Causes of Cooling System Failure

Overheating usually occurs for one of the following reasons -


Insufficient amount of coolant in the system.
Defective or improperly adjusted drivebelt.
Blocked radiator core.
Dirty or restricted radiator grille.
Faulty thermostat.
Improperly functioning fan.
Improper pressure. Have your radiator cap pressure tested by a gas station or repair shop.
Incorrect ignition timing.
Defective water pump.
Wrong grade of engine oil.
Inaccurate temperature gauge.

Overcooling usually occurs for one of the two following reasons -


Faulty thermostat.
Inaccurate temperature guage.

External Coolant Leakage usually occurs for one of the following reasons -

Deteriorated or damaged hoses, or loose clamps at the hose connections.
Defective water pump seals. If this happens, water drips from the weep hole in the water pump body.
Leakage from the radiator core or header tank. If this is the case, the radiator must be professionally repaired.
Leaking engine drain plugs or water jacket freeze plugs.
Leakage from damaged gaskets or small cracks.
Damaged head gasket. To verify this, check the engine oil condition.

Internal Coolant Leakage usually occurs for one of the following reasons -


Note: You can detect internal coolant leaks by examining the oil. Check the dipstick and inside the rocker arm for water deposits and milkshake-like oil consistency.


Leaking cylinder head gasket. If this occurs, test the system pressure or remove the cylinder head and inspect.
Cracked cylinder bore or cylinder head. If this is the case, dismantle and inspect the engine.
Loose cylinder head bolts.

Abnormal coolant loss usually occurs for one of the following reasons -


Overfilled system.
Coolant evaporation due to overheating.
Internal or external leakage.
Faulty radiator cap. If this is the case, have the cap pressure tested.
Engine compression is pressurizing the cooling system. A cracked head or block, or leaking head gasket(s) could cause this.

Poor coolant circulation usually occurs for one of the following reasons -


Inoperative water pump. To test, pinch the top radiator hose closed with hour hand while the engine idles, then release. If the pump is working properly, you should feel a surge of coolant.
Restriction in the cooling system. To fix, you should drain, flush, and refill the system. If necessary, remove the radiator and have it reverse flushed or professionally cleaned.
Loose water pump drivebelt.
Sticking thermostat.
Insufficient coolant.

Corrosion usually occurs for one of the following reasons -


Excessive impurities in the water. We recommend you use soft water, rainwater, or distilled water.
Infrequently flushed and drained system. You should regularly drain and flush your cooling system to maintain optimum functioning and prevent corrosion.

And much more good info on this page
Radiator Installation and Removal
How to Measure a Radiator
Radiator Cooling System Check
Causes of Cooling System Failure
How to Drain, Flush and Refill your Radiator
Common Cooling System Problems and Diagnosis
Aluminum Cooling System: A Benefit or a Headache?
In-tank Coolers - What You Want to Know
Preventing Cylinder Head Gasket and Cooling System Failures
The Cooling System: Preventative Maintenance Tips
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