1999 Oldsmobile Alero engine overheating

I had same problem; replaced the coolant temp sensor, thermostat, and reservoir cap. Didn't help. Turned out that the plastic reservoir itself was slightly warped/out-of-round where the cap screws in, so that even with cap on tightly it would not hold pressure. To test this, I put the black overflow hose that comes out of the reservoir (near the cap) into a clear plastic water bottle. With engine hot and a little water in the bottle, I could squeeze the radiator hose and see bubbles in the water bottle. No air or coolant is supposed to get through that cap until it's over 15PSI. Got a new reservoir ($35/RockAuto) and now system holds pressure. I'm leaving the plastic bottle in place, the overflow hose sort of holds it in place, doesn't interfere with nearby moving parts, and will show me if there's any more overflow (haven't seen any in 3 days)

I am so glad I read the reply from skeeter. My sister's 1999 Oldsmobile Alero was pumping all of the fluid out of the reservoir and of course over heating.I did the water bottle test and sure enough the reservoir was bad. Everyone was saying it was a blown head gasket. New tank and coolant for about $55.00. Thanks for the great information.