Air trapped in the system?

Pressure or coolant leak?

Factory recommended coolant mix?
Overflow bottle filled?

Is the shroud installed?

Is the fan in proper position in shroud?
A big spring in the lower 0r intake intake hose?
Thermostat stuck closed?

Thermostat right side up?
By-pass coolant flow blocked?

Heater core flow blocked?

Fan clutch not engaging completely?

Is the fan clutch HD?

Is a replacement electric fan under powered?
Fan belts properly tightened?
Idler pulley bearing frozen up?
Got small under-drive pulleys?

Coolant temperature Sensor having issues?
Is the temperature gauge having issues?
Is the water pump impeller, bearings or seal having issues?

Is the pressure cap tight and working?

Air dam installed?

Is there strong airflow over the exhaust manifolds?

Have scoops or louvers changed airflow under the hood?

Has the carburetor cool air intake been removed?

Bugs, dirt or wrong type of paint in cooling cores?

Is the core large enough for engine mods?

Rust or scale in the coolant?

Are rust or scale deposits lodged in the passageways of the heads or block?

Are tubes in the core blocked from old repairs?

Air baffles properly installed near the core?

Is timing (vacuum advance and centrifugal advance) and tuning properly set?

Is the catalytic converter and exhaust free flowing?

Is the exhaust heat riser working correctly?

Is an automatic transmission problem putting extra heat in the radiator?

Are A/C and oil coolers adding more than normal heat?

Is the gear-ratio/tire size proper for highway driving?

Are the brakes dragging?