Chapter 2: Basic Troubleshooting
19
PCV testing
The PCV valve is located under the flywheel 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 covered duct within
the air filter housing. See Figure 2.3.
Figure 2.3
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.
See Figure 2.4.
• Normally, small engines run with slightly nega-
tive case pressure.
• This case pressure can be measured using a
slack-tube water manometer, or an electronic
version of the same tool.
• Less than (between zero and -1”) ( -2.54cm) of
water is a typical reading.
NOTE: An adaptor can easily be made form an
old or extra dipstick. See Figure 2.5.
Figure 2.4
Figure 2.5
Electronic manometer:
Adaptor made by drilling a
hole in a dipstick /
oil cap
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