What Causes Leaking Injectors?

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What Causes Leaking Injectors?
Injectors connect to pressurised fuel rails for a constant supply of fuel. (Thinkstock/Comstock/Getty Images)

Fuel pumps create pressure in fuel lines. The pressurised fuel is forced into the head of the fuel injector held in place by O-rings. A solenoid in the injector charges and triggers a brief opening of the fuel injector valve. The fuel passes through the injector valve and is pushed through nozzles into a fine mist that is ignited during combustion. Leaking injectors create fire hazards, degrade performance and potentially damage engines.

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O-Ring Damage

Fuel injector replacement and removal may damage O-rings. Installation of the injector into the fuel rail requires pushing the O-ring through a constricted space to properly seat the injector head in the rail. Improper seating and lack of lubrication on the O-ring causes damage to the O-ring material. Small cuts and cracks leak fuel under pressure. Always replace O-rings during fuel injector maintenance. Lubricate O-rings with a coat of new motor oil prior to installation.

Heat Damage

Overheated engines may cause O-rings to deform. Superheated rings expand beyond specification, then contract leaving gaps through which fuel passes. Replace O-rings on fuel injectors showing fuel leaks at the head of the injector.


Over time, fuel injector O-rings lose their capacity to expand and contract with the heating and cooling of the engine. O-ring material gradually hardens, reducing the capacity to resist pressurised fuel. Leaks developing at the connection to the fuel rail indicate O-ring failure.

Injector Valve Failure

Fuel injectors failing to close the internal valve will bathe the combustion chamber with a steady stream of fuel. Investigate engine codes indicating rich air-to-fuel ratios in specific combustion chambers. Rich ratios may indicate failed injectors. Fuel-bathed cylinders have reduced lubrication, causing premature wear and engine failure. Rich combustion may damage catalytic converters when non-combusted fuel burns in contact with the high-temperature catalyst.

Fuel Pressure

Fuel pressure beyond specification causes leaks at the O-ring connection of fuel injectors. Test the fuel pressure to determine if the leak is a result of too much pressure or general failure of seals.

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