I have a 1996 Chevy Blazer. My temperature gauge started Rising to 210. When I parked the car there was fluid leaking underneath. At first I thought it was radiator fluid. There is no smoke only the temperature gauge is rising. The leak has stopped and it is no longer leaking but the car is still overheating. I haven't had a flush done in a while would that be all that I needed? or do I need to do something else with the coolant system. Last year I replaced the radiator and water pump and heater core + hoses.

 

I'm curious as to what the difference is between radiator fluid and coolant?

210° isn't really overheating, I'm pretty sure that's what these trucks normally run with a 195° tstat.

Also, let us know when it's getting hot. At idle, or slow speeds, like stop and go traffic, or at highway speeds?

 

so right now it's at 210 it's not technically overheating yet but its heading that way. The car typically stays at the line before 210. It never goes over. As soon as that leak started that's when it went to 210. Again the leak has stopped but when I drive on the street 35mph it starts to rise and when its idle it stays

 

I'm wondering if the pump is starting to go... or maybe a clog in your system. Has the vehicle ever been flushed? Could be something to look into, it's not too expensive and could save the system and tell you about any clogs.

Edit: been a long day and missed the part as it has been a while since a flush. Is there any obvious spots of where it is leaking from?

 

OK. So, you had a leak. Check the coolant level. Maybe it's low, which would cause it to run hotter.

If it gets hot while moving, and doesn't at idle, it's a coolant issue. If it were the other way around, it would be an airflow issue.

You just replaced water pump and radiator last year, you said? What about the thermostat? Easy way to check it is to pull it and place in a pot of water on the stove. With a thermometer, start increasing the heat, and watch when the tstat opens.

I was having overheating issues a while back. Threw a new radiator, tstat, water pump, checking for blown head gaskets, nothing was fixing it.

Finally pulled the block drains, which was a royal pain in the ***, and ram a wire coat hanger up in there to break apart the gray chunky crap that was in there

 

I am not sure where you live and what the outside temperature is but 210 degrees seems too high... I have a 98 Blazer and it never gets above 185 degrees give or take even in the dead of Summer with the air blowing on high...First thing personally I would do is replace the thermostat, very cheap to replace and if that dosen't do it I would flush and fill the entire cooling system ( radiator )... Probably a good idea to do the flush and fill even if the thermostat was the problem

 

In order to find out:

What is the outside temperature?
Run it with your A/C "off"! The A/C has a condensation drain which can leave water stains under the engine area when stopped.
In addition the A/C "radiator" is located in front of the engine radiator. A/C on "can" rise the temp of the coolant when it is quite warm outside.
IMHO 210°F is definitively on the warm side but not yet critical. As long as it does not move beyond that by much. You can see the gauge is redlined at around 225 - 230°F. (non linear scale)

Check where the coolant is leaking from.
Check if it forms a puddle regularly or if it was an only this one time issue.
Check the front of the radiator through the grille above and below the bumper - not the first time a plastic bag finds it's way there and covers part of the radiator.

When "cool"! Check the fluid level in the radiator and in the reserve tank (expansion tank).
Inside the radiator it should not drop below about 1 1/2" under the cap. (NEVER REMOVE THE CAPS WHEN HOT!)
In the expansion tank it should be filled to the "ADD COLD" line when the Blazer is cool.

You said: "Last year I replaced the radiator and water pump and heater core + hoses." Is that correct?

And a word about car's cooling systems:

Engines burn fuel and produce heat which we have to get rid of, else the engine components inside may overheat.
To do this an engine like your Blazer's uses a water pump to force water through the engine.
A radiator which offers a huge surface (with all the small fins) to dissipate heat to the surrounding air - or when moving uses the airflow to cool.
Also a small radiator is located inside the cab and can be used to warm the HVAC air, so you get warm feet in winter and can defrost the windows.
When warming up the "coolant" expands and when cooling again (when shut off) it retracts again. In order to compensate for this action we have an expansion tank on our Blazer's.

What's important to know is that our cooling systems are of the "pressure type". That means that when heating up pressure builds up which will prevent the coolant from boiling at 212°F (100°C).
So it is not yet boiling at 210 F.
This is the main reason "NEVER, EVER" to remove a radiator cap on a hot engine. Even if not boiling at the very moment as soon as the pressure is released from the cap it will boil over within the whole system at once. People got burned with 2nd and 3rd degree burns all over because it goes into "Old Faithful" mode.

About the "coolant":

Coolant, Antifreeze, Radiator fluid are all synonmys for the same thing. The mix of water and an agent to form the "coolant" we use in the cooling system for the engine.
Actually you could just run tap water in an engine cooling system. This would work on the short term but will create problems in the long run.
Depending on the manufacturer they prescribe a ratio of a "coolant" and water to be mixed.
One reason is that a coolant has to widthstand freezing, else it could destroy an engine block or burst a radiator when it freezes.
The other is that the coolant agents prevent corrosion. They slowly loose their ability to do so and should be replaced in the intervals the manufacturer states, latest every about 7 years.
If a proper amount of coolant is in the system it will feel slightly oily to the touch.
When replacing fluid because of a leak it will dilute the fluid. In the short term nothing negative will happen, but the issue needs to be addressed and afterwards filled with the proper coolant again.

A more extensive explanation can be found here:
https://www.carparts.com/classroom/coolingsystem.htm

 

If I were to guess and you can't find anything obvious then the lower intake manifold gasket could leak when the system is full.

With a lower coolant level it may not but won't hold pressure.

Going to do my third gasket in a week or two. Three different trucks...yes I am glutton for punishment.