Question: My mechanic says me engine has a "burned
valve." What does that mean?
Answer: 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.
WHY VALVES BURN
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.
VALVE RECESSION
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.
