I have been chasing a random missfire in my 98 5.4. It got brand new Motorcraft plugs about 6 months ago and that temporarily fixed the problem. When the trucks gets up to temp at idle it'll run smooth, then misfire for a minute or two then go back to running smooth. While it's misfiring, it makes the truck feel like it's bouncing up and down when you're in it. I am throwing no CEL as well. Any ideas? It's just more annoying than anything as it's not critical to the use of the truck. It's just being used as a farm truck for the time being anyway.

 

Need to know which cylinder is misfiring to give recomendations. Get something like a Bafx bluetooth/wifi scanner and a laptop with Forscan or a cell phone with Torque pro and find out which cylinder is misfiring.

When you replaced the plugs did you also replace the COPs? The above should tell you where the misfire is, move that COP to a different cylinder and see if the problem follows. If so then you have a bad COP. If not then you have a bad plug or connection to the COP.

 

How do you know its a random missfire if you get no code? You need a scan tool that will do mode 7 live data so you can see whats going on. Might be fuel pressure. Change your fuel filter lately?

 

I did not change the COP. I've had multiple scan tools on it before and they don't find anything. A bad COP does make sense though. Whould the other scan tools you mentioned for sure tell me which cylinder it is if it is indeed a bad COP?

 

I was told the fuel filter was changed a month before I got the truck almost a year ago. I believe it because the filter looks shiny and I have an old one sitting in the tool box that came with the truck.

 

A good scanner will see the misfire counts, even when below threshold to throw a code. This info is available in the pcm, so it can tell you which cyl is the problem. This is called mode 06 data.

Scanners which only scan codes cannot tell you this.

While cops are a common problem there's no guarantee that's what this is. But when you know which cylinder it is you can start swapping parts with other cylinders to figure out what the actual problem is, and no cost.

 

cheap scan tool dont show live data. There is one that works with smart phones..

 

I have a friend that has a scan tool that will tell me what I need to know. Hoping to get that taken care of sometime this week. Will update as soon as it's done. Thank you all!

 

Coils and more coils. I had one that would misfire only after the motor was really really hot. Without an expensive scan tool just replace them all, cheap to buy online and diagnosing them without a scan tool is impossible.

 

Had a friend of mine hook up a fairly expensive scan tool to the truck and that didn't even help us. I guess I'll just replace all of the coils. What brand should I stick with? Where is the cheapest place to get said brand?