How to Upgrade Ignition Coils

The Ford overhead camshaft modular V8, commonly referred to as the “mod motor”, is an excellent engine with good efficiency and performance potential. These engines have been popular since the original version that began production in 1991, and it remains a popular platform for performance. There have been several iterations of ignition systems on the mod motor, with the earlier motors using a pair of quad-coil packs to the modern style that uses a coil-on-plug arrangement for the best possible ignition control. The coil-on-plug design was put into use in 1997 on the trucks, and 1999 on the cars.

Where a single coil system is convenient, the performance is lacking, especially in fuel injected cars. Coil-on-plug systems provide better fuel economy and performance, but swapping out coils definitely seems like more of a challenge. We brought a 2004 Mustang GT with the 4.6 liter V8 into the shop to show you just how simple it really is. The Mustang had been running a little rough and was lighting up the check engine sign. A quick scan with our code scanner, and we knew what the problem was – there were several cylinders with consistent misfires. Over time, the coil packs can weaken, not producing enough spark, add to that worn spark plugs, and the problems start mounting up.

A set of new NGK platinum spark plugs and all new Accel Supercoils were ordered from our local NAPA parts store. We could have gone with the factory-type coil, but the reality is that the Accel coil packs are actually cheaper than the OEM originals, and they provide a more powerful spark. This means the engine will run better, get better fuel economy (because it burns the fuel more completely), and have more power. Why not upgrade ignition coils? Follow along as we show you how to upgrade ignition coils on our test vehicle.

The process of swapping the coils and spark plugs is fairly straightforward. It all starts by removing the air cleaner hose that runs from the airbox to the throttle body.

This gives you access to the coils. The Mustang uses metric bolts, so make sure you have metric sockets. We used a 1/4”-drive ratchet with a long extension. The coils have a single hex-head screw holding it to the intake.

Grip the coil firmly and twist it while pulling it out of the socket. Only work on one cylinder at a time. You do not have to remove the fuel rails as some suggest, the coil boots will flex enough to get the coil out.

Sometimes, the coil will come off of the boot, if this happens, just use a pair of needle-nose pliers to yank the boot.

Now you can unclip the wiring harness.

The old spark plug uses a 5/8” socket. Pull it out.

We used a spark plug gapping checker to make sure the gaps were set to spec. We opened them up to .055” for performance. The factory Ford spec is .052” to .056”

The threads of the plugs were treated to a coating of anti-seize, and then threaded back into the head.

The electrode portion of the coil needs some dielectric grease to keep it from corroding. This comes in the kit with the coil packs.

Next, the coil was dropped into the intake and seated onto the spark plug tip.

At this point, the original fastener is threaded in and tightened up. Not too much, the threads can strip out pretty easily.

Reconnect the wire harness. Repeat seven more times. Once finished, replace the air filter inlet hose.

Once all of the coils were installed, the job is done. The yellow coil packs look good under the hood, and the power was evident the second the engine fired up. The idle was smooth, the throttle response was more crisp, and on the street, the Mustang had more power than ever. While you won’t gain much horsepower, 10-15 % more spark energy makes a big difference in overall performance and fuel economy. The entire process took about 2 hours.

Check out all the maintenance parts available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on how to upgrade ignition coils, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.