A "burned valve" is a valve that has overheated and lost its ability to hold a leak-free seal. Valve burning is usually limited to exhaust valves because they run much hotter than intake valves.
The diagnosis of a burned valve is usually the result of a compression test. If a cylinder shows little or no compression, it frequently means the exhaust valve is not sealing. The valve may or may not be actually burnt (melted), but have other physical damage such as cracks or areas where pieces of metal are missing or eroded away from the valve face.
The cure for this condition is to remove the cylinder head, replace the bad valve and reface (or replace) the valve seat. As a rule, the head is usually given a complete valve job at the same time because the rest of the valves and guides probably need attention, too. If one exhaust valve has failed, the rest are probably on the verge of failure if they haven't already started to leak.
There are several reasons why valves burn. One is normal wear. As an engine accumulates miles, the constant pounding and thermal erosion wears away the metal on the face of the valve and seat. The exhaust valve sheds most of its heat through the seat, so when the face and seat become worn and the area of contact is reduced, the valve starts to run hot. Eventually the buildup of heat weakens the metal and pieces of it start to break or flake away. Once this happens, it forms a hot spot that accelerates the process all the more. The valve begins to leak and compression drops. The result is a weak or dead cylinder and a noticeable drop in engine power, smoothness and performance.
A bad exhaust valve will also increase exhaust emissions significantly because it allows unburned fuel to leak into the exhaust. High hydrocarbon (HC) emissions, therefore, may also be an indicator of a burned valve.
An exhaust valve can also burn if the valve lash closes up for some reason (improper lash adjustment, cam or lifter wear, a bent push rod, worn rocker arm or cam follower, etc.). The lack of lash (clearance) in the valvetrain prevents the valve from closing fully, which causes it to leak compression and overheat.
Valve burning can also be caused by any condition that makes the engine run hot or elevates combustion temperatures. This includes cooling problems, abnormal combustion like detonation or preignition, loss of exhaust gas recirculation (EGR), retarded ignition timing or lean fuel mixtures.
A condition known as "valve recession" can allow the valves to recede or sink into the head because of excessive seat wear. This causes the valve lash to be lost which allows the valves to leak and burn. It occurs primarily in older engines (mostly those built prior to 1975) that were not designed to run on unleaded gasoline. When leaded gasoline was still around, lead acted like a lubricant to reduce valve seat wear. But when lead was eliminated, it meant engines had to be made with harder seats. These older engines didn't have hard seats, so many began to experience valve wear problems when switched to unleaded fuel. If you're driving an antique or classic car, therefore, you should either use some type of lead substitute fuel additive to protect the valves or have the seats replaced with hard seats when the engine is overhauled.