Vehicle of concern is a 2001 Saab 9-5 2.3t. Bought used, about 2 mo ago, pretty good shape, low avg mileage. This is about one of 3 or 4 gremlin like issues I’m trying to sort out. I don’t think that it is a Saab is significant in this issue. Which briefly is:
Start with a cold engine, expansion tank is full to the cold mark.
After the engine is hot at operating temperature, the expansion tank fluid level will be low enough to trigger the warning message and is visually seen to be low. When the still pressurized system is opened (slowly, lid on the tank) pressure release is heard and the coolant level rises to the cold fill mark, where it began.
Summary: When hot and pressurized, level drops. When pressure released, level rises to cold mark on tank (the full mark).
I am trying to understand this behavior, which is the opposite I would expect from a sealed system design, so I can make targeted repairs if possible.
Possibly pertinent facts: There is no apparent loss of fluid. No water in the sump, no oil in the cooling system. The dash temp guage shows normal operating temp. Car just completed a 7-800 mile road trip, with no untoward events - except driver being sure to check coolant level frequently just in case.