Howdy,

I have a 2010 Ford Expedition EddieBauer edition with the infamous Triton 5.4L 3valve I bought earlier this year. I know I know..."Why did you buy that?" you may ask. If only I knew then what I know now. What a pain so far. 

Anyway. I started to get a misfire a couple of months ago. It strangely developed the day after I replaced one tire that had a puncture on the sidewall, but that is beside the point. However, I can't figure it out. It has about 160,000 miles on it. As I have been working on it to figure it out, I found that it is only misfiring in warmup. Once it warms up, it instantly goes away. Additionally, as I was trying to run injector cleaner through it and run it in high gears to get the RPMs pumping, I discovered that the misfire goes away if I run it in first gear for about 15 seconds and let it rev up to 4000 or 5000 RPMs. 

I was only getting a p0308 (misfire on number 8) when I first started diagnosing and the check engine light would go away after the misfire stopped and would only come back the next time it started misfiring in a subsequent trip (it doesn't misfire every warmup). Now it has a constant light with a p0316 (misfire on startup; less than 1000 revolutions)

Things I have tried with no avail

• Swapped coil pack from cylinder 8 to another cylinder and cleared codes by unplugging battery, no change in piston number.
• Swapped spark plug to another cylinder and cleared codes by unplugging battery, no change.
• Replaced plugs (Motorcraft), coils, and boots: The service manual calls for new plugs, boots, and coils at 160,000 miles, so I went ahead and replaced them all because I was almost to 160,000 when this all started. No change.
• Spark checker: I used a spark checker to check each plug and pack. It lit up on each plug and pack.
• Noid Light: Used noid lights to check the fuel injector. They all light up as well. (Don't drop those things in the engine compartment. They are impossible to find.)

At this point I took it to a mechanic to have them check it. They ran some diagnostics (I didn't tell them what I had already done because I figured I was paying, why give them the chance to skip on checking my work). They couldn't get the misfire to repeat, but they did see it had been misfiring on the same piston I had been seeing. I took it home after two days and told them I'd bring it back while it was misfiring for further diagnostics. The next morning I started it up, it was running like a pig so I ran it over to them.

• Vacuum test: They plugged in a vacuum gage. Gage ran stable. They said this would tell them if it was a broken rocker. He plugged in the guage, it ran rough for a little longer and then ran fine after it finished warming up. He seemed stumped and gave me a bottle of fuel injector cleaner. 
• Fuel injector cleaner (Chevron): I ran that through the system, still misfiring. Ran another brand (STP) with less fuel in the tank, still misfiring. 
• Replaced fuel injector for #8: The mechanic said this would be the next thing he'd try and he didn't know what to do after that, so I replaced the fuel injector, no change. 
• Pulled valve cover to check rockers and valves: after trying to do a compression test to only find I didn't have the correct adapter I decided to just remove the valve cover while I had all the coil packs and everything off to do a compression test anyway. After watching some videos and reading a little, some people have had troubles with rockers breaking in this engine, so I thought why not take a look. Everything seemed clean and in place (visual inspection only).

Additional info.

• Old spark plugs seemed clean
• no coolant or water was in any of the spark plug ports.
• It has troubles accelerating when misfiring and shakes a lot at idle. Check engine light flashes when it is bad.
• Doesn't burn oil, no leaks, no smoke
• Haven't seen any drop in coolant level
• Sprayed brake cleaner around intake manifold and vacuum hoses, no change in RPMS 
• It has a quiet ticking noise as it runs, I think it is the sound of the fuel injectors (could be something else) and it is all the time, not only when misfiring. I'm fairly certain it is not valve train noise because I had oil pressure issues in a different vehicle and the clickety clack was very apparent on the valve train. It sounds nothing like that. 
• No strong gas smells.
• Exhaust pipe outlet is not sooty.
• This has all been in the Texas summer, so no cold temps yet. 
• No metal shavings in the last oil change which was after this all started. 

I want to do a compression test next, but I need to buy a compression tester because the loaner tool at the local parts store does not fit the Ford plug sockets, but before I do that, I just wanted to check to see if I am missing anything (obvious or not) that I should check or replace as well. What should my next step be?

• Could it be phasers or any other part?
• could it be the Engine Coolant Temperature sensor (ECT)?
• If any suggestions require a code reader/diagnostic scanner, which would you recommend?

Thanks,

Q