Misfire when cold for 15 min

I have a 2010 with about 125K on it that I recently purchased. It ran pretty good, but had an occasional miss so I figured it was time for plugs... Put in new Autolite plugs and replaced the boots. It had rough idle. May have been worse than it was, or I may just have noticed it more. Ended up replacing the coils also with new Delco coils. It made no difference. I am getting a cyl 3 misfire code. I added 2 bottles of red Heat to the fuel and took it out for a 50 mile ride. The first 5 - 10 miles still had a bit of a miss, but it cleared up and ran great for 40 miles. Thought I had it fixed.

Next morning same rough idle and would throw a code when I got on it at all like entering highway. Then it clears up and runs great. Does this for 5 - 10 miles when it is cold and then runs great with no missing or hesitation even when idling.

I'm thinking it may have been exhibiting some of this behavior before since I usually run it pretty easy. I would like to get it fixed though. I'm thinking it is injector on cyl 3? Any thoughts?

On what cylinder(s) was the occasional miss before the plug, boot, and coil change? If #3, then I agree with your injector suspicion, and will add checking out both the injector and coil wiring harness, plugs, etc. Then move to compression testing.

If its the injector I'd probably replace it rather than sending it off for ultrasonic cleaning or rebuilding. You've just bought your Autolite plugs, but suspect you'll be changing those out for OEM Motorcraft plugs sooner rather than later.

I meant Motorcraft... I didn't get any codes before changing it all, but really didn't get my foot in it as hard as I have been since changing everything. If I kind of drive it gently, I don't have much of an issue now except a random miss while idling. It is when I hit 4K on the tach getting on it that it has a real miss now. That goes away in 10 miles and I can run as fast as I want.

I am going to try swapping plug and coil from #2 and see if it follows anything.

You could try this: start the truck one morning and when the miss appears, turn off the engine and pull out the #3 plug. See if the tip is wet with moisture. Good luck!

I bought a new plug this weekend to replace the one in the cylinder that was having an issue. Replaced plug, put an older coil back on, and reseated injector connector several times. Runs great now. Gut is telling me I had a bad plug since it kind of started when I replaced the plugs, not the coils, but was tired of playing with it. Runs fantastic now.

Sound like someone may have "power washed" engine.