As a mechanic, you must be able to locate and
correct cooling system problems quickly and
accurately. It is equally important that you know how to
service a cooling system.
Flushing the System
The original additives in antifreeze fight rust and
corrosion breakdown and are ineffective after 1 to 2
years. This is because of the continual exposure to the
heat in the cooling system. After the additives break
down, rust begins to form rapidly. Therefore, a rust-
colored antifreeze is an indication that the cooling-
system service is required.
The cooling system should be cleaned periodically
to remove rust, scale, grease, oil, and any acids formed
by exhaust-gas leakage into the coolant.
Recommendations vary; for example, Chevrolet
recommends that the cooling system be drained and
flushed every 2 years.
Flushing (cleaning) of a cooling system should be
done based on the manufacturers recommendations or
when rust and other contaminants are found in the
system. Flushing involves running water or a cleaning
chemical through the cooling system to wash out
Rust is very harmful to the cooling
system because it causes premature water pump wear
and can collect and clog the radiator or heater core
tubes. There are three methods of flushing-fast
flushing, reverse flushing, and chemical flushing.
Fast flushing is a common method of cleaning a
cooling system because the thermostat does not have to
be removed from the engine. A water hose is connected
to a heated hose fitting. The radiator cap is removed and
the petcock is opened. When the water hose is ON and
water flows through the system, loose rust and scale are
Reverse flushing of a radiator requires a special
flushing gun device that is connected to the radiator
outlet tank by a piece of hose (fig. 6-14). Another hose
is attached to the inlet tank, so the water and debris can
be directed to the floor drains. Compressed air, under
low pressure, is used to force water through the radiator
core backwards. The air pressure is used intermittently
to loosen scale and sediment. Excessive air pressure
should be avoided to prevent damage to the radiator.
Starting and stopping the water flow produces a
fluctuation in pressure and tends to loosen all foreign
matter clinging to the passages in the radiator core.
Figure 6-14.Reverse flushing of a radiator.
Reverse flushing can also be used on the engine
block and head (fig. 6-15). First, remove the thermostat
and disconnect the upper radiator hose.
disconnect the lower radiator hose at the water pump.
Insert the flushing equipment in the upper radiator hose.
Reverse flush the system by sending water and air
through the water jackets and coolant passages.
Following the flushing, replace the thermostat and
hoses so the system can be refilled.
When reverse flushing equipment is not available,
you can still reverse flush the system with a garden
hose. This is often effective following the use of a
Figure 6-15.Reverse flushing of water jackets.