FUEL INJECTOR CHECKS
If a misfire is not ignition-related and you suspect the fuel system, here are some checks you can perform to diagnose the fuel system.
First, make sure fuel pressure is within specifications, and that the fuel pump is delivering the required amount of fuel (typically a pint in about 15 seconds). Sometimes a pump will deliver normal pressure at idle but not enough fuel at wide open throttle to keep up with the engines appetite for gas.
Check injector voltage. Backprobe each injector connector with a voltmeter or digital storage oscilloscope (DSO). The injector supply voltage with the key on, engine off should be the same as battery voltage. If you see a difference of more than 0.4 volts, there is a problem in the supply circuit. Check the wiring and the injector power relay.
When the PCM energizes (grounds) the injector, you should see the voltage drop to near zero. This verifies the PCM ground driver circuit. If you are using a scope, you can also note the milliseconds of injector on-time and make sure it is changing when you goose the throttle.
Something else to watch for on the scope pattern is a little bump in the waveform that occurs after the ending voltage spike when the injector pintle closes. The bump should occur in the same place if the injector is working properly. If you see multiple bumps or the location of the bump is varying, it means the injector pintle is sticking or hanging up, or the injector is dirty. Note: Some scopes maynot show this bump in the injector waveform.
Another injector test is to check the current flow in the injector circuit on your scope when the injector is energized. This can be done by attaching a low-amp probe to one of the two injector wires. When the injector is energized, the current waveform should start to ramp up as the magnetic field builds in the injector solenoid. When the injector pintle pops opens, it will create a slight dip in the trace, then continue to climb. No such bump in the trace would tell you the injector is not opening.
When the PCM turns off the injector, the current waveform drops back to zero. What to look for here: A sharp vertical rise in the current trace when it starts to ramp up would tell you the injector has an internal short. Also note the peak current draw. If it is over specifications, replace the injector.
Another simple check is to measure the internal resistance of each injector with an ohmmeter. Disconnect each injector from its harness and measure the resistance across its terminals. If resistance is not within specifications (typically 3 to 5 ohms for a low-resistance injector, or 12 to 17 ohms for a high-resistance injector), the injector needs to be replaced. A difference of only 1 ohm less than specifications may be enough to prevent the injector from opening reliably under some operating conditions.
Note: Injector resistance typically increases a couple of ohms as it warms up. Therefore, you should check the values twice: Once when cold and once at normal operating temperature. If resistance goes out of specifications when hot or cold, replace the injector.