Temperature needle indicats car is hot even though no leaks and car is full of water

Sounds like you may have a bad radiator. It may be clogged and the coolant is not able to go through it causing it to come out the cap and reserve tank.

I would be thinking radiator or water pump. When the engine is warmed up and the upper hose is hot, feel the lower radiator hose. It should be hot too, just a little cooler than the upper hose. If there is some blockage in the radiator, the lower hose may feel cold or just lukewarm.
Even though the water pump doesn't leak, the impellers may be made of plastic and have considerable wear. That would restrict circulation.
The heater core should not be an issue. Heater cores do not cause overheating even when totally plugged up.

You have a water leak from the sound of it or water is draining from over heating. Things to check for, leaks at the hose connections, leaks at heater connections on engine, leaks at water pump and any leaks at head gasket. Assuming there are no leaks check or replace or adjust your water pump belt. Take the thermostat out and run the engine without out it. If the problem is solved replace the thermostat recommended for you car. You can also have it checked as well.

If it is coming out of the top of the radiator, you are getting back pressure. First, make sure there are not puddles under the engine where the water pump is at. If so, you will need to replace the water pump and gasket; as well as the thermostat. It sounds as though the thermostat is stuck closed - replace it (if no leaks from water pump) and see if this fixes the issue.
Also, start the car (radiator full) and let run to operating temperature. Feel the top radiator hose - if it is not hot, but the engine is, then the thermostat IS stuck closed. Be sure to ask for a thermostat housing gasket when purchasing the thermostat, as they do not always come together.
Please let me know if you need further assistance.
carsandcomps

The most common overlook thing is you got a air pocket, so before you go spend your money turn Heather on full blast, open the radiator cap, car running look for air bubbles, takes patience cause you got to wait for the thermostat to open up, you must refill the coolant really really slow snail pace, some car have a bleeding valve others have two chamber on the overflow bottle,good luck, if that doesn't work change the thermostat first and if that doesn't work I'm afraid you may need to replace water pump. . water pump

You need to check the fan on the radiator. Also check the nozzles for the input and output from the radiator. If the nozzle at the radiator inlet is not hot, then the problem is in the cooling system. If the nozzle from the radiator is hot, then the problem is in the radiator. Be careful not to burn yourself.

If you didn't do a thorough cooling system flush with the radiator replacement last year, then i would say you may have excessive build up of rust, scale and other debris. This will certainly clog up a new radiator quickly. You may still have a restriction. I would do a good flushing, including REVERSE flushing until water runs clear. This should help... Good luck, my friend.

replace the temperature sending unit. Its right there by the thermostat. You can get this at any auto parts store. just need an open end wrench to change it. they do go bad(found out the hard way several years back). I hope this helps you

You may need to replace the thermostat or water pump, even flush out the system. Best thing to do is to flush out the system with radiator flush and follow the instructions and replace the oil antifreeze with new one with a 50/50 mix (water/antifreeze), if you not sure how to do that you can buy them premixed. Next is to replace the thermostat and radiator cap, that should fix the problem. The last thing would be the water pump is weak and would need to be replaced but lets start with the simple., The other thing to if it's only hot when your at a stop light then check the fuse and relays and wire harness to your radiator fan to make sure it's working. If you turn on your A/C that would over ride the temperature sensor and activate the fans so you know that is working, so if's running hot all the time even when your driving, replace the thermostat and coolant. Good luck.

Flushing the radiator, replacing the radiator cap, adding new coolant, replacing the sensors and a new thermostat solved the problem. Probably didn't need to do all of these things - but since we just purchased the truck, and given the fact that is probably hadn't been done in a while, we chose to do all of these things (which really didn't cost a whole lot), it's running like a charm now.