Hi everyone,

I have a problem with a Mustang GT 2006 auto (JLT CAI, ford racing shorty headers and BBK throttle body).
I have a random misfire on 5th cylinder for a long time.
Sometimes the car run on 7 cylinder on cold start, sometimes not and sometimes the cylinder stop to work on the road...

+I changed some parts to fix the problem:
-new spark plugs
-new coil-on-plug
-interchange the injectors

=> the problem was still here

+I checked the manifold intake and found a leak between the intake and the cylinder head (the seal was damaged). I was happy to find this and changed the seal.

=> the problem was still here

+I checked the connector from the PCM to the injector and the spark plug and everything seems to be in order. I connected my multimeter to the wire of the spark plug and the injector to check the voltage and everything looks good...

At this point I have no other idea to find the cause of this random misfire.
Can you hlep me please?

 

A couple of things: Did you use a light smear of dielectric grease just inside boots before installing over plugs. This will keep moisture out of the boot. Too much though or loading the boots can cause misfire. Did you change the boots with the new coil pacs? Using old boots may not seal properly allowing moisture in between plug and boot to cause misfires. Using old boots may also cause misfire as they may have carbon tracing inside the boot allowing voltage to run down the side of the spark plug to ground. Also the spade terminal in older coil pacs, that the coil spring attaches to, may corrode and turn green.

 

Do you have a bad spark plug?


If you are running Champions they are known to be of poor quality.


You have the 2 piece design that Ford so graciously bestowed on us... when you changed the plug did you put too much nickel anti seize on it?