Coolant Leak

Pressure cap... on top of the engine... thank you

Since it was seated tightly, I would first suspect the cap itself.  They're not very expensive to replace.

The cap has two sealing surfaces, an inner high pressure seal and the outer low pressure seal.

A leak at the low pressure outer cap seal would tend to be a dribble that just flows under gravity, but because the area under the seal is under a slight vacuum, it tends to leak Air In, not Coolant out.

If the hose from the Cap to the Overflow bottle is Clear and is visably filled with coolant and not air,  the cap is sealed.

The "Spittle" is more likely caused by hot coolant being sprayed from a small leak under high pressure.

The hoses attached to the Expansion tank that the cap is attached to are under 18psi of pressure when the engine is hot and running.

You will not be able to see this happening with the engine stopped.

I would expect the "Spittle" (Good Term!)  to be coming from somewhere other than the Cap.

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At $82 each, the Caps are not inexpensive in my book!

$82?  It's just a couple tanks of gas...  :)

So, where do you suppose this high pressure leak is, which is causing the "spittle" just under the cap?

When I next fly, I will check it out.  I know I have the cap on tightly... if the condition continues, I will replace the cap.  $82 is a lot of money, but, an A&P costs that much just to review the situation.  The Annual is coming up soon, I will ask the A&P to review the whole system for vacuum leaks, etc.  Fortunately, the temps (oil and coolant) are consistent, I am not experiencing any spikes or weirdness.  Thank you for your advice.