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Another hidden cause of an overheat is a blown head gasket OR as it is better said, "a combustion leak into the cooling system".  The truth is you don't actually know a head gasket has failed until you remove the cylinder head and inspect it for damage.  It could just as easily be a cracked cylinder head or worse, a cracked block that can cause the very same symptoms.  A head gasket failure is the most common cause of a combustion leak into the cooling system but NOT the only one so keep that in mind.  Come to think of it, a leaking intake gasket can also cause very similar symptoms to a head gasket failure.  Things like a LOT of white smoke coming out the tail pipe after the engine has warmed up.  I get a lot of people coming to me saying their head gasket is bad because they see white smoke coming out the tail pipe OR they see water dripping from the tail pipe.  The truth is that water is a natural byproduct of combustion so seeing white smoke (steam) and water out of the exhaust is normal.  You need to make the determination if it's excessive or not.  If the weather is warm (above 50ºF 10ºC) and your engine is warmed up and you're still seeing an excessive amount of white smoke coming out the tail pipe this could be an indication of a combustion leak into the cooling system.  To determine if thats the case you can use the 'Block Test' shown in the above video.  That test should tell you if you have combustion gasses in the coolant which would indicate a combustion leak into the cooling system.   Another test that can prove out a combustion leak is a leak down test.  With a leak down test you run compressed air into the suspected cylinder and look for leaks.  If you see bubbles in the radiator or expansion tank when you pressurize a cylinder this can also prove a combustion leak.  Here's a link to a video I did on Leak Down testing.

The truth is tracking down internal coolant leaks can prove difficult.  The only way to truly know what the issue is is to tear into it and do a visual inspection of the suspected cause.  The above tests are a great start but as I said, not always conclusive.  When you do get things apart the coolant leak is often evident.  You'll notice that the cylinder coolant was leaking into is often very clean and free of carbon deposits.  This is due to the 'steam cleaning' effect the coolant has on the combustion chamber.