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Old 09-14-2008
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Low RPM misfire

Hello guys, I have recently been having a small problem out of my 2005 3.0. I have the 5 speed manual transmission, and recently, when I start off and the engine is below 1200 RPM, I get a misfire randomly. It does not throw any codes at all, and after the engine reaches an RPM above 1200, the problem is gone, load on the engine or not. The truck has 58,000 miles, and I just recently installed a brand new set of OEM Motorcraft plugs, but have not changed the wires. I suspected the wires were arching out under load, but I misted them down with some water (which will usually cause them to arch if they are bad) but they were fine. No sparks whatsoever. My guess is that its the low RPM circuit in the coil pack acting up, but Im not very sure on how they work. I know the Older GM HEI modules have a low RPM circuit as well as a high RPM circuit, and I am guessing that the coil packs are similar. It is just fine when I am driving it. Like I said, after 1200 RPMs it doesn't show up at all, so I am going to keep driving it for now. If it is indeed the coil pack, I am going to replace it with a screaming demon, but I want to know your opinions first. So... lets hear what you all think.
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Old 09-16-2008
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So nobody has any ideas huh???
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Old 09-16-2008
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If the misfire just started when you replaced the plugs I would put them under suspect.
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Old 09-16-2008
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i replaced my coil pack plugs and wires at 50k. i think u just need to get new pack and wires and call it a day. investment well worth it.
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Old 09-16-2008
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The problem didn't start with the plug change, they have been in for about 3000 miles, the problem just started within the last 100. Money is kinda tight right now so I will probable replace the wire first and then go from there. If they don't fix it, time for a screamin demon!
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Old 09-16-2008
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Plug wires have two different failure modes, one is the insulation (which you have tested for), and the other is the conductive core has broken down, causing the effective series resistance to be way high, or even an open circuit. I would replace the plug wires, your coilpack is probably ok.
Or, you could have screwed your truck up with some of your "performance mods"
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Old 09-16-2008
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What performance mods??? The only thing I have is JBA headers and they were put on a month after I got the truck. That isn't the problem. Why would the wires not cause a misfire above that RPM though? Everytime I have seen a set of wires go bad, (arching out or bad cores either one) they will mis at any RPM if there is a load on the engine.
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Old 09-17-2008
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New Info!!! So I noticed that it doesn't do it after the truck has cooled down. Like in the morning when you start it, the fast idle is on and it is above 1200 RPM, so it doesn't do it. Well, of course it wouldn't because it never does it above that RPM, no matter what the temp is. Well, after you drive the truck and then shut it off and let it cool down for a couple of hours, you can get back in it and it seems ok. When this happens, the engine temp isn't up to normal yet, but it isn't completely cold either. When you start it, everything under the hood is cooled down, but the engine itself is warm enough for the fast idle to stay off, and it idles at the normal RPM. However, it doesn't mis until after a few minutes of running. It does it before the engine is fully warm, but like I said, it takes it a minute or two to start acting up. Hmmm.... sounds very similar to an ignition module going bad... Its like when the coil pack cools off, it starts working correctly again, but as it starts to warm up, it also starts messing up (assuming this is actually the problem). Do you guys agree on my coil pack theory? Anything else to add? Or do you all still think the wires are the problem? Any reasoning behind your opinions? I'm all ears!
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Old 09-17-2008
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It is probably bad plug wires, since that's the last thing you touched, replacing the plugs. Did you use the grease on the boots and check the plug wires are seated?

Might as well pull those new plugs, see if one is darker than the rest, suspect that plug wire.

There are resistance checks you can do with the coilpacks, but I don't have that info. I'd put a plug in each wire and verify spark, before I even THOUGHT about replacing it.
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Old 09-18-2008
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One more time, replace the plug wires......
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Old 09-18-2008
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At 58,000, the coil pack should not be one of the first things to consider. BTW, there is no low and high speed circuit. The ground side of the primary circuits of the three coils in the pack are individually switched by the PCM to time the fire to three pairs of plugs.

I would look first for things that could be related to the recent plug change: a cracked plug insulator or a loose/damaged secondary wire. Beyond that, you could check for lean problems: a dirty MAF, vacuum leak or unmetered air leak.

Lesser possibilities: bad injector, soft carbon buildup on intake valves.

I agree with the previous suggestion that you pull the plugs to look for a significant coloration difference. That could help you narrow it down to one or two problem cylinders.
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