The gauge is your first thing to look at, but in your other post you didn't list any other symptoms so it is possible the problem is with the gauge and not the engine.
Another thing to look at is the heater. Turn it to the highest temperature, then feel the air coming out. If it is really hot and burns your hand, you'll know the engine really is getting as hot as the gauge says. If the air temperature is normal, it is likely the engine is not overheating. If the air temperature is cool, the overheating is caused by the coolant not circulating properly. That can be caused by low coolant level or a slipping belt that drives the water pump.
Another thing to be aware of is when you start the cold engine, it is going to take from three to ten minutes for it to warm up before it can even think about overheating. If the gauge goes to full hot in just a few minutes, you know the engine didn't get that hot that quickly. Suspect a defective sending unit but that is not common.
If the engine really is getting too hot, there are two conditions to look at. The hot coolant isn't going to the radiator or the heat IN the radiator is not being dispersed. Hot coolant will not circulate to the radiator if the water pump belt is slipping, if the coolant level is low, or if the thermostat is not opening. A leaking head gasket can cause the thermostat to not open in some engines. That is because they only open in the presence of hot liquid, not hot air. A leaking head gasket can leak exhaust into the cooling system, and that air can prevent the thermostat from opening if it collects in that area.
Overheating can result in some engines after performing routine service if the air is not bled out. Changing the thermostat and flushing the system are two times when this can happen. A lot of cars today have bleeder screws near the thermostat housing because when you fill the new coolant, the air can't bleed itself out from under the thermostat. That leaves that air pocket there and the thermostat won't open. On some engines such as the older Chrysler four-cylinder engines from the '80s and early '90s, there is a plug to unscrew or it's just as easy to unscrew a sensor on the thermostat housing.
If circulation is okay and the radiator is getting hot, it's not giving up the heat to the air. There are three things to consider. The most common problem is an electric radiator fan that isn't turning on. The fan is never needed at highway speed because natural airflow is sufficient. When the fan isn't working, the overheating will occur at low speeds. Next, on older cars, especially those in the northern states, look at the fins between the tubes of the radiator. When they become corroded and rotten they will crumble easily when you touch them, and they will not transfer heat to the air. The clues are the lower radiator hose will be just as hot as the top one, and the overheating will be worse at highway speeds and in warmer temperatures. This will not start suddenly. You will notice the temperature gauge will be a little higher than normal for many months, and it will continue going just a little higher gradually over a long period of time. The third thing is rather rare but should not be overlooked. If the thermostat sticks open, it normally takes a real long time for the engine to get to normal temperature because the coolant doesn't stay in the engine long enough to get heated up, but once it is hot, overheating can result when that hot coolant doesn't stay in the radiator long enough to give up its heat. Sometimes the excessive temperature will drop when you slow down from highway speed. This is easy to find on cars that have the radiator cap on top of the radiator tank, not the overflow reservoir. Remove the cap, then watch the coolant right after you start the engine. It should not be circulating yet until the engine warms up.
If the overheating only occurs in real hot temperatures, you might suspect the air is bypassing the radiator. Look for the rubber seal under the front edge of the hood to be sure it's there. If it's missing, some air will go over the top of the radiator instead of through it. Be sure the shroud is around the fan too, otherwise it can pull air from the sides instead of through the radiator. That is not much of a problem on front-wheel-drive cars because the fan is often built into that shroud.
Getting back to head gaskets, besides pushing exhaust into the cooling system, coolant will go into the combustion chamber and be burned too. You'll see a lot of white smoke coming out the tail pipe. Also look for air bubbles in the reservoir. While that air can cause the thermostat to stay closed leading to overheating, a good clue is if you see those bubbles when the engine is still cold. There will be no steam with those bubbles. The bubbling will also stop as soon as you stop the engine. If you see bubbling or hear boiling sounds for half a minute after stopping a hot engine, you can be sure it really is overheating.
A partially plugged radiator will make the engine run hot but that won't happen suddenly. The temperature will keep getting hotter over many months and will usually be worse at highway speeds. If you suspect that, run your hand over the front of the radiator from one side to the other. It should be the same temperature all the way across. If you find one area that is cooler, hot coolant is not circulating through those tubes.
There is a real good test if you suspect a leaking head gasket. That involves drawing air from the radiator through a glass cylinder with two chambers partially filled with a special dark blue liquid. If combustion gases are present that liquid will turn bright yellow. You'll need to have a mechanic perform that test. It only takes a few minutes.
Saturday, June 4th, 2011 AT 7:10 AM