What is this thing under the carburetor on my 1981 Ford 300ci straight six engine? What is it and what does it do?


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Comment by Jimco_W001 on March 8, 2020 at 10:34pm

Jack is talking about ported or main engine vacuum.

Comment by Jack Wasmuth on March 8, 2020 at 10:15pm

Kevin, I don't want to bust your bubble but, your truck has thermal vacuum switches. That means that they operate at different temps. The vacuum to your EGR does not work until the engine reaches a certain temp. No vacuum to the EGR at idle at any time, Then off idle ONLY when warm. If the thermal vacuum switch is bad OR miss routed vacuum lines the EGR will either not work or it will operate at the wrong time

Comment by Kevin F Smith on March 8, 2020 at 8:51pm

Thanks Jimco!  I found it.  It is right on the top of the radiator mount, and it is still legible.


Comment by Jimco_W001 on March 8, 2020 at 1:07pm

Kevin, there should be a decal under the hood showing the vacuum lines and where they go.

Comment by Kevin F Smith on March 8, 2020 at 9:44am

Also...do you know where the vacuum line is supposed to hook up to?  Mine is just sitting there on top of the valve cover, not attached to anything.


Comment by Kevin F Smith on March 8, 2020 at 9:43am

Thanks Jack!  I just looked up the part, and it is cheap...so I'll just replace it and the gasket.  Apparently, the big finned casting under the carburetor is a part of the exhaust manifold, and the EGR assembly is just the part on the end.


Comment by Jack Wasmuth on March 8, 2020 at 8:26am

Legally you can not disconnect it. It is a pollution control device. To tell if it is working you can have the engine warmed up to operating temp and with a vacuum pump apply 20 inch of vacuum at a idle and the engine should run rough or die. This will open the valve to allow exhaust to enter and lean down the fuel mixture. Its purpose is to lower the "knox". That is the yellow haze that you see over a large city.
Just a little bit of info, the gas that we have now a days is junk. I recommend that you do not use the cheapest gas that you can find. Nothing below 87 octane. If you are not hauling a load up hills, 87 is ok, but loaded you should use 89 or higher. Without the EGR not working you are more likely to ping under a load, so use higher octane. Set your timing to factory setting at Idle and CHECK the TOTAL advance at 2500 rpm. Total should not be more than 30 degrees
Hope this helps

Comment by Kevin F Smith on March 8, 2020 at 12:10am

So this entire contraption, the big finned cast assembly with the big vacuum thingy on the end, is the EGR?  Seriously?  On my old 454 it was tiny, sat on top of the intake, and I capped it off with a small plate.  So, how do I test if it is good, can it be eliminated, and what is the small vacuum line coming off of it supposed to attach to, since mine is just hanging there not attached to anything?  And these aren't actual pictures of my engine...the big hose from my oil filler cap goes back to the top of the air cleaner assembly.


Comment by Jack Wasmuth on March 7, 2020 at 11:51pm

Oh and Kev, that oil filler cap with the line from the carb bowl vent should not go to a pcv. You have a pcv in the back of the valve cover. If you put one in the front it won't work. They will fight each other. Make sure that the front one is not a pcv.

Comment by Jack Wasmuth on March 7, 2020 at 11:40pm

EGR vale. Exhaust Gas Recirculator.



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