Head Gasket


Sometimes a leaking head gasket or a cracked cylinder allows a concentrated mixture of ethylene glycol coolant to leak into the crankcase.

An internal leak can result in coolant entering the crankcase.

The result is varnish-like oil that can plug oil rings and ruin valve guide seals. This sticky substance can actually seize the crankshaft (called thermoplastic seizure). The problem will happen again if the source of the leak is not found and repaired. Then, the engine and cooling system must be flushed. The condition will be very evident when a valve cover is removed.

A blown head gasket caused the oil to take on this appearance.

When a leak occurs between an oil and water passageway, pressurized oil (approximately 30 psi) will force its way into the cooling sytem (approximately 15 psi). The engine will overheat and pour a messy oil and water mixture from the radiator overflow.

A leaking head gasket or a crack in a cylinder head or bore can result in an internal leak.

When there is an internal leak, coolant will flow into the cylinder when the engine is off and during the intake stroke. During combustion, exhaust gas migrates into the cooling system.

When there is an internal coolant leak, the coolant level often drops. Leakage from the outside of the engine is not evident. When there is an internal leak, coolant will flow into the cylinder during the intake stroke and when the engine is off. During combustion, exhaust gas is forced into the cooling system and can appear as bubbles in the radiator.

Exhaust gas leaking into the cooling system can result in bubbles in the radiator.

There are several tests that can be done to confirm an internal leak.

  • Look for bubbles in the radiator when the engine is warm and under a load. Rapidly accelerating the engine is usually enough of a load to produce the bubbles. Internal leakage can be spotted by installing a pressure tester on the radiator filler neck of a warmed-up engine.
A pressure tester installed on a radiator filler neck.
  • A leaking head gasket will not always show up on a pressure test. ablock check tester or an infrared exhaust analyzer can also be used to check to see if there is exhaust gas in the coolant.

Cracks tend to leak more when the engine is cold. After warm-up, the crack closes. The radiator cap will blow off when the pressure from the combustion leak exceeds radiator cap pressure. To see if combustion pressure is indicated, put the radiator overflow hose into a container of water while the engine runs.

Air leaking into the system results in air bubbles coming out of the overflow tube.

If bubbles are evident, combustion pressure is getting in. Bubbles could also be present because the cooling system is drawing in air. To eliminate this possibility shut off the engine, loosen the drive belt to the coolant pump, and repeat the test. If the bubbles disappear, air was getting into the pump. When there is air in the system, corrosion occurs at about three times the normal rate. Air can leak into the cooling system through a leak in the lower radiator hose. The lower hose is the suction hose, where coolant is drawn into the pump. Air can leak in even though water may not leak out. To test a cooling system for air leakage, tape the filler neck of the radiator closed. Put a hose from the radiator overflow pipe into a jar of water. With the engine running, look for bubbles in the jar.

Sometimes an internal leak can result in one or more cylinders filling up with coolant after the engine is shut off. This happens because the radiator cap continues to exert pressure on the coolant, even though the engine is off. If the engine stops with a piston down in the cylinder while both of its valves are closed, the engine will be hydrolocked and the crankshaft will not be able to turn. If the spark plugs are removed, the engine will be able to crank. Water will pour out of the offending plug hole.

After disassembling the cylinder, inspect the head gasket. Look for evidence of coolant or oil leakage. If the gasket was sealing properly, there will be a well-defined line of thin carbon around the combustion chamber on both the head and the block. Carbon deposits on the metal rings of the gasket or a poorly defined combustion seal indicate possible compression leakage.

Coolant leaking from a bad head gasket or cracked cylinder head can also contaminate an oxygen sensor. Coolant leaves a white flaky deposit that sometimes has the sweet smell of ethylene glycol.