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Old 05-20-2013, 08:16 PM   #1
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Default Misfire in cyl #3, possible causes?

EDIT: See post #2 for latest diagnostic info






Hey guys,
I've been chasing a misfire of some kind for the past week now on my 2000 Miata. Just a bit of history:
I swapped in a set of ID725 injectors a few weeks ago, running with my MegaSquirt 2. Things have been great with the new injectors until a week ago, getting off the freeway the car was down on power exhibiting symptoms of running on 3 cylinders (rough running, very down on power). My coil pack is ~10 months old (bought new from Mazmart). I was only a mile from home so I babied it back, and about 1 block away it suddenly cleared up and everything was fine.

Now in the past week since then, once in awhile I'll get a miss under random throttle conditions (sometimes just part throttle, sometimes WOT). It happens maybe once or twice every 20-30 mins of driving.
This afternoon I started the car up after having last driven about an hour before (coolant temps were ~145 or so after cooling from sitting), and it ran very rough immediately. I started pulling plug wires, and pinpointed the problem to cylinder #3. There was spark from the #3 wire on the valvecover though. I limped the car home, and again about 3/4 of the way home it cleared up on its own and runs fine again.

A common symptom of both times is the wideband reports the engine running very lean, normally at cruise I run ~15.4, with the misfire it was running in the 18-19 AFR range. This kinda makes me think it's not the coil pack, or the wideband would show it running rich, and there'd be a lot of fuel smell from the exhaust from the unburned fuel.

I've read sometimes the cam angle sensor can go bad? Is there any other testing procedure I should try? I thought maybe I could check the injector wiring to make sure I have everything plugged in, but jiggling the #3 connector as best I can through the manifold doesn't cause any issues. If it were loose I'd think that would be enough to unseat it fully.

Any ideas/suggestions would be appreciated!

edit: I just went out & started the car after ~20 mins of sitting/heat soaking the engine bay in the garage. It starts right up with no issues...

Last edited by SJP0tato; 05-20-2013 at 09:50 PM.
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Old 05-20-2013, 09:49 PM   #2
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Well, an update:
Pulled the upper manifold, gave a small pull to the #3 injector -> harness adapter, and it came off of the ecu-side plug. Plugged it back in, made 100% sure it was fully seated. Got the manifold all back together, started it up... and no fire on #3 100% of the time now.

It's gotta be something with the wiring or injector #3 at this point. Too aggravated to do any more diagnostic work tonight.
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Old 05-20-2013, 10:24 PM   #3
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Another update: went out & started it, and it fires right up and idles/runs perfectly. No clue where to look now.
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Old 05-21-2013, 12:27 AM   #4
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Quote:
Originally Posted by SJP0tato View Post
A common symptom of both times is the wideband reports the engine running very lean, normally at cruise I run ~15.4, with the misfire it was running in the 18-19 AFR range. This kinda makes me think it's not the coil pack, or the wideband would show it running rich, and there'd be a lot of fuel smell from the exhaust from the unburned fuel.
What makes you think the WBO2 would show rich during misfire conditions?

Think about it. Is the WBO2 a fuel sensor? Or is the WBO2 an oxygen sensor? Do you have more or less oxygen in the exhaust when you misfire?

As for procedures, thing to do now is swap and see if the miss follows a particular component. You can swap injectors. You can swap adapters. Etc.
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Old 05-21-2013, 01:14 AM   #5
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Dilelectric grease on each injector connection. Win.

Edit: I have had a flaky connection on my #2 before. Harness was clicked in but was making a poor connection. Dielectric grease solved it. Put it on every connection.

Last edited by Mobius; 05-21-2013 at 01:25 AM.
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Old 05-21-2013, 03:07 AM   #6
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Thanks guys, swapped the 1/3 injectors. Fired up just fine, it's just a waiting game now to see if/where something happens.

Appreciate the feedback!
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Old 05-21-2013, 10:48 AM   #7
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Started up fine this morning, didn't seem like anything was wrong so I drove it into work. In my workplace parking lot it smells faintly of fuel. So I pop the hood and look around... nice slow drip from the #4 injector. Looks like I get to pull the manifold/rail again. This'll be the 6th time I pull it, I'll soon be the most experienced manifold/fuel rail re-installer known to man.
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Old 05-21-2013, 10:57 AM   #8
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From you build thread, didn't you do something non-OEM on your fuel rail mounting (lost a bushing or something?). Maybe return that to OEM parts?
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Old 05-21-2013, 10:59 AM   #9
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Quote:
Originally Posted by hornetball View Post
From you build thread, didn't you do something non-OEM on your fuel rail mounting (lost a bushing or something?). Maybe return that to OEM parts?
That's a good point, not sure if that's the cause, but it couldn't hurt to go back to OEM just incase.
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Old 05-21-2013, 06:24 PM   #10
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Left work today, immediately it was running on 3 cylinders again. Pulled the #3 plug wire, and it's still not firing. So it's not the injector or the interim harness. Sounds like either the coil pack (gonna swap them when it cools down and I get the rail spacers in to stop the fuel leak), or the OEM injector wiring itself.
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Old 06-02-2013, 04:28 PM   #11
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Well after a bunch of other diagnostics I finally figured out what this was last week:

After swapped back to the MS2 from the stock ecu, I guess I hadn't snugged up one of the screws that holds one of the wiring looms to the MS2. So after a week or so of driving the upper half worked itself loose enough to intermittently lose connection on a few pins. Explains why it'd be totally fine sometimes, and not working others.

Pulled the MS2, snugged up all the connectors, and it's been fine for over a week now. Lame cause, wish I'd checked there first, but you live & learn.
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