Thanks for the replies. But I don't think a lack of coolant addresses my question.
Bare Bones of the question, sequence of events:
- Cold engine, normal full expansion tank.
- Drive car, engine at operating temperature, in a while SID gives warning.
- Stop, check level: No visible fluid in the still closed tank. SID was right.
- Engine still hot, open tank, pressure releases, level returns to #1's level. So, no apparent need to add coolant. SID quits complaining.
- Question #1: Where does the coolant in this sealed system go to (#3) and come back from (#4)?
As you say, the expansion tank level is expected to rise when hot. The level falling when hot (#3) suggests a pressure leak expelling fluid from the system. The level then returning to normal (#4) does not. ( I know, that doesn't mean I don't have any leak. But first questions first.)
Have you actually verified that the coolant level is normal when it's cold?
Have you verified that, when it's cold, that the cap is still sealing the system? There should be a minor change in the level and a hiss when you pull the cap. In my experience, the level rises a bit.
If it truly is up to the KALT level when cold, and then drops when you drive the car, I would figure that air or some kind of gas is getting into the system while the car is off. Which points to head gasket.
When the system is pressurized the level in the tank is lower than when the system is depressurized. This is completely normal & also the reason why the "cood" fill line is so high in the tank. By removing the cap and watching the level rise again you are depressurizing the system, thus seeing the coolant level "rise" in response.
That is true, see my statement above. However, if the system is not leaking, the level in the coolant tank will rise as the engine heats up and the system pressurizes. The coolant expands, squeezing the air at the top of the tank; and the air pushes back, of course, so the system is pressurized. What the OP is describing is something quite different and unusual.
The level will absolutely appear to drop at operating temp and rise back when cold/depressurized. This exact occurrence causes the system to suck up air bubbles if the cold level is too low. On the Saab it's not as dramatic as on my other car which will almost instantly overheat if this happens. When you look at the level in the tank with the car at temp it will appear empty, yet once you slowly open the tank (to prevent a geyser loss of fluid) the level rises & a fluid line is seen in the tank again. Again, on my other car the system is much more sensitive to this change in coolant level between when the system is hot/cold but regardless it occurs in both systems.
That's absolutely not the case.
You may be confusing this with when you first refill the system after a flush. When the thermostat opens, coolant starts circulating through the rad, which displaces air pockets. The level in the tank drops, possibly dramatically.
Once it's topped up, the coolant should never drop below KALT in a properly working system, whether cold or hot.
