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.