Cooling System Strange Behavior

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.

You either have an air bubble trapped somewhere in the system or you have a hose that is ballooning out ready to burst is my guess. It is also possible, the head gasket (or a crack in the head or block) has a small leak from one cylinder into the cooling system that is letting gas into the coolant but not letting coolant out into the cylinder.

I owned one of these cars… the exact year and engine, an Aero in Black with a bit over 100K miles when it was sold. Never had cooling problems. AC problems, blend door problems, sure, but not cooling.

Bad coolant reservoir cap and the hoses are expanding.


I think Mustangman has the correct idea. Under pressure there is a hose expanding enough to show a coolant drop, when you release the pressure, hose contracts, coolant rises.

Mustangman and all:
Thanks for the assessment. Hose(s) ballooning and trapped air was all I could come up with but I was not confident, given my tendency to overlook the obvious. So thanks again.

I’ll add the cap to my thinking. To start, my list of parts is the upper hose that feels soft and aT-stat. I’ll test the cap or maybe just replace it, I suspect it’s original. And check the other hoses.

I’m thinking that if those renewals solve the problem, the newly re-pressurized system may show a leak, likely at the heater bypass valve.

dont forget to bleed the air out of the system when your done.

There is air in the engine, the cooling system is low.

The level when hot should be several inches above the cold mark, add coolant.

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