Chapter 11: Failure Analysis
106
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 proper-
ties, 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 crank-
case or cylinder head indicates that it has been over
heated. See Figure 11.11.
Another sign of an overheat failure is warped parts. As
metal parts heat up, they expand. In an engine a cer-
tain 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 cylin-
der 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.
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 11.12.
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 maybe caused by
lack of air flow or exhaust system issues. Typical dam-
age 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 indicators
of an over heated engine.
Figure 11.11
Discolored rockers
Fi
ure 11.12
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