Due to component access issues and electrical configurations, it’s important for the technician to adopt a diagnostic strategy that’s best suited to the vehicle platform that he’s working on. With that said, several different methods can be used to test fuel injectors. The easiest test is to place a stethoscope on the body of the injector to check for a clicking noise produced by pintle valve activity. If the injector pintle is inactive, the PCM’s injector driver should be tested for activity by using a multimeter to display duty cycle or pulse width at the injector connector. The multimeter can also be used to compare injector winding resistance to a published specification or to a known-good injector.
Although multimeters can display pulse width or duty cycle, a lab scope provides a graphic display of injector circuit voltages, pulse widths and even pintle closing patterns.
If you’re interested in pursuing lab scope analysis in detail, a number of manuals that cover fuel injector waveforms in detail are available in the aftermarket or contact your parts distributor to get enrolled in a labscope class hosted by Standard. You can also find out more at their Standardbrand YouTube channel. If you’re going to use a lab scope to diagnose faulty injectors, it’s also wise to store known-good waveforms in the scope’s database for comparison.
Scan tools may also indicate fuel injector activity by indicating fuel injector operating status on a data screen, storing fuel injector trouble codes, or by retrieving long- and short-term fuel trim numbers from the PCM. Although interpreting fuel trim numbers requires specific training, negative fuel trim numbers generally exceeding 10% indicate that injector pulse width is being decreased to compensate for fuel supplied from an outside source, such as a leaking fuel pressure regulator. Positive numbers exceeding 10% indicate that the PCM is increasing injector pulse width to compensate for the lack of fuel from clogged fuel injectors or from excess air supplied by an unmetered air source such as a vacuum leak. Unmetered air may also include air leaking into the intake duct on vehicles equipped with air flow sensors. Whatever the case, a scan tool can provide the fastest analysis of a cylinder misfire or poor fuel distribution
This video is sponsored by Standard Motor Products (SMP).
