Finding a CoP Misfire with a Scope

With wide spread use of coil on plug ignition systems, testing secondary ignition components with a scope has become a bit more of a challenge. With no plug wire to attach your leads to, getting a secondary pattern is all but impossible.

A good way to verify a coil is misfiring with a any Coil on Plug(CoP) system is by testing the primary ignition circuit with a scope. Below is an example of a good coil on a 2000 Mustang with an intermittent misfire. This pattern looks very similar to the standard secondary ignition pattern you are used to viewing except that the voltage scales are a bit different. Instead of measuring voltage in KV or kilovolts, the scale is from 0 to about 400 volts for most systems.

This vehicle uses a multi-spark or msd ignition set-up. As long as the RPM are below a certain value (usually around 1200 rpm or so) it will fire 3 times per cylinder event instead of just once. Many Ford vehicles use a multi-spark ignition system for low RPM ranges to improve emissions.

The capture below is of the misfiring coil, cylinder #3. There is not any significant difference between image below and the one above. That is because the misfire is NOT occurring at idle, only under a heavy load. The easiest way to reproduce this is with a hard snap.

Below are a couple of captures from one of the good ignition coils on this vehicle taken on a hard snap of the throttle from idle.

Notice that the multi-strike went away and now the coil is only firing once. This is normal since the engine RPM has risen above a predetermined value. Also note that the burn line is not perfect; a lot of hash on both signals. A good coil CAN look even worse than these two screen shots with excessive hash in the burn line. The point is that a good coil does not have to look perfect under a hard snap throttle situation.

A coil that is misfiring will have its burn-time reduced to almost non-existent. Take a look below at the offending coil.

While a good coil can spike the voltage up over 300 volts the burn time will usually be at least 300 to 400 microseconds or longer, the misfiring coil will have almost no burn time (usually around 100 microseconds).

Take another look at the good coil (below) under a hard snap throttle. It has about 400 microseconds burn time.

The faulty coil, as seen below, has very little burn time on a hard snap. Although it is difficult to see , the burn voltage is also much higher (which is why we get such a short burn time). In the picture above, the burn voltage is only around 40 volts. (do not confuse firing line voltage spike with burn voltage)

It will take a little practice to get good scope captures, but this the best way to verify an ignition misfire with a scope when dealing with coil on plug ignition systems. Although this article deals with a Ford CoP system, these testing procedures should work with any coil on plug system.

Some shops or techs may only replace the boots on the coils instead of replacing the entire coil, but I do not recommend that practice when dealing with Ford vehicles. The coils are not very expensive and can have carbon tracks or other internal damage from the misfiring that can generate a comeback. In any case, you should replace the spark plug for that cylinder, at the very least, as it will often have a carbon track along the side of the ceramic. Be especially careful with the 4.6L and the 5.4L Ford engines when removing and installing spark plugs. These engines are very fragile having few threads at the spark plug hole and can strip easily. I do not recommend changing them on a hot or warm engine.

Practice makes perfect, so use any spare time wisely and practice capturing snap-throttle primary waveforms on any CoP vehicle. Once perfected, you will be able to find misfires quickly and accurately on many newer vehicles.