BASIC TROUBLESHOOTING
18
3. Interpreting compression readings. PCV testing
The PCV (Positive Crankcase Ventilation) 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 heat box
through a molded rubber hose. The rubber hose
directs crankcase fumes to the heat box assem
-
bly. See Figure 2.3.
2. When functioning properly, the PCV valve works
with the inherent 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.
Table 2:
Readings in
psi
Possible causes
<20 (1.4
Bar)
Most likely a stuck valve or
too tight of a valve lash,
provided the starter rope
pulls with normal effort.
20-40
(1.4-2.8 Bar)
Valve seat damage or pis-
ton ring and/or cylinder
wear.
40-75
(2.8-5.2 Bar)
Normal readings
>75 (>5.2
Bar)
Excessive valve lash, a
partial hydraulic lock, a bad
cam or a bad automatic
compression relief.
Figure 2.3
crankcase
air chamber
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