First, see if there is dirt or debris clogging up the radiator fins. This can severly limit your cooling system's performance. If there is, spray the radiator down on both sides with a garden hose and nozzle.
Next, confirm that the radiator cap is holding pressure. For every psi that the cap isn't holding minus from the 14ish psi it's supposed to, you will lose 3.25 degrees Fahrenheit of protection. That means the coolant will boil that much sooner ending in an eruption of coolant. To do this, you simply put your ear near it with the engine cold. Open the cap one quarter turn and listen for a vacuum like woosh that lasts 1 to 2 seconds. If you do not hear this, the cap is likely not holding pressure. Another possibility is there are internal leaks. That will be covered later.
The third thing you need to figure out is if the thermostat is opening or not. You can let the engine warm up to normal operating temperature (til radiator fan cycles) then feel and compare the temperature of the upper radiator hose versus the lower. If the lower feels alot cooler than the upper this means the thermostat is not opening. -or- For proof you can see, take the thermostat out and boil it in water and watch for it to open a few moments before the water starts to boil. If it doesn't open on your stove then obviously it isn't opening in your engine. If the thermostat is good, reinstall and proceed directly to the next section below. If the thermostat is found to be bad, replace it with a new one then proceed.
If the fan does not ever cycle at all even when temps are above the half way mark, steps have to be taken to find the reason. Possible causes are blown fuses, pinched or cut wires, bad relays(near airbox), bad thermo-switch on the back of the block, or bad fan motor.
Of course be sure the system is properly and completely bled otherwise air pockets will be in the system causing the engine to overheat.
Once you have properly bled the system, see if the overheating problem persists. If it does, look to see that your radiator itself is in good condition. Most, at least 80 percent of the fins should still be intact and in shape, and not missing or bent over flat! If your radiator doesn't pass this standard, it's safe to say it needed replacing anyway. Do so then bleed the system again and see if the problem persists.
If it does, this is where it may get scary if you don't have much mechanical experience. Pull your spark plugs and make sure that they don't have a green tint to them. Beware of the misconception that white on your plugs means you're burning coolant. The little bit of white or actually off white that you may see on your plugs is a by-product of the ethanol and detergents in pump gas. Don't be alarmed if you see some shades of off white and trust this as it came directly from the guy who trained me the Acura dealership. Of course he may be wrong but I highly doubt it.
Next, you need to check for obvious signs of coolant getting into the oil or oil getting into the coolant. This is indicated by a cream colored milky substance either on the dipstick or under the oil cap. Another tell tale sign of this is a thin oily film on top of the coolant in the radiator. If you find this, it means your head gasket is bad. If you do not find this, it doesn't mean your head gasket is good. Reason is because coolant can be going from the water jackets directly to the combustion chamber then getting burned off and never once touching the engine oil. This could also be happening at such a slow rate that it doesn't show up green on your spark plugs as well.
In this extreme leg of overheating diagnosis, you must first perform a compression test to see which cylinder is affected (where the break in the head gasket is). This break is usually indicated by a compresion test reading on a cylinder that is 30 or more psi lower than the highest cylinder.
Note that just because a cylinder is low doesn't mean the head gasket is automatically bad. It's very possible for the compression rings to be bad or for the valves to not be seating properly.
The ultimate test for a bad head gasket once you've narrowed the problem down to the affected cylinder is to perform a leak-down test. This will require special tools and equipment and will probably be more than what you want to be involved with unless you're ready to teardown the engine and fix the problem once you've identified it.
