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 problem.
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 isn�t 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.
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