BURNT VALVE on 2003 Mercury Sable

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what is a burnt valve & what causes it? a mechanic told me that it's too soon for a 2003 to have a burnt valve. what is the average or estimated cost to fix a burnt valve?
(1) Answer
A "burnt valve" is a valve which has been heat damaged and is not sealing correctly causing a misfire in the affected cylinder. It is much more common for an exhaust valve to burn than an intake valve because cooler incoming gas help cool the intake valve. A lean running engine or out of adjustment valve clearance can cause a valve to "burn" prematurely. There are various other issues which can cause a valve to not seat correctly resulting in a misfire. A cylinder leakage test is the best way to confirm compression loss due to leaking valves. The repair can be quite expensive. You can begin with the RepairPal estimate - - for head gasket replacement and add to that a few hundred dollars for machine work and any necessary parts to repair the cylinder heads.