Stuck thermostat??

I have a 1980 Olds Cutlass 260V8 rear wheel drive. Wife just got home after a long drive in 90 degree heat and noticed it was leaking coolant from the overflow tank when she parked it. I opened the hood and the overflow reservoir was all the way to the top! It was at the proper level the other day.

After waiting 15 minutes, I restarted it and the reservoir tank completely emptied into the radiator but I noticed the upper radiator hose was being sucked/collapsed closed unitl I shut her off. The hose was not collapsed when I originally opened the hood and noticed the coolant was all the way to the top of the reservoir. Any ideas besides the thermostat stuck closed??? THANKS!!!

 

Besides thermostat, which you probably will want to check out first...............

Did she notice a water temp gauge showing hot or an idiot light flashing? As the motor gets hotter, more coolant will expand and go into the resevoir.

Radiator fan not turning due to loose belt, or fan seized on shaft (applicable with belt driven blade) - or - electric fan not coming on (fan motor, wires, sending unit, relay, low on water causing sending unit to not register accurately, as we have discused in another thread here recently.

Least likely to be fan issue if motor is hot when cruising down highway.

If engine gets hot while city driving AND highway driving, more likely to be radiator clogged.

Head gasket.

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Collapse of upper hose: Maybe hose repeatedly hotter than normal for longer than you might realize, land lost it's resiliency. And maybe something is wrong with the overflow reservoir/tube, that when the motor cools and tries drawing coolant back into the engine, it is encountering a resistance at some obstruction in overflow tube/reservoir. See how hose feels when engine has cooled dowen, if you feel it is too spongy and might need replacing.