Thread: P0300 (Random misfire) Shows on 3 & 6 only. looking for help.

So here's the story. I recently purchased a 2001 Regal GS with a P0300 (random misfire) code. Misfires only showed on 3 & 6. I got it for a decent price because of the code and I figured it would be a simple fix. Misfire on 3 & 6... Must be a coil.

Wrong!

Let's step back to the original diagnosis procedure. Car idles great. ZERO misfires at idle. When I wind it up to 2k rpm, HPT starts counting misfires by tens on 3 & 6.

1.) I swapped the coils around. Misfires stay on 3 & 6. I figure it must be the ICM.
2.) I replace the ICM with a brand new Delphi unit. Still misfires on 3 & 6.
3.) I replace plugs and wires. Still misfires on 3 & 6.
4.) I swap injectors 3 & 5. Still misfires on 3 & 6.
5.) I tested for 12v at #3 injector with the key on. Lit a test light nice and bright.
6.) 12v test light on the #3 injector connector barely pulses. Very faint light. However, it is the exact same intensity as #5.

Not sure where to go from here. Maybe a compression test? What that will tell me? Again, zero misfires at idle, no excessive blowby, and it's still on a stock pulley.

Rapid fire: What to I check next?

7.) CASE relearn done. Still misfires on 3 & 6 (P0300) with and additional P0303 (misfire on #3) code.
*Edit. CASE relearn fixed it. Read on for details.

Sounds like injector wiring (or injectors, maybe swap those around, too?) is the only thing left, except of course grounds which I assume you verified, along with firing order. Plug wires can be hooked up all wrong and due to the design, a guy may never notice. I don't know that cranking pressure would tell you anything unless theres stuck valves. What did the plugs from those holes say? I would think comparison to healthy fuel trims might offer a hint. Connecting an analog (plain old regular) vacuum gauge would point out a manifold or valve problem.

I do know I am not fit to advise but one reply beats none.

Originally Posted by idrivejunk

Sounds like injector wiring (or injectors, maybe swap those around, too?) is the only thing left

So I did swap injectors 3 & 5. No change. I believe it would be worth checking resistance on the signal wires for injectors 3 & 6. I just need to dig up a wiring diagram.

except of course grounds which I assume you verified, along with firing order.

I mean, I did a visual inspection and everything looks good. Didn't see any obvious missing grounds or chaffed wires. I triple checked that the plug wires are in the right spots. Even printed out a cheat sheet. Maybe I'll swap 3 & 6 just for grins.

I don't know that cranking pressure would tell you anything unless there's stuck valves.

Or broken valve springs? Then again, zero misfires at idle...

What did the plugs from those holes say?

Great question! Those plugs looked exactly like the other 4. Kind of like a toasted marshmallow. Light brown. No crust. No soot.

I would think comparison to healthy fuel trims might offer a hint.

Agreed. I haven't let it run long enough to go into closed loop.

Connecting an analog (plain old regular) vacuum gauge would point out a manifold or valve problem.

Worth a shot I suppose. So strange that it only misfires on 3 & 6 though.

I do know I am not fit to advise but one reply beats none.

Nope, I appreciate the feedback. Gets me thinking in multiple directions.

got to be a bad wire. off the icm or to the injectors. check that metal harness loop on the do bone. see if theres any damaged wires inside that area.

Originally Posted by SgtMarshal

could it be the O2 sensor or Maf?

I mean, I really don't think so with the misfire isolated to 3 & 6 only.

Originally Posted by Scottydoggs

got to be a bad wire. off the icm or to the injectors. check that metal harness loop on the do bone. see if theres any damaged wires inside that area.

I'm leaning toward this. Especially on the spark side of the equation. I just can't get past the fact that 3 & 6 share a coil. It is just too convenient to be anything else in my mind. Especially since the misfires on 3 & 6 are pretty much equal. Within 10% I'd say. I'm seeing if I can find some diagrams for the PCM and ICM. I'll do some resistance checks and peel back some wire loom.

I'm thinking the control wire for the 3/6 coil is chafed and/or missing a bunch of strands somewhere. Enough remains that everything is fine at idle, but can't handle any kind of load like higher RPM. That's my current theory anyway. Probably won't know anything till next weekend. I only have time to mess with it between 60e builds. Gotta rake hay while the sun is shining.

a broken valve spring wouldnt cause a RANDOM misfire,would it?

i was wondering the chances that Chip has visited you twice.

Originally Posted by tomsteve

a broken valve spring wouldnt cause a RANDOM misfire,would it?

I mean, random as defined by the PCM. I'm not sure what the exact parameters the PCM needs to see to set the MIL.

Originally Posted by mguzzo

i was wondering the chances that Chip has visited you twice.

You shut your mouth with that negativity!

dumb question, are you sure that 3 is going to 3 and 6 to 6?

Originally Posted by mguzzo

dumb question, are you sure that 3 is going to 3 and 6 to 6?

Valid question. Correct me if I'm wrong... #3 wire is going to the center cylinder on the front bank. #6 wire is going to the furthest left (drives side) cylinder on the rear bank.

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Originally Posted by mguzzo

dumb question, are you sure that 3 is going to 3 and 6 to 6?

Doesn't actually matter. Since the packs are 1/4, 2/5 and 3/6. Both towers of each coil fire at the same time. Hence the term waste spark system. One spark is always wasted during the exhaust stroke the other goes to the other cylinder for detonation. 3/6 would fire both at same time.

Since it's mad only on that coil and you've swapped coils, wires, plugs and ICM, try a crank learn. I know it seems like it's not needed and you'd never consider it. I've seen some really odd behavior and typically a crank relearn did the trick. Had a few that started and ran horrible, you check everything and can't find anything.. crank learn and it purrs. Can't hurt to try.

Originally Posted by BillBoost37

...try a crank learn. ... Can't hurt to try.

You're absolutely correct. I had enough time this evening to do just that. Unfortunately didn't change anything. Going to order a noid light kit and if I get time tomorrow I'll do a closer visual check of all the wires.

You know.. you can just pull a 194 bulb out.. straighten the tabs and use that

i hipped him to that trick in da chat......

Originally Posted by BillBoost37

Since it's mad only on that coil and you've swapped coils, wires, plugs and ICM, try a crank learn. I know it seems like it's not needed and you'd never consider it. I've seen some really odd behavior and typically a crank relearn did the trick. Had a few that started and ran horrible, you check everything and can't find anything.. crank learn and it purrs. Can't hurt to try.

Originally Posted by BrandonHall10

You're absolutely correct. I had enough time this evening to do just that. Unfortunately didn't change anything. Going to order a noid light kit and if I get time tomorrow I'll do a closer visual check of all the wires.

Once again Bill is spot on. Apparently I wasn't doing something correctly. Had a buddy do the CASE relearn with a pro scan tool. As soon as he did that... ZERO misfires. Going to put some miles on it now and report back, but as of right now all is well. Hope I'm not speaking too soon.

More like got lucky again.

In the past I had a couple of cars I worked on and they just wouldn't run good for anything from start up. You check over everything and you wonder.. what happened???? Tried a crank relearn and sure enough the engine purrs. Only thing I can think of is that the pcm etc is off on it's information and the relearn straightens it up. Kinda like a pc reboot.

Happy to hear it's running good.