the tester until the pressure gauge reads radiator cap
Do not pump to much pressure into the
cooling system or part damage may result.
With pressure in the system, inspect all parts for
coolant leakage. Check at all fittings, at gaskets, under
the water pump, around the radiator, and at engine
freeze (core) plugs. Once the leak is located, tighten,
repair, or replace parts as needed
A pressure test can also be applied to the radiator
cap. The radiator pressure test measures cap-opening
pressure and checks condition of the sealing washer.
The cap is installed on the cooling system pressure
Pump the tester to pressurize the cap. Watch the
pressure gauge. The cap should release pressure at its
rated pressure (pressure stamped on cap). It should also
hold that pressure for a least 1 minute. If not, install a
Combustion Leak Test
A combustion leak test is designed to check for the
presence of combustion gases in the engine coolant. It
should be performed when signs (overheating, bubbles
in the coolant, rise in coolant level upon starting) point
to a blown head gasket, cracked block, or cracked
A block tester, often called a combustion leak tester,
is placed in the radiator filler neck. The engine is started
and the test bulb is squeezed and then released. This
will pull air from the radiator through the test fluid.
The fluid in the block tester is normally blue. The
chemicals in the exhaust gases cause a reaction in the
test fluid, changing its color. A combustion leak will
turn the fluid yellow. If the fluid remains blue, there is
no combustion leak.
Combustion leakage into the cooling system is very
damaging. Exhaust gases mix with the coolant and
form corrosive acids. The acids can cause holes in the
radiator and corrode other components.
An exhaust gas analyzer will also detect
combustion pressure leakage into the coolant. By
placing the analyzer probe over the filler neck and
accelerating the engine, the probe will pick up any
hydrocarbons (HC) leaking from the system, which
indicates combustion leakage.
To check thermostat action, watch the coolant
through the radiator neck. When the engine is cold,
coolant should not flow through the radiator. When the
engine warms, the thermostat should open. Coolant
should begin to circulate through the radiator. If this
action does not occur, the thermostat may be defective.
There are several ways to test a thermostat. The
most common is to suspend the thermostat in a
container of water together with a high-temperature
thermometer (fig. 6-16). Then by heating the container
on a stove or hot plate, the temperature at which the
thermostat begins to open, as well as when full open,
can be determined. If the thermostat fails to respond at
specified temperatures, it should be discarded.
Specifications vary on different thermostats. For
example, a thermostat with an opening temperature of
180°F to 185°F, full-open temperature is 200°F to
202°F. If the test is satisfactory, the thermostat can be
A digital thermometer can also be used to check the
operating temperature of an engine and thermostat.
Simply touch the tester probe on the engine next to the
thermostat housing and note its reading. If the
thermostat does not open at the correct temperature, it is
defective and should be replaced.
Figure 6-16.Testing a thermostat.