Cooling System Diagnostics (008-020)
B3.9 and B5.9 Series Engines
Section 8 - Cooling System - Group 08
The water pump pulls coolant from the bottom of the ra-
diator and pumps it through the engine back to the top of
the radiator for cooling. Reduced or interrupted flow will
result in the engine running hot.
The pump is belt-driven from the crankshaft pulley. An
automatic belt tensioner is used to prevent the belt from
slipping on the pump pulley. A malfunction of the tensioner
will cause the water pump impeller to rotate at a slower
speed, reducing the amount of coolant flow.
The coolant flow can also be reduced if the inlet hose to the
water pump collapses. A hose will usually not collapse
while the engine is running at low speed. Check the hose
while the engine is running at rated speed.
NOTE: Be sure the engine is warm, a minimum of 95C
[203F], so the thermostat is open.
A worn or malfunctioning water pump will not produce the
flow required to prevent the engine from running hot. How-
ever, be sure to check the other possibilities indicated in the
Troubleshooting Logic before checking the flow or replac-
ing the pump. Refer to Procedure 008-062.