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Head Unicorn
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Could one bad coil cause the rest of the cylinders to misfire intermittently?
I don't have an answer to that question.... but using the cylinder contribution test you'll be able to single out each and every COP individually and visually see what they are doing. In my experience, when running the contribution test and you have only one misfiring COP...... replacing that COP and running the test again will identify another misfiring COP (or more than one). I don't know that one causes the other, but one can mask the other.
 

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Discussion Starter · #22 ·
Interesting. Thanks for your input.
 

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Head Unicorn
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Hope the info helps you...........
 

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ZYZZ Brah
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Yup! I had a misfire due to a bad COP. It missed above 3000rpm and WOT. Replaced all 8 cops, ran perfect.

Only code I was throwing was from my offroad X.
 

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You replaced all of them, or just the bad one?
I replaced just the one that was bad. After about 2 weeks of misfiring, it finally set a code, so I was able to pinpoint which cylinder it was.
 

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Captain Slow
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I don't have much experience with Mustang OBDII systems, since I just got mine.. But I do have experience with older BMW systems. Pertaining to "random" misfires, the computer has a threshold of number of misfires over time before it would actually illuminate the CEL. Before it would, you could scan the computer and pull out shadow codes or pending codes if you had the right scanner. If it were a single cylinder, say a failed COP, you would get a fairly immediate CEL with fuel cutoff for that cylinder. So, under that type of system you could theoretically have random misfires without a CEL. I would take a look at your fuel trims, or maybe try a smoke test or spray around a can of carb cleaner to try to rule out a vacuum leak. The cam might exacerbate a rough idle due to a vacuum leak, but I'm not really sure of that. Start with the simple things and work your way up.
 

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Discussion Starter · #28 ·
I've got an SCT tuner that I can datalog with and pull real time data from the car's computer, I'm not just pulling codes. You're right, there is a misfire threshold before the CEL lights up. It's showing random misfires on all cylinders, at a rate of 1 every couple seconds which could be robbing me of power but not enough to trip a CEL.
 

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Discussion Starter · #29 ·
If anyone knows their ****, I can do some test runs and send them the live link file
 

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Discussion Starter · #30 ·
Pulled the plugs after less than 100 miles, they all pretty much look like this

They're all dry, with an off-white on the grounding strap that changes color right at the 90* bend, and sort of a half-ring of dark brown around the lip where the threads start. The porcelain is spotless.



 

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Discussion Starter · #31 ·
Am I correct in thinking that they look pretty good?
 

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M112'd Sixxer
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That plug is fine.

Ive had a random misfire at idle on my 5.4 3v since i bought it. Id bring it to work to test somethings and it wouldnt misfire, next time it would. 100% PITA

Sent from my droid using my fingers.
 

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Just fixed my misfire. Clean your MAF and check your vacuum lines. I pulled all of mine out and inspected them. Try unplugging all of your cops one by one. see if there is a change is idle. I really think you have a vacuum leak... if anything replace all your vac lines its cheap and will ease you mind. Lmk if anything of this worked for you.
 

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Discussion Starter · #34 ·
Cold, dry compression check results:

Drivers side:

150, 150, 150, 140

Passenger side:

150, 140, 150, 140

If I did a test after running the engine for a while to expand everything, I'm sure they would jump up a good 10psi


Looks pretty good. At least I know the valves didn't touch.
 

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Discussion Starter · #35 ·
Just fixed my misfire. Clean your MAF and check your vacuum lines. I pulled all of mine out and inspected them. Try unplugging all of your cops one by one. see if there is a change is idle. I really think you have a vacuum leak... if anything replace all your vac lines its cheap and will ease you mind. Lmk if anything of this worked for you.
Roger. I'll check vacuum and clean the MAF next, then I'm just gonna bite the bullet and go get the COPs tested the right way. I'm pulling the coil plugs off one by one, and they all do something, but some change the idle more than others.
 

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Discussion Starter · #36 ·
I took the boots off all of the coils and cleaned them, then put them back together with some grease around the bottom of the boot. It misfired like a bitch all the way to work, CEL came on, then the misfire went away as the engine warmed up.

Pulled 2 codes: P0356 and P0306

Coil primary/secondary circuit malfunction cyl 6
Misfire on cyl 6

Pretty sure I found my problem. Gonna get some new Motorcraft OEM coils.
 

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Head Unicorn
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Yep, hopefully your problem is solved.
 

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Have a 2006 Mariner that was mis-firing at 3/4th, and no service engine light. Fords, Mazdas won't immediate pop a service engine lite, you need a Solis device to dig into Mode 6 to see which cylinder is misfiring. 2005 Mazda did same thing, had replaced cylinder 5 coil with a pulled coil, and was misfiring again, replaced with a new coil, and runs fine, all the while, no service engine lite, no OBD2 Code. Ford products take a while before the lite will come on as compared to GM.
 

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my car will miss under low RPM high load (35mph in 4th gear) It never threw a code, but the car will shake\miss. However that is the only time it does that, idle is smooth, WOT no problems. The engine has to misfire a certain amount of times in a given time to set the Check Engine Light.
I know this thread is about 8 years old, but man I have to ask, did you ever figure out the cause of this problem? this is the EXACT same thing i’ve been struggling with for months. I’ve replaced a couple emissions components, fuel filter, spark plugs, cleaned MAF and nothing has fixed it. No trouble codes either
 
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