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The most common types of fuel injector failure

When fuel injectors fail, many are left wondering what happened. The truth is, fuel injector failure can be caused by a number of things that can be difficult to diagnose. However, arming yourself with an understanding of how and why fuel injectors can fail, appropriate diagnostic procedures, and important technical tips can go a long way in preventing future injector failures.

The fuel injector is an electro-mechanical device, so both electrical and mechanical failure modes are possible. We'll take a look at each type.

Mechanical Failure

As we discussed in our Fuel Injection 101 article, the fuel injector can be pulsed up to 80 times a second. Each time the injector opens, fuel flows out of the injector and into the intake stream where it mixes with air and then enters the cylinder. An injector must precisely meter the fuel during every injection event and will fire billions of times in its life. Because of this, injectors are manufactured to extremely precise tolerances and are susceptible to certain types of mechanical failures including, contamination and internal o-ring failure.

Contamination and Restriction

All of the fuel the engine burns ultimately passes through the injectors. As a result, the injector is susceptible to contamination and carbon build up which eventually restricts the flow of fuel. There are many factors that contribute to this, some of which include fuel quality, driving conditions and injector design.

Restricted Injectors

Two issues occur with restricted injectors. First, the fuel flow through the injector is reduced resulting in less fuel flow for the same injection time. Usually the vehicles on-board computer will see this via the oxygen sensor and increase the injection on-time to compensate. However, the computer cannot control fuel flow to individual injectors; so unless each injector is restricted the same amount, imbalances in cylinder to cylinder fuel flow will exist. This causes high emissions, poor fuel economy and poor performance.

The second issue is that when an injector becomes restricted, it adversely affects the spray pattern. This results in poor atomization of the fuel. On a multi-port system each cylinder will have different atomization properties resulting in high emissions, poor fuel economy and poor performance.

Diagnosing Restricted Injectors

Diagnosing restricted injectors can be tricky. On newer vehicles that can detect individual cylinder misfire, a diagnostic code may be present. Symptoms for restricted injectors include poor idle quality, hesitation or stumble on acceleration, failed emissions test, poor cold engine performance and poor fuel economy. You'll need to eliminate other possible causes before you suspect a faulty injector. This includes ignition related issues; spark plugs, secondary wires, ignition coils, as well as mechanical failures; low compression, leaking intake and exhaust valves, etc.

Internal O-Ring Failure

Most injectors contain internal o-rings which keep the fuel from leaking out of the injector where the mechanical parts of the injector are assembled. If these o-rings fail, the injector will leak fuel externally, usually around where the plastic connector is molded to the metal injector body. Often this is just a small amount of fuel seepage causing no drivability symptom. You may be investigating this because the customer may be complaining of a fuel odor on initial start-up or after the vehicle sits for a while.

Electrical Failure

The injector contains a coil of wire wrapped around a bobbin. When an electrical current is passed through the coil it creates a magnetic field which causes the pintle to lift off of its seat, allowing fuel to spray. When the current is shut off the magnetic field collapses and a spring pushes the pintle onto its seat shutting of the fuel flow.

Resistance values for injectors vary between manufacturers but usually are either low impedance or high impedance. Low impedance injectors usually have a resistance value of 1.8-3.0 ohms depending on the injector. High impedance injectors usually have a resistance value of 11-15 ohms depending on the injector.