Misfire after changing spark plugs

[Cortcase]

Ive got a 94 4.0, new to me, 137k miles. It had a bit of a rough idle and slight hesitation when cold. I knew the spark plugs and leads were probably original, so I figured that was the best place to start.

I purchased new plugs and leads from Napa. Gapped plugs, changed one at a time, comparing lead length as I went. Everything by the book.

But after I changed everything, the idle and hesitation got worse. The engine is now clearly misfiring. The misfire is the worst when the engine is cold and it's raining. When the weather clears up, and the engine is warm, you almost wouldn't know there was an issue.

My first suspension was the leads. Perhaps they aren't getting a good connection, or maybe one is faulty. I ohmed them all out, and they were all good. The old leads also tested good, but barely within spec. My second suspension is the coil. This is where we get to my first question.

I tested the primary and secondary, comparing values against Chilton's manual. Spec for the three primaries is .3 to 1 ohm. All three tested .6. spec for the secondaries is 6.5k-11.5k ohms. This is where things get odd. All three coils tested at 14.13k ohms, ±.02, post to post.

This is the first oddity. If two pairs of posts had been between spec I'd definitely reace the coil pack, but since all three are out and even, I'm suspicious of Chilton's spec.

Also, Chilton's testing procedure produced no reading. The manual says to test from pin to post. But the connector pins are part of the primary, therefore they have no connection to the secondary, therefore no continuity. Testing post to post produces a reading, but it's out of spec. But all three are identical.

I'm at a loss. Am I doing something wrong? Should is replace the ignition coil? What are the odds I got a bad plug or two? Is there something else I should be looking at?

 

[Uncle Gump]

What brand of plugs do you install?

If you say something other then Motorcraft or Autolite…. that is where I would start.

 

[Dirtman]

I agree with unckie gump, fords hate anything but motorcraft/autolite ignition parts. The part where it gets worse when it rains leads me to think wires or coil though. One of the old school tests for wires/coils was to wait till dark and then mist the engine with water. If the wires or coil were bad the water will light up like christmas.

I don't know how to perform a "real" test on the coil packs.

 

[Cortcase]

I used Bosch platinum finewire plugs and Belden leads. Should I use different leads as well or start with plugs?

 

[sgtsandman]

Do one thing at a time to figure out what is causing the problem. Bosch in general does not play well with many brands of vehicles. Honda and Ford seem to be a couple of them. So, I would look at the plugs first.

 

[Cortcase]

So, tell me if this would be a reasonable test. When I took the old plugs out I was surprised at how good of condition they were in. Really the only thing wrong with them was they were out of gap spec and I would not be surprised if they were quite new. Would it be reasonable to clean the old ones up, re-gap them and put them in temporarily to see if that helps the issue?

 

[Dirtman]

It would certainly tell you if you're on the right track... bosch plugs are effing terrible.

 

[Kira]

38 days have past and we're no closer to....knowing.