Auto Service World
Feature   August 1, 2007   by CARS Magazine

Catching Failing Fuel Injectors

Fuel injectors usually fail in one of three ways. First, the injector nozzle can fail to seal properly in the rest position for a number of reasons, which will cause it to drip fuel into a cylinder af...


Fuel injectors usually fail in one of three ways. First, the injector nozzle can fail to seal properly in the rest position for a number of reasons, which will cause it to drip fuel into a cylinder after the engine has been shut off. This can cause internal damage to the engine. Second, the injector nozzle or its filtering screen can become clogged with deposits or debris and the spray pattern along with the amount of fuel entering the cylinder suffers. Finally, the windings that make up the electrical coil can become shorted or open causing the injector nozzle to open too late or not at all.

A leaky injector nozzle requires a visual inspection to verify the cause. The other two conditions can be diagnosed with some instrumentation. One quick, easy way to “look” inside the fuel injectors and test their internal coils is to use the Snap-on Low Amp Probe with the Snap-on MODIS or Vantage PRO.

You clamp the jaws of the low amp probe around the main power source to the injectors or by using a fused jumper wire at the fuse box. Vantage PRO and MODIS both provide the technician with many preconfigured tests for analyzing amp patterns. Since most injector windings have resistance levels around 12-16 ohms, and the voltage supply is roughly 14 volt while running, the amp draw should be approximately 1 amp. The pattern should “ramp-up” as the injector coil saturates with electricity. When the magnetic field forces the pintle of its seat, the waveform shows a fluctuation called the pintle bump. When the PCM turns off the injector, the amperage draw drops immediately to zero.

All of these attributes have to be there or else the injector is defective. The elapsed time to verify the integrity of the electrical portion of the injectors: about five minutes. This five-minute investment quickly confirms the electrical portions of the injectors are in good shape, or condemns them. Then you are on to the next phase in the repair fast.


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