BASIC TROUBLESHOOTING
18
PCV testing
The PCV valve is located in the valve cover and allows
the crankcase pressure to escape.
Leakage and blockage are the two failure modes for a
PCV system. Either mode will cause crankcase pres-
sure to build-up, though the effects of a blocked PCV
are generally more dramatic. Increased case pressure
will result in oil entering the combustion chamber.
1. The PCV chamber is vented to the air filter
through a molded rubber hose. The rubber hose
directs crankcase fumes to a chamber within the
air filter housing.
See Figure 2.3.
NOTE: On snow blower engines the breather
hose connects to a chamber inside the lower
half of the heat box assembly.
See Figure 2.4.
Figure 2.3
crankcase air chamber
Figure 2.4
Bottom of heat box
crankcase air chamber
2. When functioning properly, the PCV valve (Posi-
tive Crankcase Ventilation) works with the inher-
ent pumping action of the piston in the bore to
expel pressure from the crankcase.
NOTE: Normally, small engines run with slightly
negative case pressure. This case pressure can
be measured using a slack-tube water manome
-
ter, or an electronic version of the same tool.
Less than (between -3 and -4”) ( -7.6 - 10.2cm)
of water is a typical reading at idle.
3. An engine that fails to purge extra case pressure
in a controlled manner will build case pressure.
The pressure will find it’s own way out of the
engine in undesirable ways.
• Oil will be forced by the rings and valve guides,
being burnt in the combustion chamber.
• The cause of this oil burning can be mistaken for
a worn-out engine, if proper diagnosis (compres-
sion, leak-down, and case pressure) is not per-
formed.
4. Experimentation by MTD’s Training and Educa-
tion Department has revealed the following char-
acteristics of MTD engines:
• A leaky PCV system will not build-up substantial
case pressure.
• A leaky PCV system will allow the engine to
ingest contaminants through the system, accel-
erating engine wear.
• A blocked PCV system will allow crankcase
pressure to build very rapidly. Noticeable oil
fumes will be evident in the exhaust within sev-
eral minutes of normal operation.
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