MTD Engine - Series 350/450/650
31
11. CRANKCASE VENTILATION
11.1. 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 11.1.
11.2. 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 extra pressure from the crankcase.
See Figure 11.2.
• Normally, small engines run with slightly nega-
tive case pressure.
Figure 11.1
Figure 11.2
• 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.
11.3. An adaptor can easily be made form an old or
extra dipstick. See Figure 11.3.
11.4. 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.
11.5. Experimentation by MTD’s Training and Educa-
tion department has revealed the following char-
acteristics:
• 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.
Figure 11.3
Electronic manometer:
Adaptor made by drilling a
hole in a dipstick /
oil cap
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