I just threw a code for cyl # 4. Comment was "VERIFIED". Had this same problem a year ago: Replaced the plugs-ng, replaced the wires-ng, Replaced the coil assembly-seemed to cure the problem. Back then the problem would go away and after the plugs i thought it was fixed but wasn't. After the wires it seemed to be fixed but then the miss came back. After the coil it has stayed gone for about a year and 10kmiles.It is back. New observation is that in the morn it might run OK. After sitting after a warm-up it will start acting up. Whats next? Start over with the plugs? Oh, and this is a new observation: When it is acting up if I go into the throttle the miss worsens. Nurse the throttle and it subsides. Just like I would think a bad plug might do. I have also had a bad wire do this and a couple times I had a "weak" coil that acted like this.

Any help?

 

My 1994 4.0l eats spark plugs, motorcraft or autolite last a bit longer but they still fail
I have had them fail while driving, once at highway speed, misfire and bad spark plug
But often first noticed at startup cold and then it starts working again when warmed up, then will start misfiring 'when ever'

I used original spark plug wires for 15 years, new motorcraft set in 2008, and still on there and working fine, and still have original 1994 coil pack
But spark plugs just do not last in this engine, I only use regular copper now since the spark plugs are easy to change on this engine and the Double platinum only lasted a bit longer

Drivers side bank uses Reverse spark(center to tip spark) so more likely to notice problem on that bank first, so 4, 5 or 6
With the waste spark system it shouldn't matter but my experience seems to be that it does

Pull out #4 spark plug and #5, compare them
If #4 is a cleaner white then you may have the start of a cracked head.
If #4 is darker color then injector could be sticking

Nice very light brown is expected
Swap #4 and #5 plugs if no cracks are seen in #4, then see if misfire code P0304 changes to P0305, gap is larger 0.054 is spec

If possible pull all the spark plugs and do compression test, just to take that off the table for further diagnostics, you can end up chasing your tail and spending money on non-fixes when compression is the issue, compression is rarely intermittent, so once tested its off the table, it IS the cause of the misfire
4.0l OHV runs 9.0:1 ratio so expect 155-165psi, but actual numbers are not the issue since testers and situations vary, the comparison of all cylinder is the point of a compression test

 

Outstanding advice. I never considered plugs being "eaten" but I am buying that experience. Thank you. Your trouble shooting and eval info is great. Old school, like me.

I was told that this coil pack will self destruct if a wire or plug opens up and that an individual cylinder can fail while all others jugs go on working. Possible?

Thanks,

John

 

No, there is no self destruct for the coil pack, must be a Chevy or Dodge thing.....................lol.

There are 3 coils in the V6 coil pack, each coil fires 2 spark plugs at the same time
To balance any multi-cylinder engine you need to setup piston and crank to have Matched Pairs, to counter-balance the movement of the pistons
With V engines 2 cylinders on opposite sides will be at TDC at the same time, on a V6 engine that will happen every 120degrees, 3 sets of pistons, 1 set every 120deg = 360deg, crank has 1 full rotation
Engine is balanced

The 4.0l V6 has these Matched Pairs, 1/5, 2/6, and 3/4, 3.0l is the same Matched Pairs

The firing order on the coil pack should make sense now
3 4
2 6
1 5
front

1 and 5 spark at the same time, as do 2 and 6 and 3 and 4

You can look at any Firing order from any engine and tell its Matched pairs
302(5.0l) uses 1-5-4-2-6-3-7-8
To get the Match Pairs split it in 1/2 and put one 1/2 above the other
1-5-4-2-
6-3-7-8

Matched pairs are 1/6, 5/3, 4/7, 2/8

The 351 has different firing order but same Match pairs, 1-3-7-2-6-5-4-8
1-3-7-2-
6-5-4-8
1/6, 3/5, 7/4, 2/8

They just used different Cams, the Cams set the firing order,

So back to the coil packs and Waste spark
Since there are 2 spark plugs firing at the same time one spark is wasted, hence the name, on the cylinder that is on its exhaust stroke, nothing left to ignite.
The 2 spark plugs are wires in Series
So electricity travels from + to -,(actually the other way buts thats another story, lol)
When the coil sparks, the high voltage from the + side that coil travels to the spark plug and jumps the gap from center to tip, then travels thru the engine's metal(block and head) to the other spark plug's TIP, it then jumps the gap to the CENTER of spark plug and up thru the spark plug and wire back to the coil's -, so there is a completed circuit and both cylinders got spark

In theory if one spark plug or spark plug wire is disconnected/broken then it should disable BOTH spark plugs, but high voltage will travel to lower voltage, + or -, i.e. lightning, it jumps a LARGE gaps, lol, but not just to ground, its closest lower voltage, like power lines or airplanes.
And lightning travels in BOTH directions, cloud to earth and earth to cloud, so + or - are not the prerequisite, its the voltage difference
So because of the high voltage being used the "still connected spark plug/wire" stays working, usually
I don't know enough about high voltage physics to explain it better than that


The Waste Spark system was used on the very first gasoline/alcohol engines, they used a flywheel/magneto with points, so spark plug fired at every TDC, compression or exhaust stroke, so oldest spark system there is and easiest to set up.