Changing Spark Plugs and Plug Wires on a 1999 Mercury Cougar V6

The old Cougar developed a misfire. Sometimes when I hit the gas, it would hiccup or go chug-a-chug, especially if it was below 2000 RPMs. It would also lose power when starting from a stop, so it seemed like I didn't know how to work the clutch right. I bet my kid was wondering why I couldn't work the clutch right. Once, the check engine light started flashing at me when I didn't let off the gas while it was misfiring. At night, in the dark, without any lights on, you could see blue rings of electricity going up and down the spark plug wires, especially when I revved the engine.

Cylinder 4 (front of the engine, front of the car) had some misfires counted up on the computer when I scanned it. Also, the O2 sensor #1 on bank 2 (same side as cylinder 4) was taking longer to warm-up than bank 1. I figured I'd just do plugs and wires together. But before I could get to it, the check engine light came on for P0420, catalyst efficiency below threshold.

I decided on a set of NGK wires and NGK Laser Platinum spark plugs. I was going to try the NGK laser iridium plugs that had an iridium center electrode with a platinum tipped ground electrode, but RockAuto.com was out of stock. I really like the performance of double platinum plugs, so I'm still happy I got the Laser Platinum plugs.

Judging by the wear on the old Motorcraft spark plugs, I'm guessing that they were in there since the car came from the factory. If the plugs were original, then the Motorcraft wires on the car were probably original too.

The new plugs were pre-gapped to 0.044". The old plugs were worn much wider.

I measured them with a ramp gauge after I took them out.
Plug #1: 0.075"
Plug #2: 0.071"
Plug #3: 0.067"
Plug #4: 0.092"
Plug #5: 0.089"
Plug #6: 0.089"

I was a little worried about cylinder 6 after it was still tight when I turned it 1/8th turn, so I squirted some WD-40 down the hole and turned the plug back and forth until it was a little looser, before backing it all the way out. This is just one of those things I do by experience. When you're loosening any bolt and it stays tight, or gets tighter, don't continue loosening it. Work it back and forth with WD-40.

Here, you can see the plugs from cylinders #4, 5, and 6 had about twice the gap they should have. I normally wouldn't post about such a routine job, but I had to show you the huge gaps on these plugs.

The new spark plugs are threaded longer, but that just means they fit more cars. Since there's no washer on these (just a tapered seat), the spark plug box recommended 1/16 turn to tighten them.

Before installing the new plugs, I put a little anti-sieze compound on the threads. This should prevent the new plugs from binding and damaging the threads the next time they need to be removed.

Before installing the plug wires, I put dielectric grease (included in the plug wire box) on both ends of the plug wires. This will keep moisture out, and also prevents the plug wires from gluing themselves to the plugs.

Even though it's a sideways V6, getting to the back three cylinders wasn't too bad. The worst part about the job was that it was freezing cold outside.

Now that it's done, it's really nice to drive without any misfires.