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.
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.
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. Many technicians will perform on-car cleaning in an attempt to clean the injectors, without really knowing the condition of the injectors. Although this can improve some conditions, it is rarely effective on injectors that are restricted to the point that they are causing a drivability symptom. On-car cleaning is not as effective as the marketing hype would lead you to believe. Refer to our article on on-car versus off-car cleaning techniques for a more in-depth discussion.
There are a few methods to determine if the injectors are restricted, none of which provide a complete picture of injector health. For instance, without removing the injector from the vehicle it is impossible to examine the spray pattern. The best method is to remove the injectors and have them Reconditioned and Returned (R&R). GB's R&R service consists of 14 steps and ensures you'll have a balanced and fully tested set of fuel injectors to reinstall in the vehicle, eliminating any doubt. This service includes ultrasonic cleaning, flow testing, leak testing, spray pattern testing and will include new seals and o-rings.
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.
Diagnosing Internal O-Ring Failure
Although it may seem like a simple problem to diagnose it can be a bit tricky. The customer may report this as an intermittent symptom. This is because the fuel may only leak under certain temperatures or may only be present under certain driving conditions. For instance, a small seepage may evaporate if there is adequate under hood ventilation, such as with higher speed operation on a freeway.
The best course of action to pinpoint this problem is to clean the exterior of each injector thoroughly, then let the engine idle for a period of time, rechecking for fuel odor and seepage periodically. Driving a vehicle under normal conditions puts the engine “under load”, which in turn increases fuel pressure. Increased fuel pressure often reveals the source of the fuel seepage. On the other hand, under hood air currents increase when the vehicle is moving, which aids in the evaporation process. The bottom line here is patience.
Injectors found to be leaking externally need to be replaced as there is not a suitable field fix for this type of 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. These injectors are also referred to as “peak-and-hold” injectors. This is because low impedance injectors are fired differently than high impedance injectors. On vehicles that use peak-and-hold injectors the electronic control unit pulses the injector on by applying full current to the injector. This creates a strong magnetic field which lifts the pintle off its seat. Once opened, the ECU reduces current flow to hold the injector open for the duration of its pulse time.
High impedance injectors usually have a resistance value of 11-15 ohms depending on the injector. These injectors are also referred to as “saturated” injectors because the ECU turns the injector completely on (fully saturated) during the entire duration of the pulse, without reducing the current flow.
A typical injector coil contains about 35 feet of wire that is approximately .010” in diameter. The wire is a special alloy which is insulated with a very thin coating of enamel. Some injectors, particularly Nissan, are prone to have the enamel break down over a period of time. This causes the injector to short-circuit resulting in lower than normal resistance. The break down in enamel is a design flaw in particular injectors and is caused by the constant thermal cycling of the injector. A resistance (Ohms) check of the injector can identify a shorted coil. This test should be done when the injector is cold as well as hot (if the vehicle will run). It's possible that the injector will only exhibit this change in resistance at a specific temperature and checking at operating temperature will verify this. If you check the injector's resistance when the engine is cold, don't assume it will be the same when the engine heats up, especially if you're diagnosing an intermittent symptom. You'll need to compare the injector's resistance to the OEM specification which is usually available in vehicle specific diagnostic manuals. On a multi-port fuel injection system, you can compare the injector's resistance values to one another. Be careful though; don't rule out that several may be off spec, especially on high failure rate injectors.
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