The first thing you must know in regards to a vehicle over heating is that most modern cars use dummy gauges. A dummy gauge is a type of gauge that only generally reflects the actual data. For example, the coolant temperature gauge will show cold, normal and hot, but will not show the many temperature fluctuations an engine goes through while driving. The problem with this type of gauge is that by the time the temperature gauge shows a hot reading, the engine has already reached a very high temperature, and has in many cases caused major damage to the engine.
Next you must understand the anatomy of a modern engine. Modern engines consist of a cylinder block, and one or two cylinder heads, depending on whether the motor is an in-line motor or a V’ed motor. Generally, an inline four-cylinder engine looks like this:
The cylinder head sits on top of the engine block and a cylinder head gasket seals the space between them.
Furthermore, most modern engines use an aluminum cylinder head and a cast iron engine block. Some engines, such as Subaru, use aluminum cylinder head and an aluminum engine block. The aluminum cylinder head is in many ways more efficient than the old style cast iron cylinder head.
The main draw back of an aluminum cylinder head is that when the cylinder heads becomes too hot, they easily warp. A cast iron cylinder head can be warped, but it must reach a much hotter temperature, and must reach that temperature many more times.
The warped head no longer has a proper mating surface in order to maintain a tight seal between itself and the engine block. Because the tight seal is lost, the cylinder head gasket eventually goes bad. The bad cylinder head gasket causes the vehicle to continue to overheat.
You see, the anatomy of a cylinder head gasket is such that there are many holes in it. Some holes are for the pistons to pass through, some for oil to pass through, and some for coolant to pass through. When the cylinder head gasket goes bad, it allows some of the exhaust gas to go from the cylinder area into the coolant. The exhaust gas is much hotter than the coolant, and also forces air into the cooling system. The combined effect of the hot gas and the resulting air pocket in the cooling system results in a vehicle that overheats, despite the otherwise proper cooling system.
Symptoms of a bad head gasket and or a warped cylinder head include:
Overheating while under load, cruising down the highway, or in stop and go traffic despite and otherwise working cooling system.
Rough idle at first start up after sitting for an extended period of time. While the vehicle is not running, coolant may drip into the combustion chamber and cause a misfire until that coolant is burned off.
White smoke out the tail pipe at first start up. Again, the coolant may drip into the combustion chamber while the vehicle is off, the burning of that coolant results in smoke.
Vehicle continually needing coolant added despite no visible leaks. The engine consumes the coolant and burns it; therefore it constantly needs to be topped off.
Excessive pressure in the cooling system while the engine is running. The same combustion force that drives the engine pushes the exhaust gas into the cooling system, resulting in very high pressure in the cooling system.
Continual boiling over and pushing fluid out the overflow reservoir. Because the exhaust gas is forced into the coolant, the pressure becomes so high that it forces the radiator cap to open and pushes excessive amounts of coolant into the overflow reservoir.
Hydrocarbons (a component of exhaust gas) in the cooling system.
Oil in the coolant, and coolant in the oil. If the cylinder head is warped badly enough, the coolant and oil will mix. This symptom is very rare, and only occurs in the most extreme case of a bad cylinder head gasket.
Here are the two worst over heats I have seen. An Audi that melted a piston, and a Subaru that melted the block!
A bad cylinder head gasket is costly to repair because major components of the engine must be dissembled. The intake and exhaust manifolds must be removed. In the case of overhead camshaft motors, the timing belt or chain must be removed. Lastly the cylinder head itself must be unbolted and removed from the engine block.
Once the cylinder head is removed from the engine block, it must be thoroughly cleaned and taken to a machine shop. The machine shop then inspects to see how much damage the heat has caused, because the valves and the valve seats may also warp due to heat. In the case of warped valves, not will the cylinder head need to be milled back to flat, but the valves may also need to be replaced.
Once the cylinder head has returned from the machine shop, the engine may be reassembled. The end result is a very labor intensive and expensive repair. The positive note being that an engine that has had the cylinder head gasket replace, and the cylinder head properly repaired, will run like a top.