the tester until the pressure gauge reads radiator cap pressure.

CAUTION 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 tester.

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 new cap.

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 cylinder head.

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.

Thermostat Test

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 180F to 185F, full-open temperature is 200F to 202F. If the test is satisfactory, the thermostat can be reinstalled.

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.