FAILURE ANALYSIS
85
Overheated
The MTD engines are air cooled engines. Because
of this, cleanliness of the engine is very important to
the life of the engine. Dirt, grass and sludge all form an
insulating layer on the engine. This will trap the heat in
the engine and cause it to over heat.
As metal parts heat up enough to change their
properties, they will take on a yellowish or blue cast.
As oil is heated to the point that it evaporates,
black deposits are left behind. This is called “coking”.
An engine with lots of coked oil deposits inside the
crankcase or cylinder head indicates that it has been
over heated.
See Figure 11.9.
Another sign of an overheat failure is warped parts.
As metal parts heat up, they expand. In an engine a
certain amount of expansion is expected. Engines are
built so that when parts are at operating temperature,
the parts will expand to be within the tolerances
needed for the engine to run. A problem occurs when
the parts are over heated. They expand more than they
were designed to. Some parts are mounted firmly, like
cylinder heads (the hottest part of the engine). As they
try to expand, they fight against the head bolts. The
head bolts will not move to allow the expansion so the
head warps to allow the expansion.
Figure 11.9
Discolored rockers
This warping of the head allows the head gasket to
leak. A leaking head gasket allows the compressed
gases in the engine to escape, lowering the compres
-
sion in the engine and hurting engine performance. As
the cylinder head cools, it shrinks back down to its nor
-
mal size, but there will still be some warpage of the
head.
See Figure 11.10.
Localized over heating will leave localized “hot
spot” indications, such as discoloration.
Rapid over heating of a cylinder, like when there is
a cooling air flow obstruction, may cause hot spots and
metal transfer between the piston skirt and the cylinder
wall.
Over heating of the cylinder head may be caused
by lack of air flow or exhaust system issues. Typical
damage from this kind of over heating is a dropped
valve seat. A dropped exhaust valve seat combined
with coked oil in the cylinder head would be sure indi
-
cators of an over heated engine.
Figure 11.10
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