P0300 - The Case of the Mystery Misfire - Mazda 6 Forums : Mazda 6 Forum / Mazda Atenza Forum
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post #1 of 11 (permalink) Old 04-20-2017, 11:52 AM Thread Starter
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P0300 - The Case of the Mystery Misfire

This is going to be long.

03 Mazda 6s 3.0 V6 (~130k)

Things I've replaced

- Precats
- Oxygen Sensors
- Ignition Coils
- Ignition Coil Plugs
- Fuel Pump
- Spark Plugs
- Intake Gasket
- Cracked Hoses
- Rebuilt Transmission
- PCV Valve

Mods

- EGR Valve Mod

I took it to mazda after I replaced all this.

They said it's misfiring on Bank#1, Bank #2 , Bank #3

They said all the following were fine.

- Everything I replaced
- Fuel/Air ratio was fine
- No vacuum leaks
- No fuel leaks
- Fuel Injector resistance is good
- Compression test came back fine (190 PSI on all cylinders (save bank 4 which is 185, still fine)
- They reset the crankshaft adjustment and keep alive light

I spoke with the service tech and he said at this point they're just trying things and seeing if they work. I should take it and do the labor myself if I can. The case was so odd for all they did, they charged me basically nothing in labor.

He said I should try swapping the fuel injectors, I'll probably just buy new ones as I have to take the intake off to do so anyhow. After that he said it might be the PCM or harness, but he said he can't really see anything wrong with either.

Things that crossed my mind... Possibly replace purge solenoid valve before new fuel injectors and see if that helps?

Any advice/thoughts/suggestions/conspiracy theories would be much appreciated. Been fighting with this thing for months.

Throwing a wrench or two is part of the repair process.
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post #2 of 11 (permalink) Old 04-20-2017, 12:31 PM
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I would focus on the wiring harness that goes over the intake manifold. I know my harness is sketchy and I painted it with liquid black tape last year hoping it will last a bit longer. I'm not having problems and I was not having problems when I did that, but my harness is very hard and brittle and I don't even touch it if I don't have to.

Injectors hardly ever fail.

Does it actually have a misfire that you can feel? Is the check engine light flashing when you are driving? What are the physical symptoms of misfires you are feeling?

2004 Mazda 6s 3.0 5dr. never ending money vacuum project
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post #3 of 11 (permalink) Old 04-20-2017, 01:17 PM Thread Starter
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Quote:
Originally Posted by b1lk1 View Post
I would focus on the wiring harness that goes over the intake manifold. I know my harness is sketchy and I painted it with liquid black tape last year hoping it will last a bit longer. I'm not having problems and I was not having problems when I did that, but my harness is very hard and brittle and I don't even touch it if I don't have to.

Injectors hardly ever fail.

Does it actually have a misfire that you can feel? Is the check engine light flashing when you are driving? What are the physical symptoms of misfires you are feeling?
They said the 4Cyl injectors fail frequently, but the 6 generally never do.
The misfire doesn't shake/rattle/roll, that's kind of what makes it so hard to diagnose.
The light goes off and blinks when it does misfire, it's around 60mph when it does it. Acceleration doesn't seem to matter hard/slow, it's always around 60mph.

No physical symptoms, it runs just as smooth as it did before the misfire.
So it's weird. /shrug

Throwing a wrench or two is part of the repair process.
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post #4 of 11 (permalink) Old 04-20-2017, 01:22 PM Thread Starter
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Quote:
Originally Posted by b1lk1 View Post
I would focus on the wiring harness that goes over the intake manifold. I know my harness is sketchy and I painted it with liquid black tape last year hoping it will last a bit longer. I'm not having problems and I was not having problems when I did that, but my harness is very hard and brittle and I don't even touch it if I don't have to.
This harness?
NEW OEM ENGINE HARNESS FUEL INJECTOR WIRING 3.0L 2003-2008 MAZDA 6 #GN3C-67-080C | eBay

Throwing a wrench or two is part of the repair process.
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post #5 of 11 (permalink) Old 04-20-2017, 03:25 PM
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Yeah, that's the harness I was talking about. Take a good look at yours and look for dried out insulation on the wires. I could see the copper wire through the cracks in the insulation in my harness.

2004 Mazda 6s 3.0 5dr. never ending money vacuum project
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post #6 of 11 (permalink) Old 04-20-2017, 04:18 PM
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I have a similar mystery misfire but I haven't replaced even half of the things you have, I have also done the EGR mod though. I'm going to the junkyard to hopefully find a salvageable EGR mount and replace my modded one, l'll let you know if that helps. Also interested in replacing the harness as suggested.
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post #7 of 11 (permalink) Old 04-20-2017, 05:03 PM Thread Starter
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Quote:
Originally Posted by b1lk1 View Post
Yeah, that's the harness I was talking about. Take a good look at yours and look for dried out insulation on the wires. I could see the copper wire through the cracks in the insulation in my harness.
It's pretty dried out, I know when I was replacing the coil plugs, it cracked a few times and I had to fix it. I would think it would most likely be the harness than the injectors themselves. They did a pressure check on the injectors and it came back fine.

Quote:
Originally Posted by ExNylo View Post
I have a similar mystery misfire but I haven't replaced even half of the things you have, I have also done the EGR mod though. I'm going to the junkyard to hopefully find a salvageable EGR mount and replace my modded one, l'll let you know if that helps. Also interested in replacing the harness as suggested.
I was getting this code long before the EGR mod, I just included it as additional info that's been done to do the car.

Throwing a wrench or two is part of the repair process.
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post #8 of 11 (permalink) Old 04-20-2017, 05:33 PM Thread Starter
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Alright, so I called Mazda 6 double check the part number on that harness just to be sure.

The 03 and 04 Mazda 6s has a different harness than the 05-08 Mazda 6s.
For future reference the part number for the '03 and '04 Mazda 6s is
GK2C-67-080G

05-08 is GN3C-67-080C

It is anywhere between $150-200

I'll replace that and report back probably in about a week.

EDIT: Probably be the week of May 1st, they have to order the harness.

Throwing a wrench or two is part of the repair process.
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post #9 of 11 (permalink) Old 04-20-2017, 09:51 PM
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Quote:
Originally Posted by Bazgrim View Post
Alright, so I called Mazda 6 double check the part number on that harness just to be sure.

The 03 and 04 Mazda 6s has a different harness than the 05-08 Mazda 6s.
For future reference the part number for the '03 and '04 Mazda 6s is
GK2C-67-080G

05-08 is GN3C-67-080C

It is anywhere between $150-200

I'll replace that and report back probably in about a week.
Good Luck Mate, I am excited to hear how it turns out.
I had a random misfire after replacing quite a bit of the usual offenders as well. Mine turned out to be bad secondary cats that I did not replace when I did my pre-cats. It was a misfire for a while then it progressed to a full on drive-ability issue. I couldn't even get to 60mph at the end.
May the Force be with you.

2004 Wagon S 3.0, 5-speed ATX, Performance White (A2N) 167,000mi

Last edited by DrFeelGood; 04-20-2017 at 10:08 PM.
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post #10 of 11 (permalink) Old Yesterday, 03:56 PM Thread Starter
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Update:

For shits and giggles, I threw some fuel system cleaner by Chevron when filling up for gas today.
Can't get it to throw the misfire, but haven't driven it around much. (<50 miles) I usually can get it to do it instantly though.
Based on this I went ahead and ordered some new fuel injectors in addition to the harness, maybe some weirdness going on with them too. Might as well replace them since I have to take the intake off anyhow.

Hoping everything will come Friday and I can put it all in this weekend.

Throwing a wrench or two is part of the repair process.
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