What Are Fuel Injectors?
Fuel injectors are electronically controlled valves that spray a precise, atomised mist of petrol or diesel into the engine's intake ports or directly into the cylinders. They replaced carburettors in modern vehicles because they provide far more precise fuel metering, leading to better fuel economy, lower emissions, and more responsive performance.
A typical NZ passenger car has one injector per cylinder — four injectors on a four-cylinder Mazda Demio or Suzuki Swift, six on a Subaru Impreza six-cylinder, and so on. Diesel vehicles (like the Toyota Hilux) use common-rail direct injection, where injectors operate at very high pressure (up to 2,000 bar) and can inject fuel multiple times per combustion stroke.
How Do Fuel Injectors Work?
The ECU calculates exactly how much fuel the engine needs based on inputs from the MAF sensor, oxygen sensor, throttle position, engine RPM, and coolant temperature. It sends a precise electrical pulse to each injector's solenoid, opening it for a calculated number of milliseconds to deliver exactly the right amount of fuel.
The fuel pump maintains constant pressure in the fuel rail. The length of time the injector is open (pulse width) determines how much fuel is delivered.
How Do Injectors Fail?
Clogged Injectors
The most common problem. Petrol contains wax and carbon deposits that accumulate on injector tips over time, especially in stop-start urban driving. A clogged injector sprays poorly — a dribble or distorted spray pattern instead of a fine mist — causing incomplete combustion.
Leaking Injectors
A leaking injector allows fuel to drip into the cylinder when it should be closed, causing a rich condition, poor fuel economy, and potentially washing oil off cylinder walls (damaging the engine).
Electrical Failure
The solenoid winding or connector can fail, causing the injector to stop opening at all. The cylinder it serves stops firing, causing a rough-running "misfire" condition.
Signs of Injector Problems
| Symptom | Likely Cause |
|---|---|
| Rough idle or misfire | Clogged or failed injector |
| Poor fuel economy | Leaking or clogged injectors |
| Engine hesitation or stumbling | Inconsistent fuel delivery |
| Check Engine light (misfire codes) | ECU detected abnormal combustion |
| Fuel smell inside or outside car | Leaking injector seal |
| Hard starting when cold | Insufficient fuel at start-up |
A workshop can perform an injector balance test or use a scan tool to identify which cylinder is misfiring and whether the injector is the cause.
WoF and Injector Issues
While injectors won't cause a direct WoF failure, an engine with misfiring injectors may produce excessive exhaust emissions that fail an emissions check. Some WoF inspectors will also note an obvious fuel smell. More importantly, ignoring misfires long enough will damage catalytic converters — an expensive secondary consequence.
NZ Repair Cost Estimates
| Service | Typical NZ Cost |
|---|---|
| Injector cleaning (ultrasonic, off-car) | $150–$300 (set of 4) |
| Fuel injector service additive (DIY) | $20–$40 |
| Single injector replacement (petrol) | $200–$450 fitted |
| Full set replacement (4-cylinder) | $600–$1,400 |
| Common-rail diesel injector (per injector) | $400–$1,200 fitted |
Ultrasonic cleaning by a specialist workshop is often the best first step for clogged injectors — replacement is only necessary when cleaning fails.
When to Book a Mechanic
- Check Engine light is on with misfire codes (P0300–P0304 range)
- You notice rough idling or stuttering under acceleration
- Fuel economy has dropped noticeably without explanation
- You can smell fuel from the engine bay
Have the diagnostic scan done first — a proper scan will point directly at the faulty cylinder and help distinguish an injector problem from a spark plug or ignition coil issue.