engine over heating 1997 olds aurora, 135,000 miles.

What kind of engine do you have??

Explain water pushing back out of the Reservoir

Do your coolant fans work??

I have a 1997 olds aurora that I recently purchased with about 135,000 miles on odometer. This car has the 4.0 litre north star v8 engine made by cadillac. This is a 32 valve dual overhead cam engine. This engine runs very smooth and has lots of power. But my problem is that it overheats in about 5 to 10 minutes of driving at 55 mph. The radiator cooling fans seem to be operating properly and there are no leaks of coolant except it comes out of coolant tank overflow after about 5 minutes of driving. This coolant comes out fast and and with lots of force when temperature gauge gets up to about 260 to 280. There is no water in the oil and no oil in the water. Water pump belt and pulleys are fine. I have not replace engine thermostat. I have been told that it might be a blown head gasket or a cracked cylinder head, but I don't want to do this work if it is not needed. Please advise.

Sadly that is a sympton of a bad head gasket or cracked head. The engine compression is by passing a gasket and forcing the water out of the coolant jug.

The North Star engine damages easily if overheated. At 260 + degrees the engine is already damaged. Both heads need to come off, and checked. Hope fully the block assemby is OK. When you mix aluminum and heat in an engine any variance in temperature causes engine damage.

You can purchase a CO2 kit at any parts store that allows you to test is CO2 is in the coolant system. I think it would be a waste of money because the top of the engine is already ruined. It probably was bad before you got the car and the owners knew it. Hence the sale of a nice car.

You may want to consider a low mileage salvage engine.

Good Luck