Modern engines are designed to run within a narrow range of temperature. Because the metals used in an engine, expand at different rates, excess temperature causes a great many problems. Above a certain temperature, permanent engine damage will result.
Upper engine damage caused by an overheat
Cylinder heads warp when subjected to overheating. Once the cylinder head warps, the sealing surface is no longer flat. A warped cylinder invites a leaking head gasket, but that is not the only problem. Camshafts, that pass through overhead cam cylinder heads, also depend on the head being straight and true. A warped cylinder head may cause the camshaft to bind and cause damage. A binding camshaft may also cause the timing belt or chain sprocket to slip. When the sprocket slips, valve timing is altered.
On many engines, the piston and exhaust valve, occupy the same space in the cylinder, just at different times. If cam timing changes, the piston will strike the exhaust valve. This can cause a huge amount of damage, including bent valves, broken pistons, damaged cylinder heads and engine blocks.
When a cylinder head is warped, combustion pressure forces its way into the gap, that was sealed by the head gasket. The situation gets much worse over time. Cylinder heads may also crack from excess heat. Once cracked, the cylinder head may leak coolant into the cylinders and cause other problems. Combustion gas may also enter the cooling system through a cracked cylinder head. Combustion gas is many times hotter than engine coolant and can produce steam. Steam easily burns through plastic components like radiators and intake manifolds. Steam also breaks up circulation by the water pump, creating further overheating and damage.
Damage to pistons, cylinder walls and rings
Pistons are fitted to cylinders with very close tolerances. The spacing between the piston and the cylinder wall may only be .002 of an inch or less. This Clearance is critical, as oil fills the gap and protects the cylinders from wear. At normal temperature the pistons and rings ride on a very thin film of oil. Under these conditions there is next to no wear.
Elevated engine temperature causes the aluminum pistons and cylinders to expand. As the pistons and cylinders expand, the gap used by the oil, is taken up. At temperatures of 280 degrees Fahrenheit or more the pistons may actually contact the cylinder walls. With no lubrication and metal to metal contact, friction is drastically increased. When this occurs, the piston is damaged as well as the smooth surface of the cylinder wall.
The piston on the left shows normal wear, for a well maintained engine. The piston on the right has been subjected to engine temperature of 280 degrees Fahrenheit during a single engine overheat. The piston is destroyed, by contact with the cylinder wall.
The upper rings of the piston, seal combustion pressure in the combustion chamber. When the piston is damaged, the rings stick in their grooves. Combustion pressure leaks by the rings and enters the engine crankcase. This excess pressure and heat quickly overwhelms the positive crankcase ventilation or PCV system. As pressure builds, seals blow out and oil leaks develop. Corrosive combustion gas also breaks down the lubricating oil and may cause damage to the crankshaft and bearings.
The lower rings on the piston help to control oil and keep it out of the combustion chamber. Damaged oil rings allow oil to travel into the combustion chamber. Here it is burned, which reduces engine power and increases oil consumption. The burned oil enters the catalytic converter, greatly raising the temperature and causing failure. A smoking engine or an engine that consumes oil, after an overheat, often suffers from such piston damage.
The engine in the example above, had to be replaced, due to piston noise and excess oil consumption. Damaged cylinder walls, caused by overheating, may have been prevented by normal cooling system maintenance. A very expensive repair could have been prevented.
Overheating an engine, even one time, can cause irreparable damage. Temperature over 280 degrees Fahrenheit is almost always damaging. Automatic transmission fluid is also cooled by the radiator. When the engine overheats, the automatic transmission temperature is also greatly elevated. An overheated engine, may cause transmission damage, causing even further, unexpected repair.
If the temperature of an engine approaches 235 degrees Fahrenheit, the wisest plan is to turn the engine off and allow it to cool naturally. Never add cool water to a hot engine, as thermal shock can cause additional damage.
Stop driving any vehicle that is starting to overheat. Usually there will not be a second chance. Rental cars and wreckers are MUCH less expensive than engine repair.