Cub Cadet MTD P90 Series - Manual (Page 117)

Failure Analysis
111
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 crankcase or cyl-
inder 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 expan-
sion.
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 compression in the
engine and hurting engine performance. As the cylinder
head cools, it shrinks back down to its normal 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
• Exhaust system issues.
• Recirculation of cooling air caused by a modifica-
tion that restricts air flow.
• Debris build up on the cooling fins
Typical damage from this kind of over heating is a
dropped valve seat. A dropped exhaust valve seat com-
bined with coked oil in the cylinder head would be sure
indicators of an over heated engine.
Figure 11.9
Discolored rockers
Figure 11.10