My 98cadalac eldarodo is over heating .took thermostat out the water is circulating . All of a sudden it will go from normal to the red .is this common and what else can it be .

Cooling fans may be staying on constantly also .

You need to bleed the system as it's air locked

replace your thermostat and check the waterpump. The ac "pulls" heat out of the air in the car thereby making it cooler, at the expense of making your coolant heat up even quicker than normal. Try this to see what's going on.. Open the radiator cap, start the engine and wait until the car gets up to operating temperature. You should be able to see the water circulating through the radiator...if the fan comes on, and the water isn't moving, then replace the thermostat. If it still isn't moving after that, the water pump may be no good. Id bet on the thermostat. They're cheap and need to be replaced every so often. If the water is circulating and the fan never turns on, then you check the fan motor, relay, etc. There is also a temperature sensor located somewhere next to the thermostat. It's rare to have to replace that, but if it's faulty, then the fan won't turn on and thus overheating occurs. I am assuming here that if you drove your car up a hill without the ac on, it will overheat or get hotter than normal...So....go test your circulation first and let me know what you find. Oh yeah, when you do the circulation test, turn your heater in the car on full blast too...does it get hot inside the car?

The thermostat is a critical part of your coolant system, the system that circulates water and antifreeze (coolant) through the engine to keep it from over-heating. It is called a valve because it can open and close, depending on how hot the coolant is acting upon it.
The reason to put a thermostat in the coolant system is to allow the engine to reach normal operating temperature quicker (and therefore operate at peak efficiency) than without a thermostat.
When the engine is cold, the coolant is at normal outside temperature, so the thermostat is closed. This means when you start the car, the coolant starts circulating through the engine, but can't flow into the radiator. With a closed thermostat, the coolant just keeps circulating around in the engine until the car warms up, the coolant gets hot, and the heat acts upon the thermostat, causing it to open at a certain temperature (about 185-195 degrees F.), With the thermostat now open, the coolant can now enter the upper radiatior hose and flow down the radiator to cool itself down, before going back into the engine through the lower radiator hose.
So the thermostat is always found on a car in the area where the upper radiator hose connects to the engine. Thermostat closed-water can't circulate through the radiator. Thermostat open- coolant is forced through the hose into the radiator.
I hope this helps you understand the thermostat valve and why all manufacturers include them.

I had the same problem with my daughters truck. Changed everything finally changed the radiator, that was it.

Is the vehicle running at normal temperature..If its running cooler than normal,then it might be the thermostat..Feel the upper radiator hose after the vehicle hot..It should be hot to touch

make sure you get ALL the air out run it without cap until you see water circulate turn heater to hot NO FAN ABOUT 5 MIN close cap ,see if that works

Engine over heating can be caused by a number of things. When you say the water pump and radiator appear to be ok I am also guessing that you checked the coolant level to make sure it is normal.

If the level is low then you obviously have a leak. Insufficient coolant circulating will not cool the motor properly because it won't be able to draw enough heat out. If the level is normal then you need to check the thermostat to make sure it is opening properly. To do this, run the motor to temperature and check the top and bottom hoses. They should both be hot and firrm once the thermostat opens meaning the circulation loop is operating properly. If the top is hot and firm and the bottom is cold/soft or collapsed then the thermostat isn't opening.

If the thermostat is operating properly then you ned to look for some other obstruction or cause for coolant loss. Check the head/intake gaskets, hose connections, hoses, over flow tank, radiator fins, radiator side tanks etc..

Good luck.

most likely a problem with the water circulation around the heater core. if you've never used the heater, some vehicles have a hot water shut-off valve for the heater hoses. another possibility is the thermostat had been removed. the easiest way is to start the vehicle and slowly allow it to warm up monitoring the water temperature. it should heat up to normal water temperature, then heat up and then drop down. the thermostat is usually around 180 degrees or 192 degrees. it allows the water around the engine block to heat up then once it reaches 180 or whatever it opens up circulating the cooler water from the radiator into the engine block and pushing the hotter water into the radiator to cool off. no thermostat, then the water keeps constantly circulating not allowing it to properly heat up. this can slow the heater down, lower fuel economy, alot of things especially in the winter driving...

you've got a possible blockage in the head or block.