Running hot and the heat doesn't work

check fluid level --water pump -thermostat-cooling fans

Pam are you sure it's boiling? If the Antifreeze is boiling I'd think you'd see steam coming out of your reservoir as the water expands into the reservoir, but you didn't say you see anything like that.
Of course engines get hot, that's why we have the cooling systems but it doesn't sound like it's "boiling" to me with no signs of visible steam unless your antifreeze level is really low?

If it's really boiling, and you antifreeze level is good, then your reservoir should be getting filled up with expanded hot water that goes into it to catch it when it expands.

If the water isn't going into the reservoir, then maybe your antifreeze level in the radiator is too low and you need to add antifreeze?
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Best to fill the radiator with the engine cooled down. If you ever have to open the radiator when it's hot, NEVER open it with your bare hand as you could get scolded with boiling water. Instead use a big thick towel to remove the radiator cap off a hot engine. Again, if you can, just wait until the engine cools then open the radiator cap.

With the engine cool, you can fill the radiator with a 50/50 antifreeze mix. The 50/50 mix come premixed 50 percent water, and 50 percent antifreeze. So you can throw that stuff straight into the radiator.

Non mixed antifreeze needs to be mixed 50/50. Some people use 50 percent distilled water when they mix it because distilled water doesn't have the minerals in it that may get stuck in your radiator. That's being a bit picky about it though. Tap water works fine in a pinch. The other 50 percent is antifreeze. You can mix it right in the radiator too, put in a measured amount of the antifreeze, followed by the same measured amount of water. The water pump will mix it all together when you run the engine.

Put the cap back on and run the engine and if/when the water "boils" it should go into your reservoir. If it does boil in go in the reservoir then yes...your engine is getting hot and the antifreeze is boiling.

When the antifreeze boils, it's probably because your thermostat is stuck closed. That means the thermostat isn't letting water circulate through the radiator to get cooled. Thermostats are cheap...no more than $30 I'd say for most cars. Changing them is pretty easy too as they are usually right at the top of the engine. A mechanic may charge $100 to change it....but it's an easy job on most vehicles requiring just a couple sockets/socket wrench.

The car is over heating. Check the coolant level. Have a pressure test done to check for ;leaks. Have a compression test done to check for head gasket and cracked head/s. Normal operating temperature is between 85 and 95 degrees c and if as you say is 20% hotter then it is boiling

hi there.
go to this link might help you to find out why.
2004 Cadillac Deville Running hot and heat don work Google Search

have the cooling system checked for air lock
will be a gauge sender not a sensor

see this causes :
nternal combustion engines run on heat. Chemical energy in the fuel is transformed into thermal energy when the fuel burns, which produces mechanical energy to push the pistons, spin the crankshaft and drive the vehicle down the road.
As efficient as today's engines are, they still waste a LOT of the heat energy they produce. The average gasoline engine is only about 22 to 28% efficient. That means over two-thirds of the heat produced by each gallon of fuel either goes out the tailpipe or is soaked up by the engine itself. Diesels squeeze a little more bang out of each buck's worth of fuel with efficiently ratings of 32 to 38%, but even that leaves a lot of waste heat that must be managed and carried away by the cooling system.
Ironically, the hotter an engine runs the more efficient it becomes. But there's a limit because aluminum pistons and heads can only get so hot before they start to soften and melt. The same goes for cast iron. Engineers have been tinkering with exotic ceramic materials and metallic-ceramic alloys in an attempt to build high temperature, super efficient engines. They've realized some significant gains but ceramics are still too expensive for everyday applications.
HOW HOT IS TOO HOT?
Most engines today are designed to operate within a "normal" temperature range of about 195 to 220 degrees F. A relatively constant operating temperature is absolutely essential for proper emissions control, good fuel economy and performance.
A 50/50 mixture of water and ethylene glycol antifreeze in the cooling system will boil at 225 degrees if the cap is open. But as long as the system is sealed and holds pressure, a radiator cap rated at 15 psi will increase the boiling temperature of a 50/50 coolant blend up to 265 degrees F. If the concentration of antifreeze to water is upped to 70/30 (the maximum recommended), the boiling temperature under 15 psi of pressure goes up to 276 degrees.
So does this mean a cooling system with a maximum concentration of antifreeze in the coolant (70%) can run as hot as 276 without boiling over? Theoretically yes -- but realistically no. The clearances in most of today's engines are much, much closer than those in engines built in the 1970s and early 1980s. Piston-to-cylinder clearances are much tighter to reduce blowby for lower emissions. Valve stem-to-guide clearances also are closer to reduce oil consumption and emissions, too. Plus, many engines today have aluminum heads with overhead cams. Such engines don't handle higher than normal temperatures well, and are very vulnerable to heat damage if the engine gets too hot.
Anytime temperatures climb beyond the normal range, the engine is running in the danger zone.
CONSEQUENCES OF OVERHEATING
If the engine overheats, the first thing that will happen is a gasoline engine will start to detonate. The engine will ping and start to lose power under load as the combination of heat and pressure exceed the octane rating of the fuel. If the detonation problem persists, the hammer-like blows may damage the rings, pistons or rod bearings.
Overheating can also cause preignition. Hot spots develop inside the combustion chamber that become a source of ignition for the fuel. The erratic combustion can cause detonation as well as engine run-on in older vehicles with carburetors. Hot spots can also be very damaging and burn holes right through the top of pistons.
Another consequence of overheating may be a blown head gasket. Heat makes aluminum swell almost three times faster than cast iron. The resulting stress can distort the head and make it swell in areas that are hottest like those between exhaust valves in adjoining cylinders, and areas that have restricted coolant flow like the narrow area that separates the cylinders. The typical aluminum head swells most in the middle, which can crush the head gasket if the head gets hot enough. This will cause a loss of torque in the gasket allowing coolant and combustion leaks to occur when the head cools.
Overheating is also a common cause of OHC cam seizure and breakage.
Wait, there's more. If the coolant gets hot enough to boil, it may cause old hoses or an age-weakened radiator to burst under the increased pressure. Pistons may swell up and scuff or seize in their bores, causing serious engine damage. Exhaust valve stems may stick or scuff in their guides. This, in turn, may cause valves to hang open which can damage pistons, valves and other valvetrain components. And if coolant gets into the crankcase, you can kiss the bearings and bottom end of the engine goodbye.
A HOT warning lamp should never be ignored. Though a few high tech cars like Cadillacs with the Northstar engine can disable cylinders to "air-cool" the engine and keep it running at reduced power in the event of coolant loss, most engines will suffer serious damage if they overheat. So advise your customers to stop driving at the first sign of overheating. Turn the engine off, let it cool down and try to find and fix the cause before risking further travel.

I HAVE A 2001 CHRYSLER CONCORD AND THE OVERFLOW IS BOILING BUT THE TEMP GAUGE IS NOT SHOWING RUNNING HOT. DO YOU THINK THAT THE TERMOSTAT IS STUCK

Check the over flow tank located in front of the passenger side glove box for water or fluid. When driving the car, listen by the glove box to see if you car hear a boiling sound. If you can hear water boiling or moving, check your heater core to make sure it is not plugged or bad. When the car starts to over heat, does the gauge register that it is getting hot? If it doesn't then check the thermostat to make sure it is not stuck open or closed. If its over heating and the gauge is not showing that the car is running hot before boiling over, then the thermostat is defective or not working. Try replacing it again.

I would say no more than 210 F, Say you have a thermostat listed at 185, or 195, they open up at that temp, the engine under normal driving will run between 195 and 210, you really don't want to see 250. even though coolant boils at 265+, in a pressurized unit, and some cars are supposed to run that hot, I don't like the idea. Hope this helps.

Check first if the fan is running, you can do this by pulling out the temp sensor at the lower radiator hose. You should see the fan go on. If the mechanics has checked out everything, I suggest to see if there are added load on the engine such as busted bearing on the pulleys for the serpentine belt. Observe the coolant if it is boiling hard when you notice the temp go up. It might be a problem withe the sensor which triggers the ECU to shut the engine.