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Old 06-30-2013, 03:23 AM   #1
SYNY5TER
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Default P0300 missfire multiple cylinder

so i got a p0300 missfire multiple cylinder.

lately i have been getting poor gas mileage so i checked my scanner and read the code. my mileage is almost 70k could it be possible that i need to replace new spark plugs??

and also is this gonna be ok to drive on the street for temporary with this code goin to work and all.
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Old 06-30-2013, 05:14 AM   #2
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Driving with a multiple misfire can wash down the cylinder walls and dilute the oil with gas.

Could be plugs, but I would check the alternator too
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Old 06-30-2013, 10:36 AM   #3
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Google that code on a Ford and get the proper pin point tests to find the actual problem or take it to a shop that's good at diag.
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Old 06-30-2013, 12:51 PM   #4
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I had this issue recently and after Ford thought they fixed it, probem came back. Pulled the COPs and 3 plugs were loose, just replaced them with new ones. I'd start with plugs first as it costs $0 to pull them and visually check.

On the same note, Ford really sucked when it came to figuring out the problem. I thought they could see which cylinder was misfiring and go from there as my Scantool software has this function to monitor individual cylinders but I need the Ford specific PID add-on but that's more $$.

edit: hey jpplaw, coud you expand on the alternator causing a misfire. Mine has a big load on it and knowing this could be helpful in the future if another misfire occurs that can't be tracked down. And it could be helpful for SYNY5TER to know. thanks!

Last edited by Ricardo; 06-30-2013 at 12:55 PM.
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Old 07-01-2013, 07:40 PM   #5
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An iffy alternator, working but not quite correctly, can cause intermittent COP and therefore ignition problems. They can also cause numerous different codes to set - all sensors are dependent on a consistent clean electric supply.
The misfire monitor watches the crank position sensor (CPS) in these motors. If there is a 2% or more rpm difference between cylinders firing, it will set the code. A big cam can even cause a P0300 to set. A 10% difference will make the CEL blink. P0300 can be plugs, COP's, funky electrics, water in a plug well, or fueling problems. I have heard of broken or seriously worn timing chain guides causing the code to pop. Could also be a bad CPS.
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Comp Cams 127450's
GT500 Dual Fuel Pumps w/ dual VMP boosters
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Dad's Intercooler tank
Kooks 1 5/8” Headers, Green Cats, H-pipe
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Old 07-01-2013, 11:17 PM   #6
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so i just fixed the problem. it turns out my spark plugs were gone. and one of the spark plugs came loose. it was easy to unscrew. after installing new plugs all was smooth and quiet.
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Old 07-02-2013, 09:12 AM   #7
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Thanks jpplaw! I'll keep this in mind, like I said my alternator has a heavy load from the stereo and it's something to watch for.

Hey SYNY5TER, did you do a plug change recently before this issue popped up?

The plugs loose in mine came loose even after I had torqued them to spec and rechecked after a few engine cycles. No idea how they did this, even checked that no anti-sieze made it onto the threads. I now just make it a habit to check them once a month.
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Old 07-02-2013, 09:12 AM
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