Probably a dumb question, and I appologize for my ignorance in this field of knowledge, but I've been trying to keep an eye on a VERY VERY intermittent misfire on cylinder #3 on my car. The symptom is that it's simply not getting spark on occasion. I'll drive it all over town one day and it runs fine, then the next time I go to drive it which could be a week or so later, and it has a misfire on that one cylinder.

I have done these diagnostic tests, if you want to call them that:

-pulled plug wire off the spark plug and held a screwdriver next to it while cranking, sometimes I will get an arc of electricity, sometimes I wont (intermittent)

-swapped the #3 coil out with a coil from another cylinder, the code came back for the same #3 cylinder misfire

-swapped the coil bracket/weather pack harness out from driver side to passenger side, the misfire stayed on #3 again

-swapped another plug wire out on #3 and still had the same syptom




I can think of two reasons for the symptom:

1) short in a wire for the #3 cylinder ignition...but it's so intermittent you'd think that a short or bad wire would stay around...like I said I could go a whole day of driving, put 300+ miles on the car and it won't sputter a single bit

2) If this is even possible, maybe a clogged injector isn't feeding fuel and so the ECM is not throwing spark to that coil? I am running a 2nd hand used set of injectors, I can't verify the mileage and I didn't have them flow matched before installing them. Just kinda put them in and ran with it.



What do the pros think?

I don't believe the ECM has any method of knowing if the injector has successfully pulsed, only that it told the injector to do so.

If you can, swap the plug wire with one from another cylinder and see if the misfire follows the wire.

I forgot to mention that as well, i believe I did that test also. It was last summer but I'm pretty confident i tried swapping a wire out also...in fact I know I did because i took a wire off another running LS1 car to try it out.

ecu problem?

I would suspect a harness problem first.

Ditto.

You did swap/toss a new plug in the cylinder, correct? Might as well ohm out that plug wire and yank the connector on the ecm ohm it out as well.

Ok does anyone have a wiring schematic for the ignition on a 99-02 harness showing what color it is and which pin on the ECM?

Looks like pin R69 on the 411 PCM, wire color LT BLU. Goes to pin F on the main harness Coil connector. Pin C at the Coil itself.

http://chevythunder.com/19992002_ls1_ecm_diagrams.htm#Ignition%20Module%20bank-2%20(1-3-5-7)

Like the others have said, If that circuit checks out good, you may have a driver issue in the PCM.

Ohm the injectors, they should all be within 1ohm of each other. Next move INJ #3 somewhere else and see if the problem follows. Finally, unplug the PCM and do a continuity test from R69 to the LT1 BLU coil wire

That should cover the bases for now. If the problem doesnt show up, we'll go deeper

Sounds good, thanks bud! I'm hoping for warmer weather so I can dew werk.

Misfire isn't necessarily an ignition problem. PCM only knows that your cyl # 3 is the issue. Could also be a problem with the injector or valves. Since it's intermitent, and the problem didn't follow your coil swap test, injector or related wiring would be suspect.

So there is a possibility that a spark issue is with an injector?

If #3 is clogged/bad solenoid the PCM will log a #3 misfire. It doesnt know for sure that there is an IGN problem, only that #3 isnt firing properly all the time

Dont worry about the internal possibilities until you rule out the external stuff