I have a 2006 5.4L F150.

I am encountering a misfire on the #3 plug.

The only OBD error messages I receive are from misfire on #3, no other faults.

I have checked the plug, and it looks pretty good.

I have recently changed all COPs.

I have removed the injector rail and checked and cleaned all injectors.

I have removed the wire connector from COPs and checked the voltage while the vehicle is running.

The #3 connector is measuring around 13.6V

The other COP connectors I measure 0.17V to 0.4V.

It appears this low volt reading is due to the neg signal wire closing and opening.

I brought it in to a mechanic and he indicated the computer needs to be replaced, no signal was getting to COP.

So this appears where I am at.

I have hence gotten a LED test light and verified the other COP connector wires cause the test light to flash, while the #3 COP wire connection to just stay on all the time, mo blink signal.

Any ideas a to what the problem is?

The engine sounds pretty good, maybe a slight tick sound. But the misfire wobble is apparent.

I figure I will change the computer at this point, since it appears this is where the issue is.

What else could interfere with the COP neg signal? It is pretty clean in the engine compartment, and no visible problems with the wires fraying on the accumulator, as I have read reports happening.

Is it the cam that makes this negative signal wire open and close?

 

I don't know about your voltages, but 9 times out of 10 a miss fire is due to a bad COP. Even tho they are new, I would swap out #1 with #3 and rerun to see if the misfire stays on #3 or moves to #1.... You may be right, but I hate to see you buy a computer for nothing.

 

Swapping the COP's as suggested by @steve(ill) is easy and doesn't cost anything. Certainly if it is #3 COP, the miss would follow it. But I'm afraid the answer is in the diagnostic work you have done already.
This condition would produce a solid miss on #3.
And NO, it is not the cam that makes the negative wire open and close. The ECU has a solid state switch that completes the primary circuit of the COP based on the crankshaft position offset by the desired degrees of advance or retard. Most likely THAT solid state switch (inside the ECU circuitry) is "SORTED" to ground - resulting in the LED test light to stay on ALL the time.
You might be able to unplug the connector on COP #3 and "Ohm" the ground wire out back to the large plug on the ECU and make sure it is not shorted to ground anywhere outside the ECU PLUG. If THAT is not the case, it sounds like your (and the mechanics) diagnosis both says it is a bad circuit in the ECU.


Good luck

 

I have swapped around COPs.

Thanks F150Torqued, I believe the mechanic is probably right. He wanted 1500, but I know I can get a used computer for much less. He was not a Ford mechanic, but appears to have been right at this point