2004 3.0 Misfiring, Dieseling, Rough Idle...etc - Mazda 6 Forums : Mazda 6 Forum / Mazda Atenza Forum
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Old 11-13-2012, 08:07 PM   #1 (permalink)
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Default 2004 3.0 Misfiring, Dieseling, Rough Idle...etc

I had noticed a decline in my gas mileage, rough idling, dieseling, and misfiring a while back on my Mazda 6. I recently had my guy run some tests and we found that 2 coil packs were duds. This happened to be the ones under the intake manifold so of course I had to pay for the couple hours of labor, 3 coils (might as well change the other one too), and 6 spark plugs. After getting the car back today it starting working great, but then tonight it started back up again. It's rough idling at every red light, dieseling after every time I turn the car off and its misfiring still. And it usually misfires when shifting at about 2500 rpm in 3rd gear. Even if I lightly touch the throttle. It hasn't thrown me any CEL's since I got it back and this problem has really plagued me for a while. If anybody could possibly identify the problem it would be a great help to me.
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Old 11-13-2012, 08:13 PM   #2 (permalink)
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Please try refraining from posting duplicate threads as its not fair to other members & pushes their threads from the active topics list. Also try beaking up large posts for ease of reading.

More details about the car would be great. MTX or ATX? Any mods? Have you checked vacuum lines?


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Old 11-13-2012, 08:25 PM   #3 (permalink)
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Eek, sorry about that. I thought the one on general had not gone through and it wasn't showing up before. My apologies. As far as mods go, the resonator was out when I bought the car and I stripped the airbox, but that is it. It's MTX. And my mechanic looked all over the bay for any other problems. He would have told me if there was a leak, and like I said it hasn't thrown me any codes since. I could always go back over it a second time.
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Old 11-13-2012, 08:36 PM   #4 (permalink)
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Hopefully it spits out another code so you have a starting point. Easy & inexpensive things to start with are cleaning you MAF, checking all vacuum lines for leaks, check acordian tube for cracks.

Personally i would have replaced all coil packs at once. If 2 are bad, how long will the rest last? Im not telling you to run out & replace the other 3 in hopes it will fix your issue i just would've done them all.

Were your plugs gapped properly? How did your mechanic pinpoint your misfire to those two coils?


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Old 11-13-2012, 08:49 PM   #5 (permalink)
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Hopefully. I will have to check the MAF, the lines, and accordion tube in the morning. As far as I know the plugs were installed properly.

My mechanic is a Miata specialist in the area and he always works on my cars, pretty skilled. I couldn't tell you for sure as to whether they are gapped properly due to inexperience.

He took the evidence I gave him and deduced it was the coils. He also tested them and two failed the test. Maybe he missed something? The front 3 coils were perfectly fine he said, and the other codes that came up disappeared once he replaced the back three.
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Old 11-13-2012, 08:58 PM   #6 (permalink)
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What other codes were present?

If he unhooked your battery to do the plugs (which he should have) then all codes would be cleared.

To me coil packs are like plugs. Replace as a set.

All mechanics miss stuff from time to time. Having a second set of eyes helps out huge sometimes.


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Old 11-13-2012, 09:04 PM   #7 (permalink)
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He didn't express to me the exact codes; however, some emissions codes were thrown as well. And yes, it was. Well unfortunately I didn't have the funds to replace all 6. They aren't exactly the cheapest thing out there.

I do know that some gas was mixed with oil and a lot of fuel has been dumped through the exhaust because of this problem. Changing the oil this weekend, but don't think it really coincides with the misfiring.
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Old 11-13-2012, 09:25 PM   #8 (permalink)
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Having lots of fuel go through the exhaust isnt good. Especially for the cats. Hopefully nothing like pre-cat failure is coming for you.


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Old 11-13-2012, 09:35 PM   #9 (permalink)
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Swapping out the manifold for MSDS headers down to a catback pretty soon here. I happen to live about 60 miles away from Nautilus. Until this problem gets solved though, I can't.
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Old 11-15-2012, 12:07 PM   #10 (permalink)
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I replaced all 6 coils Dec 2010 due to hesitation-herky-jerky engine issue between 40-50 mph at low rpms. I purchased the BWD coils st Stockwiseauto for $35 each. I just checked and they are out of stock but list at $30. While I had the manifold off I noticed a lot of "gunk" on this silver tube extending into or out of the manifold, can't remember. I cleaned the tube as best I could and put it all back together with 6 new plugs. Now I'm having the knocking-dieseling. I cleaned the MAF & throttle body & seemed to go away for about a week. I noticed some "fluid" in the flexible tube between the air cleaner box & throttle body directly below the tube connection which extends from the valve cover. After cleaning the "fluid" (didn't have a distinguishable smell) I disconnected that tube, plugged both ends and drove it for a few days. Not noticably better. Knocking returned but seems like knocking only occurs after driving the car for 30 minutes, when engine is hot.
So yeah, don't know what to do. Replace PCV valve?
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