Injector back leakage and blow-by

Back leakage and blow-by

Is your diesel engine difficult to start?

Your injectors maybe at the root of the problem.

siemens common rail diesel injector

Injectors are at the centre of a diesel engine. Their settings determine the amount of fuel injected into the engine.


What are the kind of problems that you get with modern common rail injectors?

In the modern common rail injector problems usually originate in 3 ways

  1. Back leakage occurs due to wear in the injector itself and in the internal seals -fuel escapes back to the fuel tank starving the engine of fuel

  2. wear, contamination and pin-sticking in the injector nozzle

  3. blow-by which occurs when the injector does not seal properly on the injector seat in the cylinder head.


Symptoms of faulty injectors include:

poor starting,

black smoke (unburnt fuel)

lumpy irregular running

Worn burnt nozzles – nozzle is burnt and deformed so spray pattern is deformed

Sticking nozzles – fuel contamination and wear causing nozzle pin to jam in the nozzle body


Back leakage – loss of pressure – what is it?

Common rail diesel injectors operate at very high pressures. Excessive wear on internal surfaces and seals will allow fuel under high pressure to take the easiest way out. When this happens we call it back leakage. It means that fuel in the injector does not spray out of the nozzle but goes back out the return channel back to the fuel tank. Consequently the engine is temporarily or permanently starved of fuel.

Back leakage – Loss of pressure – what are the symptoms?

Back leakage and loss of pressure in the injector can give the following symptoms:-

Hard or difficult starting/ difficult or impossible to start

Erratic or uneven tick over or idle / engine idles on tick over erratically or unevenly

Lumpy running

Hesitation on acceleration

Smoke on tick over or acceleration /black smoke when ticking over or when accelerating

Lack of power or even a total shut down of the system / engine lacks power / engine cuts out entirely

Check the back leakage in your injectors. Click here for more details

Back leakage and the high pressure diesel pump.

Common rail diesels operate by delivering very high pressure fuel. This highly pressurized fuel is provided by a high pressure pump. At low engine speed the pressure delivered by the fuel pump will be lower than the pressure delivered when revs are high. Excessive back leakage in the injectors means that the high pressure pump cannot provide enough pressure. This is particularly pronounced at starting or tick-over because of the low engine and low pump speed.


Blow-by can result when the injector does not seal properly.

Common rail injectors need to seal properly in the cylinder head. The injector should seal against the seat in the cylinder head. When the injector does not seal properly injector blow-by is likely to result. Gases are escaping from the cylinder and producing a chuffing sound. Also black tar can be seen forming around the injectors. Sometimes as a result the ECU can compensate by ordering more fuel. This can result in diesel wash which removes the oil lubricating the piston; serious damage can occur. If the injector seat in the cylinder head has been eroded or excessively carbonised the seat may need to be recut to restore the proper sealing of the injector.