1998 Cadillac Catera Problem:

check the coolant level. you may have a stuck thermostat

It should be on the rear passenger side of the engine.

posible if purchased new

May go to the radiator / tranny cooler next to radiator.

If you have a head gasket problem, you should see dense white smoke coming out of the exhaust pipe if you have antifreeze (ethylene glycol) in the cooling system, especially right after starting the engine.

I don't have a diagram, but your problem is the oil cooler underneath the intake on top of the engine. Remove the intake, and the cooler is right there in between the heads. Easy to change once you get the intake off. This is a VERY common problem on Cateras, and the symptom of it is oil in your coolant. I have a 1998 Catera and had the same issue.

First check for any leakage from your radiator, hoses or overflow tank.

If it were a freeze plug you'd not be able to miss the puddling spillage. Where if it were a head gasket the coolant and fuel / oil would mix and you'd be driving a mobile smoke generator { very noticeable while smoke }

If you can't see the leak take it in for a coolant system pressure test. Which will pinpoint where your vehicle is losing coolant.

Remove radiator cap while cold. Don't open hot ever! Start car and allow to warm till thermosta opens and watch for coolant flow thru cap. If not to bad can be flushed.

try flushing the system.you may have some restrictions in the motor, but first remove the radiator cap before it gets to hot,but hot enough for the thermostat to be open,and make sure you have flowing coolant.If not your water pump is bad.The fluid should be moving quite a bit.

You should be able to, so yes. If you have a coolant leak the pressure will build and slowly decrease or it will not get to the correct pressure. Unless the leak is in the radiator cap itself as the pressure tester generally uses a different cap.