A hot running engine is can be caused by a variety of problems:
- Defective thermostat
- Scale & rust accumulation
- Blocked hoses
- Incorrect ignition timing
- Cooling System Leaks
- Wrong Coolant Concentration
- Bad cooling fan
- Bad water pump
- Defective fan/water pump belt
The above calibration will determine at what temperature the thermostat opens. It is possible that, even though it may open at its rated temperature, it may not fully open.
Especially with older cars that have had multiple owners, there may be some accumulation of scale in the radiator and within the engine. This accumulation could block off the passages within the radiator which can severely downgrade the radiator's ability to transfer heat. Rust and scale on the internal surfaces of engine will reduce the heat transfer to the cooling water. The only way to remove both rust and scale is with an cooling system flush that contains an acid safe to the metals used in the engine. A good commercial flush is Gunk Super Radiator Flush, PN C2124C. You can also use citric acid (available through DIY wine shops) and oxalic acid (wood bleach) - both are safe on cast iron, brass, and aluminum used in older automotive engines. A chelating solution (like Evapo-Rust or Metal Rescue) will also remove iron rust from automotive cooling systems.
It is important to keep the coolant from boiling within the engine. The coolant strength should be kept at a concentration of 50% (vol) and any leaks must be repaired. A pressure tester will help determine how well the cooling system is sealed.