Check the fuel injector

The injector is an electric valve. When the ECU switches it on, a coil lifts a small valve and pressurised fuel sprays as a fine mist into the intake. It clicks many times a second while the engine runs. A blocked injector (varnish from old fuel, dirt past a damaged filter) gives too little fuel: lean running, hesitation, poor power. A leaking injector drips fuel: rich running, black smoke, fuel smell, hard hot starts and flooding. This page covers the checks a rider can do and what the workshop does.

Safety

• The injector sits in a pressurised fuel line next to a hot engine. Do not remove it at home. The K-Light 125 manual requires releasing fuel pressure, disconnecting the battery and fitting new O-rings; a small O-ring fault can cause a fuel leak and fire.
• Never clean an injector by applying battery power to it with jump wires.

What you can check safely

1. Click test: with the engine idling, touch the tip of a long screwdriver to the injector body and put the handle to your ear (or use a stethoscope). A steady, fast clicking means the ECU is driving it. On a bike that cranks but will not start, have a helper crank while you listen: no clicking points to the injector circuit, the crank sensor or the ECU.
2. Connector: key OFF, unplug and inspect the injector connector for corrosion and loose pins; refit it until it clicks. Do not twist the injector while doing this; the K-Light 125 manual warns it can shift the spray direction.
3. Fuel smell or wet spot around the injector with the key ON (pump primed) and engine off: an external leak. Do not ride. Workshop.
4. Codes: read the ECU's codes. Injector open or short codes (K-Light 125: P0201, P0261, P0262) point to the coil or wiring. See Read engine fault codes.
5. Fuel quality: if the bike stood for months with old fuel, drain it first: Drain old or contaminated fuel.

What the workshop will do

• Measure the injector coil resistance. K-Light 125: about 10 Ω; infinite means an open coil.
• Check fuel pressure first, because low pressure looks like a blocked injector (K-Light 125: about 300 kPa; fault below 270 kPa).
• Check the oxygen sensor reading when warm. The K-Light 125 manual says a reading always above 500 mV at normal temperature suggests a leaking injector.
• Remove the injector with the pressure released, clean it with an ultrasonic cleaner and the right fluid, and test the spray and leak-down on a bench.
• Refit with new O-rings, lubricated only on the seals (never on the nozzle plate), in the original direction. Turn the key ON without starting and check for leaks before running.
• Replace a leaking or failed injector with one of the same part number.

What to ask for

• "Please check fuel pressure and injector resistance, then clean and flow-test the injector, and fit new O-rings."

Reference values (K-Light 125)