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Need Help Cant figure out rough idle!

February 13, 2008
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City, State
Bizerkley Mi
Year, Model & Trim Level
2002 Limited
Ok, so my 2002 4.6 Limited has a rough idle or "miss" when its @ 1000 rpm. I have changed the plugs, downstream O2 sensors, cleaned MAS, cleaned IAC, cleaned TB, cleaned injectors (done by mechanic), adjusted TPS, ran seafoam through TB, all with no change.:confused: Also sounds like it has a light nock especially when I let off the gas. Almost sounds like there might be a exhaust leak 2. So should I change the upstream o2 sensors? Could it be a bad coil or injector? If so how do I test the coils and the injectors? I really need to get this figured out, its driving me mad! Thanks in advance for your help guys!:confused:

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Did you replace the plug wires? How many miles are on it?

Yambig sounds right, the plugs can cause a lot of trouble for the idle..sounds like that's all you haven't replaced yet:)

Is the truck throwing any check engine lights?

No codes

No codes, NO PLUG WIRES either. The 4.6 has coil on plug. I would need to replace the entire wireing harness for the injectors and the coils. I havent even seen them for sale. You guys are right though, it could be the source of my problem. A wierd thing happened, after I posted yesterday. I was driving up to the local gas station, listening to the truck. When I just decided to stomp on it from a dead stop. (never really did that since I got the truck) When I came to the next stop, the idle was smoother and the sound was gone! I drove some more, still no noise. I will test drive it some more today and see how it sounds and idles. :confused:

You might be all carboned up. Driving like a little old lady isn't the best for them. I really wouldn't think your coil on plugs could be bad.


Well that could still be the case. I did run seafoam through th TB, crank case, and fuel tank. But no smoke show when I did it. Not that I drove Miss Daisy for the month that Ive had the truck, but I never matted it to the floor cuz I knew it wasnt running right. This time I did and it did help. :confused: Any one ever replace the wiring harness for the coils and injectors on thiers?

Any one ever replace the wiring harness for the coils and injectors on thiers?
Not necessary for yours (Coil on plugs). The wires are low voltage signal wires.
When you changed your plugs, did you clean your coils and apply fresh dielectric grease? The spark could be arcing to ground through the old dirty grease if you didn't.

No dielectric

There was none to begin with. I didnt know that was required. Are you talking about where the spark plug slides into the boot for the coil?:confused:

Also STILL running rough.

Mainly between 850 & 1000 rpm, is when I hear the noise. Coasting to a stop, it sounds like a light nocking sound. Goes away when I stop completely. Also have a loud injector, does this mean its bad? Also in that rpm range is when it idels the roughest. @ 550 rpm(in Drive, warm) it idles so smooth you cant feel the truck run. (like it should) Help!:confused: