Here's the background, 2004, 5.4 w/ 145k. Last month I changed the spark plugs and the engine developed a rough idle. So then I went and changed all the COPs too. But the engine is still rough at idle, and ran fine at normal driving RPM ranges. And since it isn't throwing any codes I just wrote it off.

Well I was driving in the mountains this weekend and when the engine was under a serious load there was a real miss and even a clicking sound. But since it isn't throwing any codes, I don't really know what is wrong. I did the system checks with my Innova scanner, but still nothing is showing up. I think there is a bad plug, but is there any way I can diagnose it without pulling each plug again? Is there maybe something else I am missing?

 

If I had just changed the my plugs and immediately noticed a rough idle afterwards I would assume its probably one of the harnesses I disconnected during the plug change. Check all the injector and COP connections. With that many miles all the clips are bound to be brittle and tend to break making the connector not completely seat. Also, there are usually a couple vacuum lines that one might unhook to make it easier to get at the plugs, make sure they are hooked back up.

 

What brand plugs did you use?

 

I got a similar problem, but did plugs about 30k ago and just did all cops. Is it possible that an old coil fouled a plug and should I replace all plugs again? Thanx

 

Thanks, everything looked secure when I put everything back together, but I will double check everything again. I'll also hook up my vacuum tester to see if there is anything going on.

Thanks for the suggestions.

 

I got Autolite plugs from NAPA. They are the same design as the Motorcraft ones.

 

Worked on it today. Checked the vacuum with the gauge and it was reading a steady 20 Hg from the power steering vacuum port on the manifold. Cleaned the MAF. And then I started going through each plug again, one by one, pulling them and checking them. Actually found one bad one on the driver's side. Got through 6 of the 8 (still need to do the last two on the passenger side) but I am still having the rough idle. I even pulled the connections on each of the COPs one by one to see if that made the engine worse, and when I did that each time I disconnected it the engine ran worse. I am running out of ideas. I am going to check the last 2 plugs when I get the time, but I don't know of anything else to do. Anyone have any ideas?

 

Run the Motorcraft plugs. These engines are real finicky when it comes to the plugs. I can't stress that enough, use the correct motorcraft plugs. Go to motorcraft.com and look up the correct plugs.

 

this is intresting. if you go back to the motorcraft plugs and it works. i would have to take the autolite out of my truck and go back with the motorcrafts also. but wouldnt suprise me one bit. Im learning that most say designed to meet oe spec but dont really.

 

humm, try this. turn off the engine and disconnect the negitive battery terminil for two minutes. then reconnect it and drive for 15 miles and reevaluate the run rough. by the way why did you decide to tune it up anyway, was there a problem?

 

I changed the Autolight plugs to Motorcraft plugs tonight and it seemed to work. The truck is running like it always used to. No real rough idle anymore. It was a pain, but honestly I am glad it was this instead of something mechanical.

 

Sorry I missed this before I made the change. What would disconnecting the neg terminal do?

 

It would have reset the PCM, but I doubt it would have worked as the plugs were the problem. Glad it is solved

 

About disconnecting the negitive battery terminil does is that within the pcm there is a program that collects information about the performance of the engine. In the trade it is called the "adaptive stratigy" when there has been a defect with the way the engine has been operating it stores that info and it will make adjustments to compensate for wear. like in this instance you may have made a change of spark plugs and perhaps the old plugs were worn to the point where the fuel trims had to compensate to maintain engine performance, now with the newer plugs they are burning more efficently and the pcm is still making the compensation of perhaps a less amount of fuel to each cylinder. now the new plugs are actually burning leaner, ie spark knock, rough idel, weird kind of stuff. once you disconnect the neg. cable it can take up to 15 miles before the engine starts to run "normal" again. but that action will erase any bad stratigys. in the trade I perform this action everytime i do a tune-up on a high milage vehicle but i do it in another way which is called "clearing the KAM" or otherwise known as the "keep alive memory", doing it this way i dont have to reset the radio stations or the clock. Hope this helps, good luck

 

Thanks for the info. I will definitely try that. By the way, I originally changed the plugs because I was up to 140k and thought it was time to do it even though I wasn't having noticeable engine problems.