Fuel Injector Operation and Balance Testing
Port fuel injectors are solenoid-operated valves that receive a precisely timed electrical pulse from the ECU. The pulse width (measured in milliseconds) determines how long the injector valve opens and how much fuel is delivered. A healthy fuel injection system delivers equal amounts of fuel to each cylinder. When one injector delivers less fuel (due to coking/carbon buildup on the pintle, a worn solenoid, or internal debris), that cylinder runs lean, contributing to a misfire and triggering positive fuel trim corrections that affect all cylinders. The injector balance test measures how much fuel pressure drops when each injector is pulsed a fixed number of times at a standardized pulse width (typically using a scan tool command or a dedicated injector tester tool). The technician records fuel pressure before and after pulsing each injector. A larger pressure drop = more fuel delivered = injector is flowing freely. A smaller pressure drop = less fuel delivered = restricted or clogged injector. All injectors on a healthy engine should produce pressure drops within ±2 PSI of each other. An injector that produces a 10 PSI drop while all others produce 14 PSI is delivering approximately 30% less fuel — it must be cleaned or replaced. Before condemning an injector electrically, always check fuel pressure at the rail (typically 43–58 PSI for return-style systems; 58–65 PSI for returnless systems) and confirm proper injector duty cycle using the scan tool.