Engine Maintenance
85
Crankcase Breather System
The crankcase breather system ducts crankcase
gases formed in the crankcase directly to the air
intake. Harmful vapors that would otherwise
collect in the crankcase and contaminate the oil,
or escape to the outside, are drawn back into the
engine and burned.
The crankcase breather is located in the cylinder
head cover, which is a combination of valve cover
and intake manifold. A restrictor in the cylinder
head cover limits the flow of gases from the
crankcase to the intake and keeps the crankcase
pressure from getting too low.
Normal crankcase pressures with a new air
cleaner are shown below:
The vacuum will increase as the air cleaner gets
dirty and becomes more restrictive. Remove the
breather cover and the baffle plate and check to
make sure nothing is plugged or damaged.
See the TK270, TK370, TK374F, TK376, and
TK380F Engine Overhaul Manual (TK 53163) for
the procedure to check the crankcase pressure. If
your readings are significantly more positive than
0 to 11 in. (0 to 279 mm) H
2
O of vacuum, you
may have excess blowby past the rings. A
compression check should be performed to
confirm this.
The following items can effect the crankcase
pressure readings.
1. Breather Cover
2. O-ring
3. Baffle
4. Baffle Plate
Figure 160: Crankcase Breather Components
Unit/Speed
in. (mm) H
2
O of vacuum
Truck/Low 0 to 8 in. (0 to 203 mm)
Truck/High 2 to 11 in. (51 to 279 mm)
TriPac 0 to 8 in. (0 to 203 mm)
Crankcase
Pressure
Effect
Typical Cause
Increase Piston Rings Stuck or Worn
Increase Restrictor Plugged
Decrease Air Cleaner Dirty or Plugged
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