Before you try and solve an overheating issue, you need to make sure there actually is a problem.
A problem is typically defined as an engine that won't run under 210 degrees or is blowing water out of the cooling system (i.e. visibly overheating).
Make sure your gauge is accurate. If you don't have a gauge, temporarily install a quality mechanical temperature gauge to verify the coolant temperature. A hand held heat sensor can also accurately gauge temperature.
When is the vehicle is overheating - sitting still or moving? This will help you narrow down the cause.
1. Low coolant
2. Incorrect coolant mix - should be 50/50 anti-freeze & water. The more anti-freeze you add, the less efficient the coolant mix will be in dissipating heat.
3. Loose or slipping belt - check for squealing or glazing of the belt and correct tension on belt.
4. Radiator cap faulty
5. Thermostat stuck closed, or not installed.
6. Air trapped in coolant system - purge coolant system
7. Ignition timing to far advanced - over advanced engines cause the engine to run hotter.
8. Incorrect heat range of spark plugs.
9. Collapsed hose - typically observed when car is in motion, the water pump will suck the lower hose closed and block flow. To solve this you'd install a spring inside the lower hose.
10. Trash in between radiator/condenser - often times you'll have a lot of leaves and other trash in between the condenser and the radiator - blocking air flow.
11. Inadequate airflow through radiator (missing baffling, shroud).
12. Insufficient cooling from radiator (either too small, clogged up or fins bent) - have radiator serviced and make sure it's in good condition and correct for the application.
13. Fan Clutch (if equipped) faulty - a fan clutch will allow the fan to free wheel when it's not needed. When the car is sitting still and the engine is hot, the clutch should engage and drive the fan.
14. Impeller or pulleys incorrect for application - under-drive pulleys will turn the water pump slower - and reduce the amount of cooling. Aftermarket water pumps may spin the impeller slower as well. If you overheat in traffic - this may be the culprit.
15. Blown Head Gasket or cracked head - leaking exhaust into water jacket - You can check for hydrocarbons in the coolant and perform a compression test to determine if the head gasket is blown. Also, your oil may have coolant in it turning it into what looks like chocolate milk.
Cooling system Tech:
The more pressure the system is under - the higher the water can get before boiling.
With a 0 PSI cap water will boil at 212 degrees. For each PSI you raise the cap, you raise the boiling point temperature about 2 percent. Typically with a 15 PSI cap you boil at 252 degrees.