2005 Ford F150 King Ranch Edition 5.4 Misfire

This vehicle came into the shop with a customer complaint of a rough running engine, a misfire.  Customer said he was driving along on a trip and all of a sudden the truck started to misfire and run poorly.

First things first, we verified the customer's complaint.  Took the truck for a test drive and confirmed the misfire.  Next, we connected our diagnostic tool and checked for stored codes.

We pulled three DTC's.  They were P0171 System Too Lean (Bank 1); P0303 Cylinder 3 Misfire Detected; P0316 Engine Misfire Detected on Startup; and P0353 Ignition Coil C Primary/Secondary Circuit Fault.  We checked the freeze frame data and started our diagnostic.

We weren't concerned with the first code, with an active misfire on that bank, the lean code is explainable.  We needed to know what was causing the misfire.  Just a head up, on the last code P0353, the Ignition Coil C . . . C is number 3 cylinder.  Ford uses letters to correspond to the cylinders.  So "A" is 1, "B" is 2, "C" is 3 . . . "H" would be cylinder 8.  Okay, you get the idea.

Another FYI for you, Ford commonly mistakes cylinders.  So it may throw a cylinder 3 misfire, but when you start investigating you determine it isn't number 3 that actually has the misfire.  So be careful with that too.

Now, we needed to see what was causing this misfire.  Was it a plug, coil, wiring, or PCM?  With the P0353 code, any of these could be possible.

So, we hooked up our lab scope and sync probe and did some investigating.  First, let's rule out the PCM and wiring.

We checked battery voltage at the PCM with key off and with at KOEO, both showed good.  So wiring seems to be fine.  What about the driver in the PCM?

That's where our lab scope and amp probe come into play.  The picture below shows two traces (one red and one green).  The red trace is through the amp probe connected around the power feed.  This wire was accessible in the steering column and was fairly easy to get to.  It is a dark blue wire with light green tracer.  The green trace is our sync probe, allowing us to know which cylinders are which.  In this case, we were connected to number one cylinder.  With that info we could go by the firing order if we saw any problems in the patterns.  Here, what matters, is that all 8 cylinders are accounted for.  You will notice there are 8 red "lines" between each green "line".  This would suggest the PCM is onboard and functioning.

Now, we need a closer look to see where the problem lies.  The below pic shows a zoomed in view of the above pic.  Notice the difference in appearance of the two sets (3 spikes each) of red traces.  The first is nice and pointed, the second image (to the right) sort of curves over to a point and then drops straight down and then straight back up.  The first one is a known good pattern.  The second is a view of a shorted secondary ignition component: either plug or coil.  I know these trucks and engines are known for bad coils, but you have to follow the evidence to make a proper diagnosis and the right repair.

The next pic is an even more zoomed in look at the known good pattern.

The pic below is the faulty pattern . . . again, notice the differences between these last two images.

As I said earlier, these vehicles are known to mis-identify the offending cylinder.  So, we moved our sync probe from cylinder number 1 coil (control wire) to cylinder 3 coil (control wire), the cylinder that is supposedly the misfiring cylinder.  Below is the pic of this.

Based on the above pic, we knew it was indeed cylinder 3 that was the misfiring cylinder.  So, in this case, the PCM had the right cylinder pegged all along.  Now it was a case of determining if it was the spark plug or the coil that had shorted.

It turned out to be the spark plug.  The plugs had been replaced about a year ago.  So, we removed that one plug and replaced it with a new one.  We also ended up replacing another coil because it showed signs on the scope of weakness.

Started the truck and it ran like new.  Took it for a test drive, allowed the monitors to run, double checked no misfires being reported, lean situation straightened out with the misfire no longer present, cleared the codes, and shipped the truck.