FAILURE ANALYSIS
79
3. Particles that enter the intake system travel at
great speed and act like sand blasting media
inside the engine. This causes wear to the parts
affected.
NOTE: Choke and throttle shafts are very vul-
nerable to this wear. If an air filter becomes
clogged, the vacuum produced by the engine will
try to draw air in by any means possible. This
usually happens around the throttle and choke
shafts. Because the throttle shaft moves more
than the choke, it will wear faster.
4. The particles can pass through the intake sys-
tem to the valves and valve seats.
5. When particles enter the combustion chamber,
the up and down motion of the piston grinds the
particles into the side of the cylinder walls and
damages the cylinder wall, piston and piston
rings
6. This can be identified by the scoring along the
vertical axis of the piston and cylinder wall or the
cross hatch on the cylinder wall being worn off.
NOTE: To help in the lubrication of the cylinder
walls, and help with the seating of the piston
rings, a diamond cross hatch is honed into the
cylinder wall. Debris entering the cylinder will
polish the cross hatch off of the cylinder wall.
See Figure 11.3.
NOTE: Abrasives that enter the engine through
the intake system will cause the upper portion of
the combustion chamber to wear more than the
lower portion. Measurements of the cylinder
bore at the top and bottom will show this.
Other sources of abrasives that get into the engine
includes carbon that builds up on the top side of the
piston, metal shavings from the wear of engine parts or
dirt entering through the oil fill port. leaking gaskets and
seals also have the potential of allowing debris to enter
the engine.
A symptom of abrasive ingestion is smoky exhaust. As
the cylinder walls wear; pressure from the combustion
chamber blows by the piston and pressurizes the
engine sump. This overpowers the PCV valve and
allows oil to build up in the combustion chamber.
See Figure 11.4.
7. Abrasive materials that enter the engine get
absorbed by the oil and thickens it.
See Figure
11.5.
Figure 11.3
Cross hatch polished off
Figure 11.4
Figure 11.5
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