P0300 random misfire - Mazda3 Forums : The #1 Mazda 3 Forum
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post #1 of 20 (permalink) Old 10-28-2010, 04:02 PM Thread Starter
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P0300 random misfire

As I posted weeks ago on the bad idle, it's caused by a misfire. No more rpm jumping but idle will misfire, worse when it's cold and it's completely gone when on the throttle. had it checked out and the mechanic doesn't know but said possibly intake manifold gaskets. The car is a 07 ms3 with almost 60k on the clock. Only mods are cobb intake, forge bpv and two resonators deleted from exhaust. Does anyone have any idea what could be causing this, my performance is completely unaffected so I've been driving it but the misfire still concerns me and the CEL it's still really annoying. Help would be great, thanks!
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post #2 of 20 (permalink) Old 10-31-2010, 01:27 PM
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Re: P0300 random misfire

Well if it's a random misfire it could be numerous things, first thing i'd check is the spark plugs myself, then deal with the other Gaskets as they come, could also be Vacuum leak as well... It's Air, Fuel, starving or too much it all depends.. check plugs first.

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post #3 of 20 (permalink) Old 11-01-2010, 02:27 PM Thread Starter
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Re: P0300 random misfire

just got the denso plugs a couple of weeks ago when it first started happening. I kept their pre gap, is it possible there those plugs aren't gapped right even though they're pre gapped?
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post #4 of 20 (permalink) Old 11-01-2010, 05:42 PM
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Re: P0300 random misfire

check the MAF seal, see if thats broken, Also check to make sure that all of your grounds are grounded, another thing could be that the BPV isn't adjusted properly too , To answer your questions though i think that it'd be a good idea to check those plugs to make sure they are in correct specs for sure.

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post #5 of 20 (permalink) Old 11-01-2010, 09:32 PM Thread Starter
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Re: P0300 random misfire

alright. the misfire came before the bpv. what should the plugs be gapped at?
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post #6 of 20 (permalink) Old 11-03-2010, 01:43 AM
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Re: P0300 random misfire

when did it start happening? what did you replace before it happened or what did you do right before it started happening, within about a week period. Then I can help you but, until then, there's quite a few things that could cause this condition.

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post #7 of 20 (permalink) Old 11-03-2010, 07:44 PM Thread Starter
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Re: P0300 random misfire

the CEL came on the day after I got back from a 2,000 mile highway road trip. I had the Cobb SRI on for about a month at that point. Though at first it was because of elevation changes throughout the trip so I disconnected the battery for a couple of hours. Light came on two days later so I checked the plugs and they were used looking so I replaced them and it didn't change anything. I had it checked out by a local shop and they couldn't find anything. They thought since it's only at idle that it was vacuum which it didn't feel like to me and they saw no signs of it.
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post #8 of 20 (permalink) Old 11-04-2010, 03:04 AM
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Re: P0300 random misfire

if it's only at idle it's probably like they said a vacuum leak. Since they "couldn't" find it I would likely lead toward the BPV or maybe even a pin hole in the intake couplers. this is where I would start. you can check everything except the BPV with a can of breakleen and spray all the rubber connections around the engine. IF the idle picks up or falls noticeably after spraying a certain area, then there's a vacuum leak. The oddity in your situation is that usually a vacuum leak will not cause a misfire code to be hard set.

So, before you took the trip did you recently add the BPV or have they been on for quit awhile? Do you have any performance issues with the misfire other than at idle?

BTW P0300 Random misfires are the worst codes to diagnose. They are annoying. I am going to help. I ask for information to get a background as I would if I were with the vehicle working on it.

-CoolGuy stickers, mad awesome bright lights, blackedout lenses, super wicked general tires I got from Sears for forty dollars, some random size shift boot, mad skillz shift knob with fake carbon fiber. This is the stuff you put in your sig when you have nothing done to your car but you want to feel kewl with your wire wrap and neon LED underdash lights.
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post #9 of 20 (permalink) Old 11-04-2010, 02:09 PM Thread Starter
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Re: P0300 random misfire

I did the BPV after the CEL. I was trying to replace the leaky parts the car comes stock with. The reason vacuum seems so odd to me is because you can hear it misfire. It'll make a popping noise and the rpm will randomly drop a bit very briefly, like all in a second almost like the sound of a bad idle sensor. I'm storing the car for winter so it'll be much easier to tear things apart for the next 3-4 months
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post #10 of 20 (permalink) Old 11-04-2010, 09:23 PM
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Re: P0300 random misfire

do you have any low speed issues? Like hesitations misses etc. Not stuff you can necessarily hear but feel? Like jerks slight odditites? I would recommend taking off the BPV and replacing stock until the issue is solved. But that is what i would tell you if I were at the dealership. The reason I ask about the low speed issue because an EGR could cause this and since it is a Ford design it does have EGR runners built in the intake. If it were a faulty IAC you would be having more issues than a little hesitation and stumbling at idle.

after the vacuum hose spray down, like I mentioned earlier, if nothing is leaking from that, then replace the BPV with stock and disconnect the battery and touch the cables from the car together( Only after disconnecting the battery and being no where near it. This will reset the memory completely not just the immediate memory. It will discharge the stored voltage for the keep alive computer memory. In other words a complete refresh. Most people do not realize just disconnecting the battery doesn't reset the entire system. Just like the battery in your computer for the timing circuit. It will not hurt the ECU to do this but it will reset all Fuel trims etc etc. Leave the battery disconnected over night. In the morning the vehicle may start funny but it's ok I assure you it happens. It will then go through idle relearn, STFT, LTFT reset etc. etc. etc., then we will see what happens.

With a Scan tool and being there I would've fixed this days ago but, I'm not there and I can't see live shots of your ECU data.

-CoolGuy stickers, mad awesome bright lights, blackedout lenses, super wicked general tires I got from Sears for forty dollars, some random size shift boot, mad skillz shift knob with fake carbon fiber. This is the stuff you put in your sig when you have nothing done to your car but you want to feel kewl with your wire wrap and neon LED underdash lights.
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