Since I bought my 66 Coronet the Manifold Heat Control Valve was stuck. I finally got it moving again after a few days of using the Mopar solvent, part number 4318039AE. When cold it moves about 90 degrees very freely by hand from closed (clockwise) to open (counter-clockwise). The spring pulls it closed cold as expected.

1. When I started the engine and ran it to operating temperature it smoked a bit while burning off the excess solvent but tightened up somewhat again so it did not move freely when warm/hot. When cool again, it moved freely again. With a little more treatment will it stay free when hot?

2. When looking at Figure 27 of Section 0 in my Shop Manual I see a picture of this valve from below with front to the left and back to the right. The counterweight and return spring are clearly seen on the left but there is a another "carburetor-like" spring on the right (back) side of the manifold between the heat control valve rotating shaft extension and another fixed rod up and outboard of it. Each of these rods has a groove where the spring sits. See picture below. What is this spring and why is it there? Mine has no spring. Do I need one - why?

That's your anti-rattle spring. It keeps a bit of side load on the valve shaft so it won't rattle, and also serves to keep the valve shaft centred so it doesn't edge too far forward or backward which would tend to bind up the valve, though for the most part that is the job of the hairpin clip you see in the pic there in the groove nearest the manifold—make sure you're not missing any clips.

It may take a few cycles of spraying the valve solvent and operating the (cold) valve as well as push-pulling the shaft fore and aft and then spraying and rotating again, to clean out all the crud. If the valve still wants to bind up when hot, you may be dealing with a bent shaft, but you can try lubing the valve with a graphite-in-liquid-carrier spray. Name brands escape me at the moment, but it shouldn't be hard to find.

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Thanks so much for the reply. After I thought about it awhile I figured it must be for anti-rattle like you say. I found a very similar type spring in my box of old carburetor parts and adapted it to fit (I had to shorten it and make a new loop on the end). So, now I have the spring I was missing. The clips are still present and the shaft really has no slop in it at all. I will continue to work the valve with spray including moving the shaft fore and aft as I have been. Maybe soon it will stay free upon heat up. For me it was a satisfying success to get an old part working again.

Jon