Dirty and low-flow injector nozzles cause many driveability problems like rough idle, cylinder(s) misfire(s) and poor performance. A very common problem is a low flow condition caused by dirt accumulation in the injector inlet screen and injector poppet valves that stick closed. These conditions can be observed on a secondary ignition pattern during a snap throttle test. When the throttle is opened rapidly, a momentary lean air/fuel mixture is present in the combustion chamber. You should expect to see all cylinders show signs of the lean mixture by observing an increase in voltage at the end of the spark line, this is normal. When the end of the spark line rises to over half the voltage as the spark line, that cylinder or cylinders is probably too lean.
The examples below are from a GM Vortec that had a P0302 fault code and obvious low performance from cylinder #2 as shown in the cylinder balance test in (Figure 1). The snap throttle test in (Figure 2) shows that cylinder #2 probably has a lean condition. Notice the voltage increase at the end of the firing line (arrow). Injector #2 was stuck closed. Increasing the fuel pressure, externally, while being activated with a scan tool opened it.
Figure 1
Figure 2
