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Old 04-06-11, 01:32 PM   #1
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What conditions need to be met before the fuel injectors fire?

doing some early morning brain storming before i try and work on my car again tonight, im trying to diagnose why my primary injectors wont fire

-My fuel pump primes
it builds 3 bar of pressure
-I have fuel in the rail, i checked when i pulled the primary rail, primed pump, fuel poured out
-I applied power to each primary injector, they click and were even squirting the small amount of fuel left in the rail.
-I used a multimeter and placed the leads in the fuel connector harness side in both connections, had a friend crank it, and the mutimeter would read voltage, ex: crank,crank,6v,4v,12v,8v repeat itsself, so this means i have power coming to the harness.
-egi fuse is good.
-i have spark, power fc turns on, etc, just wont squirt fuel.

But with the rail in, when i crank the car no fuel squirts out, how exactly do they fire, is there constant power to them all the time?

does the ecu just add a ground to complete the connection causing it to fire?

does this mean i have a bad ground somewhere if every other condition is good?
and where does the injector harness ground itsself,/ any other related grounding points.

Thanks in advance!
Adrian
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Old 04-06-11, 02:03 PM   #2
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Are you positive they're not firing with everything assembled? You can use a test light or perhaps a mechanic's stethoscope to check if the solenoid in the injector is engaging. You should be able to view injector duty cycle on the Commander to see if the ECU is trying to fire them or not. Ideally you would take logs with a Datalogit.

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does the ecu just add a ground to complete the connection causing it to fire?
Yes, the ECU engages the injector drivers (field effect transistors, wikipedia that) to pulse the ground for the injectors.

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does this mean i have a bad ground somewhere if every other condition is good?
and where does the injector harness ground itsself,/ any other related grounding points.
There are four ground wires for the ECU. The wiring diagrams show where they run to.
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Old 04-06-11, 02:21 PM   #3
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Are you positive they're not firing with everything assembled? You can use a test light or perhaps a mechanic's stethoscope to check if the solenoid in the injector is engaging. You should be able to view injector duty cycle on the Commander to see if the ECU is trying to fire them or not. Ideally you would take logs with a Datalogit.

There are four ground wires for the ECU. The wiring diagrams show where they run to.
my datalogit and powerfc say the injector duty cycle is like 2% or something not quite sure so its trying to fire them.

should i start by checking the injector pins at the ecu, 4w/4x/4y/4z? for continuitywith a ground, are these the power signals? would that show if there is a short somewhere.
are the injector pins supposed to have constant power with the ign on? i only get power once i start cranking, this seems normal, just thinking outloud

Last edited by ILoveJDM; 04-06-11 at 02:27 PM.
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Old 04-06-11, 02:33 PM   #4
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During cranking there is a fixed amount of fuel being injected. It's a special map used just for cranking, like 200 rpm. I think temperature effects how much, but besides that the ECU ignores everything else. So if you aren't getting any fuel during cranking, you have something seriously wrong.

If you really want to diagnose this, you need someone with a scope. You need to probe your primary injectors with the special probes that can fit into the connectors and making contact while the connector is plugged in. Then setup the scope to show all the injectors on the same screen and start cranking. You should get a nice digital sine wave. A multimeter won't be quick enough to give you a good reading here. That is going to tell you where the electrical problem is: either the wiring harness or the ECU.
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Old 04-06-11, 04:04 PM   #5
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Why not use noid lights to test and see if the PFC is sending a signal to fire the injectors? A steady blink indicates everything from the injector clip to the PFC is good. If not, start looking at the wirings/connections and the PFC itself.

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Old 04-06-11, 05:22 PM   #6
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You can use a led diode with resistor about 560ohm to the + of injector connector and the - of the led to the other side of connector and simple check if it light when you crank.
Just to know pfc didnt have fuse protection for the outputs so it is possible to fail the transistor array in the pfc board like arghx said.
Another test that you have to do is with multimeter negative probe to ground and the positive to constant + of the injector connector you must read stable 13,6 volt with the key to on.
If it is less under 12,5 then your problem is in the egi relay circuit and not on the pfc.
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Old 04-07-11, 01:47 PM   #7
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Another test that you have to do is with multimeter negative probe to ground and the positive to constant + of the injector connector you must read stable 13,6 volt with the key to on.
If it is less under 12,5 then your problem is in the egi relay circuit and not on the pfc.

Ok so i did this

postive connector to the injector plug, and neg to the chassis,

i get 3.0v dc on both primary plugs.
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Old 04-08-11, 02:14 PM   #8
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you will have battery voltage on one terminal anytime the key is on (it will only be 13vdc with the alt charging)

the other terminal will be "open" until the ecu fires the injector which it does be allowing a ground path

get a noid light.

or a high impedence test light

put the clip end of the light on the battery POS

probe the injector clip on the pin the is controlled by the ecu(no battery voltage) and crank the car

the light will pulse if the ecu is controlling the injector. just like a noid light

now if what you are saying is that with the multimeter neg attached to a seperate good ground (like the batt neg terminal) you only have 3vdc on the injector clip well that is the problem.

what is the battery volatge when doing the test?
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Old 04-09-11, 12:15 PM   #9
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baterry v is 12.5

found a problem: egi relay is bad. no continutiy though any of the prongs, th diode is shot.
still dont understand how i get 3v with that bad relay though.
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Old 04-09-11, 12:15 PM
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