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Ok, here's the deal. My 2000 Expedition with the 5.4L motor started running with a slight rough idle. At that time the Service Engine Light came on so I had the code read at Auto Zone. It said that cylinder 6 was misfiring. Without time to mess with it immediatly it continued to be driven, and it started running worse and worse. At the most severe point it would shake so bad it couldn't be driven past 50 mph, and was only getting 2.3 mpg. Becuase I know these trucks often have ignition coils go out, that was where I started. I removed the coil to cylinder 6 and tested it using an ohm-meter. It read a 5.4 MOhm, and the book states it should be between 4 and 10 MOhm. With this I believe the coil is ok. My next move was the spark plug. I removed the old one, and it was bad. So I bought a Motorcraft plug and installed it. Starting the truck back up I noticed the SEL went off, and the truck was running much better, however there was still a slight rough idle and it is all the time. The problem is I don't know where to go from here. Do you guys think it may have a vaccume leak, need more plugs, or even coils? I am positive it is time to replace the back four plugs, seeing that they are original and the truck has 130xxx miles on it, but if it was a cylinger mis-fire again shouldn't there be a SEL light on? Just looking for some possible answers.
Thanks,
Kyle
 

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Check on the fuel injector. That could also be your problem on #6 cylinder
 

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Mine did a very similar thing but one day I heard a slight hiss. It turned out to be a cracked vacuum T that would cause the rough idle. If it helps I found it next to the firewall near the center of the engine. The vacuum would pull the T together at higher rpms and it ran fine but at an idle the cracked portion of the T would open up and cause the rough idle. It progressively got worse.

I hope this is it because it only took a couple of dollars to fix. It also had 130000 miles on it....
 
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