the temperature at which the switch acts. I don't have handy info on how to decode the colors, though.

They are temperature-sensing switches that open or close a valve in the vacuum system, based on engine coolant temperature downstream from the thermostat. When the coolant is cold, a PVS in the thermostat housing will not switch until after the thermostat opens.

Ford also used a PVS in the water pump housing on some 335-series engines. That one senses coolant temperature inside the pump cavity, which will warm up before the thermostat opens.

There are two-port, three-port, and four-port PVSs. The two-port PVS is a simple on/off switch in a single vacuum stream. The four-port PVS is just a double two-port unit, an on/off switch for two separate vacuum streams. The three-port PVS is a "switching" valve that switches from two different sources to a single output, or from a single source to two different outputs (depending on how you connect the hoses).

 

Thanks Bubbaf250, at least now I can use the correct name whe I decide to look for replacement parts. The $80 price tag one of the local auto stores gave me has me a little scared off so maybe I'll try some junkyards.

On another note...I was wondering if you could respond to my emissions removal post. You seem to have a wealth of knowledge on the M-Blocks and your website is under construction in the areas that would address this for me. I'm not trying to be pushy, I just really want to learn some things about my truck so I can get the most out of it. Thanks in advance for anything you can tell me.