A failing injector can disrupt the quantity of fuel injected, the quality of the spray pattern or the injection timing. The consequences can range from a rough idle to the engine refusing to start at all.
The symptoms below can point the diagnosis in the right direction, but they do not always allow you to blame an injector directly. A faulty EGR valve, turbo, sensor or pump, or a compression problem, can produce similar signs.
1. The engine knocks or runs unevenly
An injector that no longer meters fuel correctly can cause a knocking noise that is more pronounced than usual. The engine may also shake or seem to be running on fewer cylinders.
- on a cold start;
- at idle;
- during the first few accelerations;
- when the engine is under heavy load.
2. The vehicle is hard to start
Long, uneven or impossible starting can be caused by excessive internal leakage at an injector. On a Common Rail system, too much fuel can be sent back into the return circuit.
- the engine cranks for a long time before starting;
- it starts and then stalls;
- it starts with difficulty when hot or cold;
- it no longer starts once several injectors are out of tolerance.
3. You notice a loss of power
Poor atomisation or insufficient delivery can reduce combustion efficiency. The vehicle accelerates less readily and may struggle to pick up revs.
Loss of power can also come from the air flow meter, the EGR valve, the fuel filter, the turbo, the particulate filter or a leak in the intake system.
4. Abnormal smoke comes from the exhaust
When an injector delivers too much fuel or atomises poorly, combustion can become incomplete. Black, white or greyish smoke may then appear depending on the nature of the fault.
The colour of the smoke is not enough on its own to identify the faulty part.
5. Fuel consumption increases
When actual delivery no longer matches what the ECU is commanding, the ECU may try to compensate for the imbalance. The engine can then use more fuel.
A smell of diesel, damp traces around the injector or a visible leak should be checked promptly.
Other possible signs
- engine warning light on;
- misfires;
- unstable idle;
- smell of fuel;
- frequent stalling;
- unusual vibration;
- limp-home mode.
How can you confirm that an injector really is faulty?
- reading the fault codes;
- analysing the correction values;
- testing the return flow;
- checking the fuel pressure;
- electrical testing of the injectors and the wiring loom;
- bench testing where doubt remains;
- checking engine compression.
Can you keep driving?
Continuing to drive can make the fault worse, increase consumption and disturb other components. Prompt action matters in particular when the engine knocks loudly, smokes heavily, loses power abruptly or shows a visible leak.
FAQ: symptoms of a failing injector
Does a failing injector always trigger the engine warning light?
No. A slight drift or a mechanical fault can exist without immediately generating a fault code.
Can an injector cleaner solve the problem?
An additive cannot repair a faulty solenoid, significant internal leakage, mechanical wear or incorrect calibration.
Do all four injectors need changing?
Not as a matter of course. The whole set should be checked in order to measure the condition of each injector.
