Chevrolet Fleetmaster Series 1947 Owner's Manual page 49

Frequent long runs at high speed, with the resultant high
engine operating temperatures, may oxidize the oil and may result
in the formation of sludge and varnish. While no de6nite drain
periods can be recommended under these conditions, they should
be more frequent than under normal driving conditions.
Driving over dusty roads or through dust storms introduces
abrasive material into the engine. Carburetor Air Cleaners
decrease the amount of
du~t
that enters the crankcase. The
fre~
quency of draining depends upon severity of dust conditions and
no de6nite draining periods can be recommended, but should be
more frequent than under normal driving conditions.
Short runs in cold weather, such as city driving and excessive
idling, do not permit thorough warming up of the engine and
water may accumulate in the crankcase from condensation of
moisture produced by the burning of the fuel. Water in the crank-
case may freeze and interfere with proper oil circulation. It also
produces and promotes rusting and may cause clogging of oil
screens and passages. Under normal driving conditions this water
is removed by the crankcase ventilator. But if water accumulates
it should be removed by draining the crankcase as frequently as
may be required.
It is always advisable to let the engine reach normal operat-
ing temperature before draining the crankcase. The benefit of
draining is, to a large extent, lost if the crankcase is drained when
the engine is cold as some of the suspended foreign material will
cling to the sides of the oil pan and will not drain out readily
with the slow moving oil.
Crankcase Dilution-Probably
the
most serious phase ';f engine oil deterioration is that of crankcase
dilution, which is the thinning of the oil by fuel vapors leaking
by the pistons and rings and mixing with the oil.
Le.akage of fuel, or fuel vapors, into the oil pan mostly occurs
during the warming-up period, when the fuel is not thoroughly
vaporized and burned.
Automatic Control Devices to Minimize Crankcase Dilution
Your Chevrolet engine is equipped with automatic devices
which aid greatly in minimizing the danger of crankcase dilution.
Rapid warming up of the engine is aided by the thermostatic
water temperature control, which automatically prevents cir-
jorty-.rel'ell