Spark Plug Caused Rough Idle

firevert

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I got a check engine light 2 days ago, I was 60 miles overdue for my regular 3,000 mile oil change, so I got my oil changed at a shop thinking it was a low oil pressure issue but the light persisted.

My car just hit 100,000 miles so my mind jumped to spark plugs which were still the factory ones, I replaced those with Autolite iridium plugs along with Motorcraft plug wires making sure to clean the coil pack connection to the plug wire, and took pictures to ensure every wire was connected to the right places, and then verifying with diagrams.

I did a full battery ECU reset and started it up after just plugs, and again after wires, then drove it for like 1 minute around my neighborhood, it was very choppy, backfiring without revving at all, but the engine was not shaking, it was smooth but loud, rumbly, and smelled of fireworks/unburned fuel. The idle will flutter between 750 and 1,000 (it's an automatic), idk what's wrong with it
 

86GT351

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Did you verify that when you pulled all 8 plugs they were all together? Meaning did any electrodes fall off? Also did you install the plugs by hand?
 

DieHarder

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I'm going to assume from your description that you have a 4.0 V6. Sounds like either a bad connection (one or more plug wires loose; bad wire; bad plug; poorly installed plug). Or, you accidentally swapped wires when you installed the new wire set (not that hard to do).

Here's a manual reference for comparison.

Coil Terminal to Cylinder Relationship: https://iihs.net/fsm/?d=210&f=Engine Ignition.pdf
Components: https://iihs.net/fsm/?d=212&f=Engine Ignition Components - Exploded View.pdf
Specs: https://iihs.net/fsm/?d=209&f=SPECIFICATIONS.pdf

If you're still not sure after checking the references or can't verify suggest visiting a mechanic or dealer.
 

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