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How to test PCM output/wiring for injector?


NAXJA Forum User

On my 98 4.0 I developed a misfire on cylinder 1, which gave the p0301 code as well as confirmed by pulling the injector connector on cyl1 to no change in engine idle.

I have confirmed good spark on cyl 1 by grounding the plug out of the cylinder, good spark.

I have swapped injectors #1 and #2, cleared the codes, and the misfire remains with cylinder #1.

I have 120psi compression in cyl #1.

I have 13.5v at idle at one of the injector wire leads. I am under the impression that the other lead is fed a momentary ground in order to fire the injector. I am unsure of what type of tool I need to test this rapid grounding, as my multimeter does not display this quickly enough.

Further, is there anything I should check at the PCM?

thanks for any help or guidance.
You could get a noid light at harbor freight. Otherwise an oscilloscope would be the only thing that would read it fast enough. It only enables thie injectors for milliseconds. You can try and test the wires continuity back to the ecu. Also you can place a screwdriver on the injector and put the other end up to your ear. And listen for a rapid clicking. If you don't hear it try swapping the wiring with cylinder 2 and see if the injector starts clicking.

It could be wiring. Or a bad or clogged injector.
Okay, I do not have access to an oscilloscope but that noid light kit is cheap and I'm happy to add that to my tools.

I did swap the injectors to no avail, so it does appear to be wiring/PCM related at this time.
On your '98 all injectors are fed 12v from the PDC so you can use the noid light or volt meter on the DG/OR side. You should get steady light on/12v. Try wiggling the harness to see if anything changes as there's a large splice inside the harness that connects them all.

The injectors are grounded through the PCM so the noid light should flicker when connected on the gnd side.

Also check the large connector (C107) along the rear of the engine compartment. Perhaps a good cleaning may help.

My Renix injectors are wired reverse. Hot from the ECU with a common ground. I had a weak splice for the gnd inside the main harness that ran along the VC making #3 intermittent. The splice was located where the harness makes the bend along the firewall. I cleaned it up and a few others in there and that fixed my injector.
I am unsure of what type of tool I need to test this rapid grounding, as my multimeter does not display this quickly enough.

Voltmeter set to a/c might catch this. Some meters might need you to swap the leads to see DC pulses. x2 on inspecting the wiring harness, especially where it crosses from the valve cover to the firewall.
You could also get a pulse tester.

Basically, you prime the fuel rail with the engine off, pulse the injector and watch how much pressure drops off. You'll need a fuel pressure gauge connected to the rail too. Do all 6 and compare them to each other. They should all be within psi of each other. This test can help you determine if you have a clogged injector.

Don't run this test repeatedly on the same injector. Once or twice, that's it. You don't want to flood the cylinder with fuel.
You could also test the injector ground wire for continuity and excessive resistance. I guess I'd backpin the wire at the ECU and at the fuel injector. On mine, I can just barely plug/unplug the harness from the back of the ECU to get testing access. I have a Renix so I don't know if this is possible on a 98.