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Discussion Starter · #1 ·
OK I replaced my sparkplugs on my 02 GT with NGK G-Power platinum plugs and all of the sudden have a single cylinder misfire under load primarily at low rpm which is very annoying going up hills and ect. There is also a spark knock that came along with the change too. I hear this happens under the initial hard acceleration in 3rd or 4th gear at about 3k rpm. I've double checked the gap already and still the same issue still there...
 

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Was it already misfiring!

---------- Post added at 02:28 PM ---------- Previous post was at 02:28 PM ----------

Meant ?
 

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Could have been a bad plug from factory or you coil is flaky. I had a coil that would fire good when I pressed on it but not when I left it alone
 

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Interesting, my 02 does the same thing. I'm wondering if it was a factory made issue; but I can't see how that would be seeing that its an 02 you must have put quite a share if miles on it. At about how many miles did you first notice the problem?
 

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Discussion Starter · #9 ·
Could have been a bad plug from factory or you coil is flaky. I had a coil that would fire good when I pressed on it but not when I left it alone
if thats the issue problem is finding that one bad plug or coil ya know:banghead:
Interesting, my 02 does the same thing. I'm wondering if it was a factory made issue; but I can't see how that would be seeing that its an 02 you must have put quite a share if miles on it. At about how many miles did you first notice the problem?
My car had about 103k or so when i changed them out and thats when the issue came
 

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It takes more voltage to ignite a lean mixture than a richer one, that is why often slight misfires and stumbling can occur at low loads and part throttle but not at higher loads.

OP have you inspected the COP boots closely, for signs of hardening/cracking and/or a "burned" smell? It is not unusual for them to harden up and lose some of their insulative property--when that happens the build up of voltage needed to ionise the plug gap can be slowed and weakened, again affecting ignition of leaner mixtures. Many COPs have been replaced unnecessarily because the boots were bad.

Most FLAPS stock new boots or you can get them online for $3 a pop.

The spark knock you are experiencing could be because each manufacturers heat ranges differ, and the NGKs are running hotter than the OEM plugs. There is no standard for spark plug heat range designations and often the cross-reference charts are not entirely correct. I would not use any but Motorcraft or Champion plugs in any n/a 4.6, and only in booted or other power adder builds if there was some specific reason to do so.

The Champion stock # 3401 single platinum (the center electrode) plug that seems to behave well in our cars, or Motorcraft's SP-493, also a single platinum plug. Unlike the coil pack EDIS used on the '96 through '98 GTs there is no need for double platinum (both electrodes being precious metal) plugs in the '99 through '04 V8s as the COPs all fire with conventional polarity.
 

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Discussion Starter · #11 ·
It takes more voltage to ignite a lean mixture than a richer one, that is why often slight misfires and stumbling can occur at low loads and part throttle but not at higher loads.

OP have you inspected the COP boots closely, for signs of hardening/cracking and/or a "burned" smell? It is not unusual for them to harden up and lose some of their insulative property--when that happens the build up of voltage needed to ionise the plug gap can be slowed and weakened, again affecting ignition of leaner mixtures. Many COPs have been replaced unnecessarily because the boots were bad.

Most FLAPS stock new boots or you can get them online for $3 a pop.

The spark knock you are experiencing could be because each manufacturers heat ranges differ, and the NGKs are running hotter than the OEM plugs. There is no standard for spark plug heat range designations and often the cross-reference charts are not entirely correct. I would not use any but Motorcraft or Champion plugs in any n/a 4.6, and only in booted or other power adder builds if there was some specific reason to do so.

The Champion stock # 3401 single platinum (the center electrode) plug that seems to behave well in our cars, or Motorcraft's SP-493, also a single platinum plug. Unlike the coil pack EDIS used on the '96 through '98 GTs there is no need for double platinum (both electrodes being precious metal) plugs in the '99 through '04 V8s as the COPs all fire with conventional polarity.
Thanks, thats helpful info...i will change those plugs out with motorcrafts and change the boots out as well while they are out but i gotta wait till this weather lightens up some first
 

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My car is a 2000 gt with 188000 miles. One owner me. Go to jegs and buy a cheap jegs coil for 17.99 to use as a test coil. Take one coil off and put new test coil on to see if it runs better. If so then buy a factory ford coil cause they are better. I just use a cheap jegs coil for trouble shooting because they are cheap and probably won't last as long as the factory. That way you have a spare in the trunk if one fails when you are out and about. Just keep moving the test coil from one cylinder to the next till you find the bad one. In my case my factory coils were still on at 167500 miles when 5 went bad in a month. Only one just stop all together. The others just got weak and would randomly misfire. So I bought all new coils. But the test coil worked great for me till my new ones came in. Glad to help man.
 

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Discussion Starter · #13 ·
Update on the mustang....swapped the plugs out with original motorcraft plugs and it fixed everything.....until the end of the weekend where I think a coil pack died on me. The car just started missing worse than before, so bad this time the service engine light was flashing. Which is good bcuz at least I know it's cylinder 3....I'm going to go buy a elcheapo coil today and if it works then go buy an oem one. Especially knowing how picky these cars are now.
 

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Thanks for the update!

Misfiring for prolonged periods can burn out a COP, if the misfire is a result of the COP dumping it's full charge into the plug well, through the boot. I have come to believe and recommend that the COP boots be replaced at 100k miles, especially after we did this 1-1/2 years ago on a neighbor's '01 at 115k miles.

He now has 165k still running the stock COPs, which all passed my 16 mm air gap test a month or so ago...
 

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Discussion Starter · #15 ·
Thanks for the update!

Misfiring for prolonged periods can burn out a COP, if the misfire is a result of the COP dumping it's full charge into the plug well, through the boot. I have come to believe and recommend that the COP boots be replaced at 100k miles, especially after we did this 1-1/2 years ago on a neighbor's '01 at 115k miles.

He now has 165k still running the stock COPs, which all passed my 16 mm air gap test a month or so ago...
Update once again....

Replaced the COP on cylinder 3 with a direct replacement motorcraft oem COP and so far all seems well....again. Lets all hope that's the end of this issue. I did drive it around the city a bit and noticed no hesitation or missing anymore...I do have a new clutch in her so I am taking it easy too so I wont screw anything up but thanks everybody for the help...I am new on here and people have been very helpful so far :rockon::headbang:
 

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Clean you iac valve I was having the same problem changed plugs cops moved injectors around to see if the misfire would change cylinders it dident looked on here seen the threads about the iac being dirty cleaned it fixed my problem instantly
 
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