Chapter 11: Failure Analysis; Abrasive Ingestion - MTD P90 series Shop Manual

A properly maintained engine will provide years of service. Occasionally an engine will fail. An important part of
working on engines is being able to recognize the root cause of engine failures. Was it something the customer did?
Was it a manufacturing defect? Did the engine just wear out? All of these questions need to be answered. Identifying
and eliminating the cause of the failure is the only way to prevent recurring failures.
Engines can fail in a variety of ways but most failures can be classified in the following categories:
•

Abrasive ingestion

• Insufficient lubrication
• Over heating
• Over speed
• Mechanical breakage/ wear
NOTE: There may be a combination of failures.
Each cause of an engine failure leaves its own fingerprint on the engine. Careful gathering of information about
the engine will identify one or more of these fingerprints. Start by getting as much information as possible from the
customer, such as: symptoms, how it is used and the maintenance history.
Abrasive Ingestion
Abrasive Ingestion is when hard particles are introduced into the engine. Particles can be introduced into the
engine by leaks in the air intake system, through a dirty oil fill plug or by particles of metal that wore off of a part,
especially during the break in cycle. Particles may also be introduced through worn or improperly installed seals or
gaskets.
Figure 11.1

CHAPTER 11: FAILURE ANALYSIS

1. Abrasive particles that enter the engine through the
intake system can be sand, hay or dirt.
See Figure 11.1.
2. Abrasive particles that enter the engine usually leave
tracking marks where the particles entered the sys-
tem. Use these marks to find the source of the abra-
sives.
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 vulnerable to
this wear. If an air filter becomes clogged, the vac-
uum 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 system to
the valves and valve seats.
Failure Analysis
105