Service Manual

Coolant Thermostat ISB, ISBe and QSB (Common Rail [...]
Page 8-10 Section 8 - Cooling System - Group 08
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CAUTION
Never operate the engine without a thermostat.
Without a thermostat, the path of least resistance for
the coolant is through the bypass to the water pump
inlet. This can cause the engine to overheat.
An incorrect or malfunctioning thermostat can cause the
engine to run too hot or too cold.
CAUTION
A missing check ball can cause the engine to run
cold, resulting in engine damage.
The thermostat contains two check balls to vent air past
the thermostat when it is closed. This is needed for the
cooling system to fill.
NOTE: Some off-highway applications use a thermostat
with one check ball. When servicing a thermostat always
be sure to replace it with the same part number. Though
an incorrect thermostat will physically fit, it will lead to
improper engine operation.
Leak Test
NOTE: The leak test portion of this procedure does not
apply to QSB5.9 keel cooled marine engines.Refer to
Procedure 008-014 for testing QSB5.9 keel cooled
engines.
CAUTION
The engine thermostat must operate properly for the
engine to operate in the most efficient heat range.
Overheating or overcooling will shorten engine life.