I've been trying to diagnose a misfire on cylinder 2 in my 97 expedition 5.4l. Running out of ideas, so I started checking everything. I've swapped plug, boot, cop, and injector with 1. Still misfire on 2. Checked voltage to injector and coil. Coil voltage on a dc reader seems to be 0.8v, but might be too fast to catch. 1 and 2 read the same.

My question is what does that yellow tab in the plug do? Pics attached. Coil 2 doesnt have one. Could this be preventing something from firing the coil? The other plugs seem to have one, but it may be coincidence. Thanks! Have not checked compression. Wanted to rule out this harness problem before I pulled the spark plug again.


Picture from harness plug on cylinder 1.



Picture from cylinder 2. No gasket or yellow insert, but voltage seems fine.

 

You're just thinking electrical and not mechanical. Do a compression or leak down test first to make sure the cylinder and valves are in good shape.

 

I have a replacement COP connector which looks very similar to the one on your #2, but has a red seal down inside.
I don't think a missing seal will cause much of a problem.
But, it does look like your #2 connector may have been replaced.
When you say "voltage seems fine" are you seeing the voltage from #2 coming out of the coil or at the PCM connector?
The voltage will go back to the PCM where it will be grounded under control of the PCM.
If the connector has been replaced check the splice.

 

I just checked the voltage at the connector. It read very similar to cylinder 1. I'm going to pull the plug and check compression soon, but if that's not good I'm likely going to pull the injector and coil power and just run on 7. No more money in this beast.

Can I test the spark with the coil out? I was always told to put it on the head, but if the pcm should ground should I just hold it in air? I assumed two connections in the plug was enough for power and ground in a DC circuit. I'm hoping electrical because cheap fix. Mechanical isn't worth it on this vehicle unfortunately. Thanks guys!

 

12v comes from the ignition switch in RUN or START and goes into the COP connector on the RD/LG wire.
The circuit continues thru the coil and out the other wire, PK/WH (?), to the PCM.
The PCM will ground the circuit causing current to flow thru the coil and create a magnetic field.
When the PCM removes the ground the magnetic field in the primary coil collapses.
The collapsing magnetic field induces a 15kv pulse in the secondary coil causing an arc across the spark plug gap to ground.
The COP is just a step-up transformer with 2 windings (coils) in it, a 12v primary side and a 15kv secondary side.
The 12v primary circuit should pulse as long as the COP is connected even with no spark plug installed.
The 15kv secondary circuit will arc if it can reach a ground which might be you so be careful.
With the COP removed insert a spare spark plug and the 15kv pulse will arc across the plug gap if the metal base of the plug is touching ground.
Be sure to pull the fuel injector connector.