I have a 05. with a 5.4 3v... 150k ... I have replaced all 8 plugs with oe motor craft, replaced all 8 cop's with motorcraft, and replaced all 8 fuel injectors with motorcraft... Truck does run much better. However, I still have a hesitation under acceleration, and I get a engine light indicating misfire cylinder 3 ... I have swapped the plugs and coils around. Misfire stays at number 3. It is under hard acceleration, about 4500 rpm... Truck idles perfect, no cam phaser noise, no oil consumption. Has a miss you can hear and feel in park at about 1600 rpm up to 1850, then cleans up at 2000... What am I missing ???
Has anyone else had anything similar ??

 

You may want to do a compression test.

 

That was on my list, have not done that... I have heard that there is a program the dealer has that can check the relative compression without removing the plugs..... ????

 

dealer might be able to do a relative compression test with a computer, but that isn't always accurate. i would do a manual compression test and also check the wiring from the pcm to the coil. make sure your terminals are tight in the connectors and are not spread. make sure you don't have any corrosion or and bare wires that rubbed against something or got burnt.

 

Wiring to injector or coil. Compression check first. I've seen many times effed up wiring to coils from people or animals.

 

Did compression check... First time around...all cylinders were 150psi, exception of #3..it was 145 psi.. Did a recheck just to verify.... This time all were at 150 psi, except # 3 which was now at 120 psi.. I checked # 3 several times, all results were 120 psi... Added some motor oil in plug hole, same result , 120 psi... I looked up the ford spec chart and 120 is within their range....Rotated plugs and coils around, test drove, same thing... Misfire # 3 under hard acceleration. The truck had a pronounced miss when it was cold, but cleared up after a couple of minutes...

 

check the wiring going to the coil make sure no mice have chewed it up or it hasn't rubbed anything. check your terminals at the coil and pcm make sure they are not spread or pushed out of the connector and not green with corrosion. you may have a failed pcm if the wiring checks and ohms out alright

 

If the cylinder that's misfiring has lower compression than the other 7, you have an internal cylinder issue. If oil doesn't raise it, that rules out rings. You might want to borescope it.

 

Most likely an exhaust valve if it's 30psi less. The psi difference will make a misfire. You could ohms the injectors and check all wiring still.

 

I did a cylinder leakdown test this weekend.... No issues... All cylinders were less than 10% drop.. Valves appear to seal off very well. Very, very minimal loss by the rings. Did not have access to a borescope though....