Bigpursuit,
I was thinking about your data points and trying to understand them. He are some ideas. Let's assume that you are right, and while you have gas (air) in the system it is not combustion gas. The water pump is not a fixed displacement pump, it puts out pressure and flow is determined by the restrictions in the system. The biggest restriction, by far, is the thermostat. Even when the thermostat is open the area for flow is far less than the hose diameters, the passageways though the block and heads and the heat exchanger. So you will always have significantly more pressure before the thermostat than after. The differential will depend upon engine rpm and how much the thermostat is open. The absolute pressure at any point will also depend upon engine temperature rise, and ultimately (and quickly) the pressure cap.
Your system should hold something in the range of 6 gallons of coolant. Expansion from 60F to 170F would be about 18 oz. Since the fluid is incompressible, if the system was rigid it must all come out. Actually the hoses provide some spring effect, so likely your should see something like 14 oz come out and 4 oz go into hose expansion (I am making this up, illustrative only). That sounds about right. The pressure cap relieves at 7 psi so that is the system pressure (to atmosphere) at that point, but depending where the cap is compared to the rest of the system (particularly the thermostat) you could have some system pressures higher or lower than that by several pounds. (Ricardo got into this on #32 so I am repeating some of his thoughts in a different form.) If you have a bubble in the system, then it will also be compressed to whatever pressure is around it. It “should” be trapped at the fill cap, so that would be 7 psi. If you slow the rpm, and your fill cap is in a “low” pressure area (I don't know your model), that is, the area after the thermostat, then that region will increase in pressure (think “the water pump is not sucking as hard”), even as the high pressure area (before the thermostat) decreases in pressure. And the gas at the fill cap will expand and bubble out to bring the pressure at that point back to 7 psi, just as you have observed.
That is a correct thing. Actually, I believe that the only time the system can be degassed is when it is heating up to temperature (and gas gets pushed out in front of that 14 oz or so of fluid that is rejected) or to a lesser extent when engine rpm change causes pressure fluctuations and tank bubbling such as you observe. This is an important point not grasped by many observers. Once you are at temperature, and still have trapped gas, you are struck with it for that cycle. Of course the real problem is that the gas should not be there, and should have been pushed out in several previous heat/cool cycles. Since your overflow fluid does not get sucked back in, you start every trip with that 14 oz bubble. And as an aside, there are some clever little hoses (in some systems) to get the gas to the fill cap where it can reside and be pushed out when thermal expansion occurs. I suppose any of those (if present) are worthing looking at for clogs.
Other thoughts: When the engine starts cold, those water pump caused pressure differentials are immediately present. If the fill cap is in a low pressure area (after the thermostat), then you will have less than atmosphere at the cap, and will immediately start sucking even more air in, if your fill system is not tight. This will stop fairly quickly as the liquid expands, and the fill cap gets to a pressure above atmosphere, but it still it could be a contributer in the first minutes of running.
And if it turns out to be one of these “magic moving bubbles” (like I have theorized in my block/head), then it could move into your left exhaust manifold and get stuck (don't ask me how). At that point the manifold would overheat, and then steam would form. This would quickly expand the bubble and raise system pressure, and since the bubble is in the wrong spot (not at the fill cap), the system would eject coolant to the overflow tank and not eject the air. Now if the bubble moves, and the steam collapses back into coolant, the system pressure will quickly go negative (unless the 4 oz the hose spring effect can give back is enough to keep it positive). A negative pressure should mean the system will suck back fluid from the overflow, but since yours does not, it means it will suck in even more air........you see where this is going.
This thing can get complicated and have weird effects, but clearly the return system must function flawlessly. It is really more important than it first appears. There really is little to go wrong except the cap, the tank hose and fitting, and fill riser the cap screws into (mine had a gouge causing a leak). In fact the fill riser is particularly suspect since I think there is a sealing surface that holds the pressure in the system, and another that keeps air from entering around the cap when the system is trying to pull back fluid though the hose. Maybe the cheap solution is to replace it all.
I suppose it is possible to have other air leaks in the system, but unlikely. The leak would be one way (fluid does not go out at 7 psi), and the air would have to go in at only 1 psi or so of system negative pressure or else the cap would open a preferentially suck from the overflow tank.
I don't claim any of this is right, but it is good food for thought. Good luck.
CaboJohn
PS I have not solved my problem, but writing this has brought back one point. One reason I was convinced I have combustion gas into the coolant as a source of heat (in spite of a negative test), was that in a cold start I could rev the engine and get an immediate pressure rise in the coolant system and most of the pressure would remain even after a quick shutdown. I could not explain it other than by leaking combustion gas since the temp had not yet moved and the pressure remained. But I now realize my pressure gage was before the thermostat (where I would see water pump pressure) and my fill cap is after the thermostat near the pump input (so it would see negative pressure on startup and suck in coolant). Then on shut down that extra coolant would remain in the system and a higher pressure reading would remain (hose spring effect). And I would be deceived. Ouch!